I have a friend who doesn't believe in the expansion of the Universe.
His main argument is that our speed relative to the microwave background
is nowhere near the speed of light, and so we must be at the centre of the
expansion. How do I answer this? And are there any good articles to read?
Submitted by sbauer1"AT"juno.com
Can I ask why your friend is anti-expansionist?! This seems to be an extreme position to take! Like deciding that you're not going to believe the earth is round, or deciding that all Moon landings were done in a TV studio!
In any case, I should attempt to explain the expansion of the Universe once more. The basic evidence is from measuring the redshifts of distant galaxies, and finding that the redshift increases with distance. This goes back to Hubble's original studies of 1926, so as you can imagine, we've come a long way since then! There is now no room for doubting that the redshifts of galaxies increase with distance from us. And the only viable interpretation is that the Universe as a whole is expanding, everything from everything else, so that *anyone* can consider themselves the centre if they like.
The CMB dipole shows that we *are* in fact travelling very fast through the Universe. Something like 600km/s for the motion of our Local Group of galaxies relative to the CMB photons. Remember also that on top of this, there is the motion of our Galaxy relative to the Local Group as a whole, the motion of the Sun round the Galaxy, and the annual motion of the Earth round the Sun - if you were to believe that we are genuinely at rest in a special place, then you'd have to decide which velocity is the right one!
I suspect that your friend may be confusing the expansion velocity with the small local velocity that each Galaxy has. You should picture the Universe as expanding smoothly in all directions. However, the individual objects in the Universe also have small gravitational effects on each other. So each Galaxy has a velocity on top of the Hubble expansion velocity, and this can be as much as several hundred km/s. This is just what you observe when you look at a cluster of galaxies - they have a whole bunch of velocities, with a spread of perhaps 500-1000km/s, but the average velocity of the cluster as a whole is the Hubble expansion velocity. This is just the same situation for our Galaxy - our local "peculiar velocity" is a few hundred km/s, while the Hubble velocity is zero here (as it is zero for all observers at their own location!).
The basic point is that the situation would be the same for an observer on any galaxy in the Universe: there is a relatively small peculiar velocity, on top of which the speeds of galaxies increase linearly with distance in all directions.
Results from the COBE satellite are one part of the clear picture we now have of how the Universe works. There are many articles you could read as background for the modern view of the Big Bang etc. I thoroughly recommend the article which appeared in Nature in 1992 (vol. 357, p288) by Peebles, Turner, Kron and Schramm or the similar one from Scientific American, which you can find here.
Expansion, as it is explained, occurs only in the voids of space,
namely the space between galaxies, not the galaxies themselves,
or the Earth, or my leg. So what's going on here? And given this
heterogeneous expansion, shouldn't there
be many directions (notably in the directions of galaxies and
large scale structure) where there will be a difference
in the CMB temperature due to travel time in areas of spacetime that
are not "expanding"?Submitted by sbauer1"AT"juno.com
The Universe as a whole is expanding, and this is a result of the large scale gravitational field of the whole Universe. However, on small scales, the gravitational effect of the Universe is easily overcome by the effects of any local concentration of matter. When our Galaxy (and other structures in the Universe) formed, the process started by having a local overdensity of matter (exactly where that came from is another story entirely!). This led to a little bit of gravitational attraction among the matter there, meaning that it was expanding a little bit slower than the Universe as a whole. Eventually the region became compact enough that it stopped expanding all together, started to collapse, and then complicated gas physics and other processes came into play and a galaxy was formed (again another story!).
This means that any object with a significant concentration of matter has locally stopped expanding with the Universe, although all the space between such objects is still expanding. Within our Galaxy the average density is around a million times the average density of the Universe, and so we are entirely dominated by "local" gravity. Hence all the objects within a galaxy, including the dust and gas clouds, the stars, the planets, the life forms, and their legs, are not expanding. But the vast amount of space in between the galaxies certainly is.
In fact there are effects on the isotropy of the CMB which come from just the sort of non-uniform expansion which you describe. However, those effects are certainly calculable - they turn out to be only important on very small angular scales, and even there they are always pretty much negligible. The reason is that if you are a photon crossing such a region, then you will blueshift on the way in and redshift on the way out, which almost cancels the effect of the local gravity. But there will be a small change in the energy of the photon due to any change in the gravity in the time it takes the photon to cross the object. This ends up being a much smaller effect than you might first have thought. So a smoothly expanding Universe containing no-longer-expanding objects (i.e. our Universe!) still has a very smooth CMB.
Does the CMB have a center or an edge? If so, where is the center and
edge?
Submitted by Pureman33"AT"aol.com
The CMB is just a uniform sea of photons, which are left over from the hot early phase of the Universe. So I'll assume that this question is essentially the same as asking about the properties of the Universe as a whole.
The simple answer is that the Universe has no centre, and no edge! And that the Universe doesn't have to be so simple that it's easy to get a clear mental picture of what's going on! In the theory of General Relativity (which as far as we know works very well for understanding gravity), space gets curved by the matter that's in it. So if there's enough stuff in the Universe, then space can be so curved that you could, in principle, set off in a straight line, go all the way "round" the Universe, and come back to where you started. Or alternatively if you could see clearly out to very large distances, you might be able to see the back of your head! If that was the case, then the Universe would be curved round on itself (in some abstract 4th dimension), so that it was finite in volume while having no edge.
On the other hand, most people's interpretation of the evidence is that there isn't enough mass to "close" the Universe. In that case, it is likely that the Universe is either genuinely infinite in extent, or at least so much bigger than the part we can observe, that it's infinite for all practical purposes. To understand that, you just have to realise that it's basically semantics: the Universe is everything, so it can't have an edge!
As far as a centre is concerned, the fact is that everyone is the centre! The whole Universe is expanding, everything from everything else, and so anyone can think of themselves as being the centre of the expansion. But in reality there is no special "centre" to the Universe. This obviously makes sense if the Universe has no edge, since it's clearly silly to think of the centre of something that's infinite.
So the best picture for the Universe is a very large (possibly infinite) thing, full of galaxies, which is expanding in every direction at once. The CMB is the background of radiation left over from when the Universe was very hot and dense. As the Universe expands it cools, and so we see the background radiation as microwaves, coming from all directions. There's no significant large scale pattern discernible on the microwave sky precisely because we live in a fairly mundane part of a Universe with no centre and no edge.
I study Physics at University and I am having trouble finding information on
things like: How are the initial density fluctuations related to the angular
fluctuations that we see in the CMB? And what would we observe if the
Universe were periodic?
Submitted by m.p.pendlebury"AT"ncl.ac.uk
If you are studying physics, then you ought to have enough background to read some of the original work in the field. You might like to start with the article in Annual Reviews of Astronomy & Astrophysics (1994) vol 32, p319, which you should be able to find in your University Library. That will give brief explanations of the questions you raised, and also point you to other relevant papers.
Of course much has changed in the last few years, but the basic physics remains the same, and this article in ARAA remains one of the best overviews of the subject (if I do say so myself!).
I am trying to get the CMB spectral data points for a plot and
data fitting routine which I will demonstrate in a graduate
thermodynamics course that I will start teaching soon. Could you
point me to a place where I can download the points, preferably with
attribution to each of the experiments?
Submitted by kevinmccann"AT"erols.com
It's always good to make your own plot, just to convince yourself that the CMB really is such a staggeringly good blackbody! There are rather a lot of different measurements now, so it's useful to have a place where you can look up a bunch of the data points. I think the best recent compilation is in George Smoot's conference review, which is obtainable at astro-ph/9705101
There's an explicit list of measurements at different frequencies, with error bars and attribution. There are also nice plots in our new web-update for the Particle Data Book (astro-ph/9711069), but we don't actually list the points there.
My question centers around the surface of last scattering. If I
understand correctly, some of the key equations that have been used to
determine the physics of the universe depend on two variables, one of
which has usually been ignored (the pressure?) because it becomes so
small as to be irrelevant. Does this assumption still hold at the time
of the surface of last scattering or should the pressure figure back
into the equations?
Submitted by rassler"AT"cleo.bc.edu
The pressure is certainly a very important part of the physics of what's going around redshift 1000 when the Universe becomes neutral and the photons last interact with the matter. So rest assured that when theorists calculate the real thing, they don't forget to include the pressure!
In today's Universe pressure on the whole is negligible. The Universe is dominated by regular stuff, which acts like a largely non-interacting fluid on big scales. Such stuff is usually referred to by the term "dust", which here has a technical meaning. So for the recent history of the Universe you can indeed ignore pressure.
But earlier in the Universe the radiation content becomes more and more important. And back when the Universe was so hot that all the matter was ionized, the photons were very strongly interacting with all those charged particles. At those early times in the history of the Universe, it would be a hopelessly bad approximation to neglect pressure effects. Indeed the effect of pressure on the generation of CMB anisotropies is very much tied up with the existence and detailed shape of the bumps and wiggles in the "power spectrum" of anisotropies, through which we hope to be able to understand all the physics of the large scale Universe!
What is the magnitude of our motion relative to the CMB (RA, Decl, km/s)?
Submitted by Wolverine"AT"netgate.net
The best number comes from analysis of data from the COBE satellite, and I will just quote those numbers. We are moving at a velocity of 370.6 +/- 0.4 km/s towards galactic coordinates (l,b)=(264.31+/-0.17,48.05+/-0.10). which corresponds to RA=11h12m, Dec=-7.2.
These numbers are specifically for the motion of the Sun relative to the CMB. Of course the Earth is in motion around the Sun, and so there is an annual variation in the Earth's motion relative to the background. In fact COBE was sensitive enough that it could detect the motion of the Earth by the changing temperature pattern in the sky throughout the year!
I'm writing a short paper for my undergraduate level introduction to
astrophysics course. The topic is on the source(s) of the CMB. What
are the most important physical processes that created the CMB photons?
It seems that baryogenesis, nucleosynthesis, and element formation are
all important, but I'm not sure where my focus should be.
Submitted by rkeyes"AT"rice.edu
The answer to this question depends to some extent on what you are really asking. In some sense the CMB photons that we detect were created in the Earth's atmosphere, as they are absorbed and re-emitted along the path of the light. But that's not a very useful answer! Along similar lines, the photons were last scattered at a few hundred thousand years after the Big Bang, and you can think of scattering as absorption of a photon and simultaneous emission of a new one in a random direction. As you go even earlier in the history of the Universe, individual photons interact ever more strongly with matter, and at times before about 1 year photons lose even energy information during scattering processes. So none of the photons we see today contain information about things happening before about the first year in the history of the Universe. What you presumably want to know is where the photons came from in the first place though!
The process of nucleosynthesis (formation of the light elements) happened in about the first three minutes. In fact there were already so many photons per baryon (normal matter particle), that the extra photons created from the nuclear energy released at this time are quite negligible.
At super-high energies particles and anti-particles were being created
and annihilated constantly -- appearing out of pure energy (photons) and
then annihilating again into pure energy. At higher energies even higher mass
particles and anti-particles were appearing and disappearing, and existed
in roughly equal numbers to the photons at that time. So early in
the history of the Universe there were lots of protons and anti-protons around,
for example, earlier than that there also lots of higher mass exotic particles,
and so on. The lightest particles we know about are the electrons, and so they
annihilated last. Before about 1 second the Universe was full of
electrons (e-) and anti-electrons (e+) and
photons (
),
in about equal amounts. Then as the Universe expanded, the temperature
dropped low enough that if you annihilated an e+-e-
pair there wasn't enough energy in the average photon to recreate the pair.
So eventually the Universe lost most of its e+'s
and e-'s, and ended up with mostly 's.
Why we have any e-'s left over is a good question
(that's baryogenesis), and I talked a little about that before.
But this annihilation process is basically what produced the photons in the
CMB, with most coming from e+-e- annihilation, some
contribution from
µ+-µ-
and 
+--
annihilation a little earlier and a tiny bit extra from nucleosynthesis.
And of course it's always possible that other unknown physical processes
occurring between the first 1 second and the first year could have
played some role in generating extra photons as well.
I'm doing an essay on "Penzias & Wilson and the discovery of CMB". I
would be very grateful if you could send me any information on the
discovery of CMB and on Penzias and Wilson themselves.
Submitted by ketal.patel"AT"ic.ac.uk
This story has been told many times before, and by people much better qualified than me to tell it, who also did a much better job than I could do here! So let me just suggest places where you can read more about the history of the discovery. One excellent place to start is the popular book "Afterglow of Creation" by Marcus Chown (University Science Books). This gives an excellent overview of the whole story, written in an exciting way, while being pretty fair on all the main players, and presented at an accessible level. This is a book I'd recommend to my mother!
For more "on the spot" viewpoints, which are necessarily more subjective, you can't do better than the articles written by Wilson himself, although they may be a little hard to obtain (in Physica Scripta, vol. 21, p599 (1980), and in the books "Modern Cosmology in Retrospect", edited by Bertotti et al. (1990), and "The Cosmic Microwave Background: 25 Years Later", edited by Mandolesi & Vittorio (1990)). The Princeton point of view is presented in Peebles book "Principles of Physical Cosmology" (1993), as well as in an article by Wilkinson & Peebles in "Serendipitous Discoveries in Radio Astronomy", edited by Kellermann & Sheets (1983).
There are even more intriguing parts of the story involving "predictions" of the CMB by Gamov and collaborators (see Alpher & Herman's article in Physics Today, August 1988 issue) and regarding whether the CMB had already been inadvertently detected, either through anomalous excitation in interstellar molecules by McKellar as early as 1941 (see Thaddeus in Annual Reviews of Astronomy & Astrophysics, 1972 for more details), or in radiometer measurements by Ohm and others in the early 1960s. There is a fairly extensive discussion of these topics in the review article (in english) by Melchiorri & Melchiorri in La Rivista del Nuovo Cimento, vol. 17, no. 1 (1994), as well as the book "3K: The Cosmic Background Radiation" by Partridge (Cambridge, 1996).
I am wondering what the redshift is to the last scattering surface.
Also, shouldn't
this give a very good number for the age of the universe? That is,
aren't the
theories about the temperature of the universe at last-scattering times
very good,
and so shouldn't the observed redshift of the CMB give a very tight
estimate? What
am I missing?
Submitted by billgr"AT"sunoptics.caltech.edu
Let me review the nature of redshift first.
Since more distant things have their light more shifted towards the
red end of the spectrum, then we can use this "red-shift", conventionally
denoted by z, as a measure of
how far away something is. Light takes longer to get to us from more
distant objects, and so when we see something at high redshift, we are
seeing it as it was when the Universe was much younger; hence redshift is
also a measure of time back into the early history of the Universe.
In the standard expanding Universe models, it is best to think of redshift
as being due to expansion rather than velocity. In other words, if you see
something shifted towards the red, that's because the light was emitted when
the Universe was smaller, and as the light travelled towards you it expanded
along with the rest of the Universe, having longer wavelength by the time
it reached you. So objects seen in the more and more distant past are
seen with their light redshifted more and more (because the Universe is
smaller and smaller as we approach t=0).
Redshift zero is today, the most distant galaxies are seen at about
z=5, the CMB photons last scattered at approximately z=1000,
and the Big Bang took place at z=
.
What exactly was the redshift of last scattering of CMB photons? Well, the last scattering surface is really a shell rather than a surface, ie there's a range of redshifts over which the photons suffered their last interaction with matter. The central redshift is around 1100, with a width of about 100 in z. This depends a little bit on the precise cosmological model (eg how dense the Universe is, or how fast it is expanding), but surprisingly doesn't change very much. It's a pretty good approximation to look at some CMB photon that you've just detected and say "this photon probably last interacted with matter back at z=1100". (What age the Universe was at that redshift does of course depend a lot on the cosmological model, which is precisely why cosmologists use redshift as their measure of time!).
The second part of the question above concerns predicting the CMB temperature. It's true that there are some rough arguments for the order of magnitude of the CMB temperature. But there is no precise prediction for how hot the radiation background should be. What I know is that if the temperature is about T0=2.73K today, then the photons last scattered at about z=1100 (when the matter was going from an ionized to neutral state). If I measured a different temperature for the CMB today, then I'd infer a different redshift for last scattering. Say for example that T0=10K instead, then I'd infer that I lived in a Universe where there was more radiation, and the last scattering redshift would have been substantially lower.
It would be great if there was an independent method to predict the redshift
of last scattering, since then I could indeed use the measured CMB
temperature today to constrain the age of the Universe at the last scattering
time. Unfortunately no such independent estimates exist, and so I can
only tell you that last scattering is at z
1100
because I've measured T02.73.
What are the significant factors that the CMB has contributed to developing
our cosmological ideas? (apart from being a large constituent
of the proof of the Big Bang).
Submitted by micky"AT"acon.com.au
So being a cornerstone of the Big Bang isn't enough for you, eh?
To summarise how important it is in this regard: the discovery and confirmation of the CMB was instrumental in building our current picture for how the whole Universe behaves with time. Namely, that it used to be very much hotter and denser, and is continuing to expand and cool.
But there is more to it than that. The precise black-body (thermal equilibrium) nature of the CMB spectrum has ruled out any other serious possibility for modelling the evolution of the Universe. Moreover, it puts precise limits on a wide range of physical processes occurring over timescales of roughly 1 year to 1 million years after the Big Bang (since these would lead to "distortions" in the spectrum, which are not seen).
Separate from that are the basic things that the CMB spatial patterns tell us. Basically, the near isotropy of the temperature is very strong evidence that we live in a Universe which is very smooth and uniform on large scales. So our Universe is pretty much the same in every direction, doesn't rotate much, etc.
The the detection of tiny differences in temperature on the CMB sky tells us about density fluctuations on large scales in the early Universe. Here we are learning a lot very rapidly, so the picture is still not entirely clear. For sure we have learned that gravity alone is probably responsible for growing all of the structures in the Universe. Secondly, it appears that there are fluctuations on the largest scales, of just the kind that so-called "inflationary" models of the early Universe predict. How far that goes towards proving inflation is still a contentious question though!
Certainly we now know the amplitude and shape of the "initial conditions"
for density fluctuations in the early Universe. We also know that general
models with a large component of Cold Dark Matter look like they're on the
right track. And in the near future we're going to start learning more
details about the values of fundamental cosmological parameters, like
0, so we can tackle questions such
as "will the Universe expand forever?", "how old is it?", "how much
stuff is there?", "where did it all come from?", etc.
Can you postulate any other plausible explanations for the 2.73K CMB
temperature other than residual radiation from the Big-Bang? Couldn't
heat radiated by our galaxy itself into a surrounding cloud of
intergalactic dust or hydrogen be the reason for the CMB?
Submitted by stardot"AT"worldnet.att.net
The assumption that the microwave radiation we observe is a remnant of a hot phase in the early universe is obviously a fairly radical one. And so we have to make sure that we have exhausted all reasonable options before deciding this is the best explanation. Because this is quite an important question, my answer will be somewhat involved.
Hot gas tends to emit or absorb in various spectral lines or bands. Dust being zapped by starlight or other radiation will cool by emitting a fairly continuous spectrum. However that spectrum tends to be characteristic of temperatures somewhat higher than 2.73 Kelvin. Typically cool clouds in the interstellar medium of our galaxy are maybe 100K, with some rare cases as low as 10K. But it is generally only the very densest cloud cores that have temperatures as low as even 10K. Although the spectrum from radiating dust grains is fairly smooth, it is generally not well represented by the standard "blackbody" shape, but varies somewhat differently with wavelength (technically modelled with an "emissivity index" which would be zero for a blackbody, but is usually between 1 and 2 for interstellar dust). There are also broad spectral features in the spectra of dust grains, which would be seen as deviations from a blackbody shape.
When the spectrum of the CMB was still rather crudely determined, it was still possible to contrive models where there was very extended dust which was absorbing starlight and re-radiating it as microwave radiation. But as the spectral measurements became more and more precise, such models got backed further and further into a corner. The last remaining notion held that perhaps grains of a special shape and composition (so called "iron whiskers") might fill the Universe and give radiation more or less indistinguishable from a blackbody. However, the exquisitely accurate blackbody shape measured by the FIRAS experiment on the COBE satellite killed even this idea. Now there is no type of dust that we can think of that would be able to fit the spectral shape which has now been measured so precisely.
On top of that, any dust would have to be extremely uniformly distributed in order not to give itself away in the spatial fluctuations on the sky. Recall that the fluctuations in the CMB temperature in different directions on the sky are about one part in 100,000 of the overall temperature. If the emitting material had anything to do with our Galaxy, then it would be expected to vary on the sky in a way which correlates with the plane of our Galaxy. No such variation is seen in the 2.73K CMB emission.
A further constraint is just to ask what other effects such dust might have. If you have enough dust around to emit that much microwave radiation, distributed in our Galaxy and presumably every other galaxy, as well perhaps in intergalactic space, then it turns out that it would have a very significant obscuring effect on distant galaxies. The fact that we see distant galaxies and quasars in every direction we look, except right through the plane of our Galaxy (where we know our view is blocked by thick dust), implies that there isn't obscuring dust all over the place in the Universe. Working through the numbers makes it very hard to have enough dust to radiate significantly at microwave wavelengths, without blocking our view of distant objects.
One last piece in this puzzle is that in fact the general emission from dust emission has been seen and that it peaks around 200 microns (or 0.2mm) in wavelength. This is almost a factor of ten shorter wavelength than where the CMB peaks, and the total energy in this "far-infrared background" (FIB?) is about 30 times lower. There is good reason to believe that this background is due to the emission from dust in galaxies distributed throughout the Universe, with the dominant contribution coming from galaxies which are undergoing periods of rapid star formation. So the cosmic emission from dust has been discovered, and it is something very different from the CMB (and of course interesting in its own right).
Somewhere I've seen the CMBR temperature stated as 2.728K; elsewhere as 2.726K.
Could you give us an idea of the latest estimates, with references? Just how
dependent is this calculated temperature upon the value of k, the Boltzmann
constant, only known to something like 8.5ppm? Is the CMBR
energy value currently more precise/accurate than the CMBR temperature value?
If so what is the value in energy units of your choice?
Submitted by jebush"AT"ridgecrest.ca.us
The best temperature for the CMB is currently 2.728+/-0.004K, where this uncertainty represents a statistical 95% confidence region. This comes entirely from analysis of data from the FIRAS experiment on the COBE satellite. The reference is D.J. Fixsen et al., Astrophysical Journal, Vol. 473, p. 576 (1996), if you are interested. So far there is little extra limitation on the temperature from the other experiments, which operated over a wider range of wavelengths but with much less precision. This situation will obviously (one assumes!) change with time as more precise data become available.
Boltzmann's constant k is what you multiply temperature by to get an energy. As has been pointed out, k is not known to infinite precision, but is 1.380658(12)× 1023J/K, where the number in brackets indicates the uncertainty in the last 2 digits. However, this constant is known much more accurately than required for getting at the CMB temperature. I suspect I will not live long enough to see uncertainties in fundamental constants being the limiting factor on getting a precise CMB temperature!
The useful quantity for the CMB is the amount of energy per unit volume, or the energy density. As to whether this energy density is better known than the temperature, I'm not entirely sure. There are probably several places that things like the Boltzmann constant enter indirectly (certainly in the calibration process I should think), which will confuse matters. But rest assured that at the moment our knowledge of the energy density, the number density of photons and all similar quantities is just limited by uncertainty in the temperature. For your information: there are about 412 CMB photons per cubic cm (with an uncertainty of about 1); the energy density is the equivalent of 0.261 electron Volts per cubic cm (again uncertain by about +/-1 in the last decimal place); the equivalent mass density is 4.66 × 10-31 kilogrammes per cubic metre; the peak of the spectrum is at a frequency of 160.4 GHz (uncertain by about +/-0.1); and the peak intensity of the background is about 385 MJy/Sr (that's MegaJanskys per Steradian, which is not a unit you meet everyday!). I hope that's enough numbers for you!
In your answer to the "How come we can tell what motion we have with respect
to the CMB?" question, there is one more point that could be mentioned.
In an expanding universe, two distant objects
that are each at rest with respect to the CMB will typically
be in motion relative to each other, right?
Submitted by fklaess"AT"pt.lu 10/98
The expansion of the Universe is certainly an inconvenience when it comes to thinking of simple pictures of how things work cosmologically! Normally we get around this by imagining a set of observers who are all expanding from each other uniformly, i.e. they have no "peculiar motions", only the "Hubble expansion" (which is directly related to their distance apart). These observers then define an expanding reference frame. There are many different such frames, all moving with some constant speed relative to each other. But one of them can be picked out explicitly as the one with no CMB dipole pattern on the sky. And that's the absolute (expanding) rest frame!
I'm looking at the physical properties of blackbodies. The graph
shows the max of the CMB curve at about 2mm.
The Wien displacement law claims T = k/
,
so T(Kelvin) 2.9/(mm).
Therefore the CMB Temperature is 1.4 Kelvin. What's the trick here?
Submitted by GASNER"AT"aol.com 10/98
A hot object emits energy over a range of wavelengths. Ideal hot objects
are called "blackbodies".
Wien's law is the simple property of a blackbody, which says that the
peak output (in wavelength) is related to the temperature: hotter objects
peak at shorter (bluer) wavelengths, cooler objects peak at longer (redder)
wavelengths. The usual formula is that
peak×T=2.9. What Wien's law tells you
explicitly is where the energy emitted per unit time per unit area
per unit wavelength peaks. This quantity is usually called "intensity".
When you deal with intensity you are free decide whether you want to use
wavelength units or frequency units (since they are just related by the
speed of light: ×
=c). For
wavelength units the natural thing is to deal with
I, the amount per unit wavelength. If
you are using frequency units (Hz), then it's natural to deal with is
I, the quantity per unit frequency.
The peak for I is shifted a bit compared to
the peak for I. Knowing the shape of
one curve you can easily work out the shape of the other. So it's
straightforward (if a little messy!) to work out the shift. Take my word for
it that the peak of the I is at
a frequency corresponding to the wavelength for which
peak(mm)×T(K)=5.1.
For this curve the blackbody peak for the CMB is at about 2mm, and so this
must have been what you saw plotted. For an
I plot, the peak would be more like
1mm.
Several years ago on some television show
(probably PBS, NOVA) they did a piece on background radiation, with an
interesting twist. The scientists involved converted the
radiation they were detecting into humanly audible wavelengths of
sound. This was an incredibly fascinating noise, besides being the
"sound of the universe" it came across as a mix of white noise and
hypnotic repetition.
I was hoping there would be someone on the internet
broadcasting this converted sound, but no luck
so far. If you know what I am talking about and have any recordings of
background radiation lying around (or know where I could be directed),
I would greatly appreciate it.
Submitted by michael_wynne"AT"yahoo.com 10/98
I dimly recall such a thing too. Anyone else remember the specifics? Let me know!
The frequencies are many orders of magnitude higher than anything that you could hear. On the other hand the wavelengths of the photons in the CMB are quite similar to the wavelengths of sound that you can hear. So if you took the waves to be sound waves and generated a range of waves with intensities given by the shape of the blackbody spectrum of temperature 2.7 Kelvin, then I guess that that would be like "hearing the sound of the Universe". Sounds like a cool idea!
The result would sound like "noise", although there would be a range of wavelengths, peaking at some particular value (roughly 1mm), so I'm not sure exactly how that would sound. Certainly it would repetitive! I think in fact the wavelengths are a little short, so you may have to cheat by a factor of maybe 100 in order to get it near the centre of your ear's response. I suspect also that, because the ear is a kind of logarithmic detector, you might have to play the intensity scale too in order to hear something interesting.
I realise there is only one answer I can possibly help you with, =
having myself submitted a proposal for a TV documentary to the company =
and inquired about the series
[abridged]
Submitted by tmgulland"AT"hotmail.com 10/03
Well here in Britain we had a programme called 'Brain Spotting' presented by Kenneth Campbell (to whom I sent a copy of my book), an it was on Channel 4, by Windfall Productions, in I think 1997 or 1998, which featured just this. I phoned Channel 4 a few years ago - they just type in the data you give them and coem up with an answer.
Could you tell me specifically what is the wavelength and frequency of the
CMB?
What is the irradiance in watts per square meter of the CMB at the surface
of the Earth.
Is there a receiver design/circuit published that one could build to detect
and monitor fluctuations in the CBR.
Submitted by amchitka"AT"email.msn.com 11/98
The CMB has a "blackbody" spectrum, i.e. its spectrum covers a range of wavelengths (or frequencies), peaking at around a couple of millimetres (or a frequency of around 150 GHz). This peak, and the general shape of the spectrum is characteristic of the emission from something (in this case the whole Universe!) which is in thermal equilibrium at around 3 Kelvin. You can calculate the flux from summing up the contribution from the blackbody spectrum. The answer is that there are about 400 CMB photons in every cubic centimeter of the Universe, all moving at the speed of light, and representing a flux of 3.14× 10-6W/m2 (at the surface of the Earth, and everywhere else!).
Detecting the CMB itself is relatively straightforward, since you can't help detecting it in any broadband receiver which is sensitive anywhere in the range from a few GHz to a few 100 GHz. However, it doesn't seem particularly exciting, since it's basically just excess noise. Basically the CMB just adds to the background hiss, but it's the the level of background which you can't get rid of by designing a more careful receiver (this is just how Penzias & Wilson detected it back in 1965. So the answer is that it's fairly easy to detect the CMB, but probably fairly hard to convince yourself you've detected it!
The fluctuations are another matter though, since they are always going to be very hard to detect. The dipole (one side of the sky hotter than the other) is 100 times harder to see than the CMB itself. I suppose this might be feasible with home-made equipment. You'd need to look for the difference in the "noise" between a couple of well-separated directions (and avoiding the Galactic plane, which tends to get in the way). Smaller scale fluctuations are about 1000 times fainter still, and are hard even for the best experiments in the world!
I have been thinking about the prediction
of the CMB temperature in the 1940s.
Is it possible to predict the CMB temperature knowing only the
Hubble constant and the ratio of neutrons to protons or do you need
some extra assumptions?
Submitted by schleif"AT"mail.desy.de 11/98
In a number of papers in the late 1940s and early 1950s Alpher, Herman & Gamov predicted that there might be a thermal background from a hot early phase in the Universe. There are many variants on the way the argument goes, but let me try to simplify.
The basic idea is that you want to start with a Universe which is an equilibrium of high energy particles (including photons), and then build up the light elements as the Universe expands and cools. To get a Universe which contains a reasonable amount of helium as well as hydrogen (all observations suggest that the average parts of the Universe have about 25% helium by weight) you need to have nuclear-type temperatures when the age of the Universe was about the time it takes a free neutron to decay. In rough numbers you want the Universe to be filled with billion Kelvin radiation when it was a few minutes old. If you also have a rough idea for how fast the Universe is expanding today (the Hubble constant), then you know approximately how old the Universe is today, and how much it has expanded since it was a few minutes old. Then you can estimate the current temperature of the radiation that was a billion Kelvin at those early times, and you get a number which comes out maybe around a few Kelvin today.
This is not an accurate prediction. It just says that you shouldn't be surprised to find relic radiation with a temperature of a few Kelvin today. You can probably refine this to give a decent guess for the CMB temperature within a factor of 3 or so. But there are too many uncertainties to get it much better than that. Really the CMB temperature is a quantity which should be measured in order to constrain other things within cosmology.
The field of "Big Bang Nucleosynthesis" is essentially the opposite of this argument. You take all the estimated primordial abundances of light elements (the isotopes of hydrogen and helium, together with lithium, and perhaps traces of beryllium and boron) and find that you can fit all the data if you have a single value of a particular parameter. This parameter is the ratio of the density of photons to baryons today (baryons are protons plus neutrons). So if you know the temperature of the CMB, and hence its density, you can use these light element abundances to estimate the density of baryons in the Universe. It's the fact that this comes out about a factor of 10 less than the total amount of mass inferred to be in the Universe, that leads us to believe that there's lots of "dark matter" out there.
Gamov, Alpher & Herman's old argument is basically a crude version of these nucleosynthesis calculations, where you assume some value for the baryon density today, and hence predict the photon temperature.
Is the CMB simply high energy photons that originated at
the origin of the universe that have been red-shifted all the way to the other
end of the electromagnetic spectrum because of the expansion of
the universe? Or is it something more complicated than that?
Submitted by bchaikin"AT"aol.com 11/98
You've got it!
Let me qualify that by saying that the CMB photons were made long after the origin of the Universe (whatever that was). Here by "long after" I mean maybe seconds!
The CMB derives from a time when the Universe was so hot that a whole bunch of particles were being created and annihilated rapidly, and so were in equilibrium with the photons. There were essentially equal numbers of all the particles you've ever heard of, including photons. As the Universe expanded and cooled various particles annihilated (there wasn't enough energy around to recreate the particle-antiparticle pairs once they had annihilated to photons), increasing the number of photons relative to matter particles. At this point the CMB photons were really CGRB photons, since they were high energy gamma-rays. The last such event (electron positron annihilation) was about a minute after the Big Bang. So the photons weren't really made in that first instant, but a lot later!
These photons stretched in wavelength along with the expanding Universe, as you rightly say. But they were still exchanging energy with the matter, and in fact there were processes which could generate new photons right up until about a year after the Big Bang. At that point the photons were low energy X-ray photons. After that, there were still slow processes that could affect some of the photons (changing their direction or mildly changing their energy), and they only really stopped interacting with the matter when the Universe became neutral around 300,000 years after the Big Bang. At that point the photons were in the near-infrared part of the spectrum. Since then they've interacted almost not at all, and have travelled through the expanding Universe being redshifted into the microwave region.
I'm a skeptic who frequently encounters arguments for the existence of
god from Christians utilizing CMB. What puzzles me is that, although
they accept CMB as a "fact," they ignore the part about the universe
being 10-20 billion years old.
Is a 10-20 billion year old universe an essential element of the modern
astronomical theory of the big bang and CMB? Or would the theory work
just as well if the universe is only a few thousands years old?
Submitted by xfaberman"AT"sprynet.com 2/99
This may be a difficult question to answer, since I usually try to avoid treading on people's religious opinions. However, there is a big difference between a reasoned set of beliefs in a spiritual entity, and some craziness which invokes half-understood scientific concepts in order to bolster some wacky theology!
My understanding is that there is no such thing as a proof of the existence of God. And that indeed such a proof goes against a basic principle of many of the world's major religions, where to have Faith is something which transcends the idea of scientific proof. But then I'm no theologian, so perhaps I'm entirely wrong here.
What I do understand is some scientifically consistent picture of how the Universe behaves. So let me stick to that, and leave all questions of how that may (or may not) relate to particular religious viewpoints to each individual to sort out for themselves. It seems like that a physical reality exists, and that it can be understood by applying the principles of rational thought and empirical testing. This is the Universe I know and love, and I have a pretty coherent picture for how the CMB fits into it!
When the CMB was first discovered, and for several years after that, it was not clear exactly what the origin was. The "remnant of the Big Bang" idea made the most sense, but for a while the possibility existed that the CMB could be produced locally, by emission from some sort of dust grains, either in the vicinity of the Sun, or perhaps distributed through the local Universe. As the spectrum was measured to be closer and closer to "blackbody", such alternative origins of the CMB became less tenable. And there are other arguments against such ideas, e.g. how come we can see very distant galaxies through all this supposed dust? So now we firmly believe that the only reasonable explanation from the CMB is that it is the radiation left over from a very hot early phase of the Universe.
There's no way to reconcile the CMB with a Universe that is very young (unless of course you just make the Universe look like it's old!) But then you can't reconcile a several thousand year old Universe with any number of other observations either: historical records and archaeological evidence that human civilization was around many thousands of years ago; everything we know about geology, e.g. the ages of various rocks on Earth, the Moon, meteorites etc.; observations of astronomical objects which are much further than a few thousand light years from us; etc. etc.
To me, the cosmic background radiation (alone) merely suggests
some past or current universal substance or phenomenon which is both
highly isotropic and highly opaque.
I am very curious as to the strict limits
current CBR evidence places on "Hat-stand" cosmological speculation. For
example, does the CBR, by itself, effectively rule out flat space? A
periodic universe? A trillion year old universe? A vast universe?
Submitted by hazelf"AT"ix.netcom.com 2/99
You are right in essence, that the CMB by itself suggests "some past or current universal substance or phenomenon which is both highly isotropic and highly opaque". But of course it can't be taken on its own, since there are many other well-established facts that we now know about the Universe. For example, we have known for almost 75 years that the universe is expanding. Together with the CMB, this implies that there was an earlier, hotter, denser phase in the history of the Universe, and that the CMB is the relic of this phase. I know of no reasonable alternative.
As for whether the CMB rules out various "hat-stand" ideas, it depends how crazy they are! Certainly there is an ongoing investigation into the curvature of the Universe as a whole, and within some bounds we are currently unsure about how curved it is. The best solution currently is flat (but expanding) space, with some amount of vacuum energy density (also called a "cosmological constant") contributing. But it could have negative curvature (an "open" universe), and the closed geometry isn't entirely ruled out either (although currently not favoured).
The space-time structure of the Universe could also be periodic in some way, but only if the scale of the periodicity is very large, otherwise you mess up the CMB in a big way.
The question of how old the Universe might be is an interesting one. There are lower bounds on the age of the Universe by finding the oldest things in it (e.g. globular clusters). But it's less clear how you might get an upper bound. The best-fitting versions of big bang models expand from a time around 13 billion years ago, and they don't work too well older than say 15 billion years (assuming nothing particularly funny happens). But I'm sure you can't currently rule out a universe that did basically nothing for a trillion years and then decided to expand rapidly, or one that might have had a previously contracting (or even expanding) phase, or indeed many phases before that. To some extent we then get into metaphysics, since it's not obvious that you could ever test such a hypothesis. Still, the simplest solution is that our current phase is the only one there's been, and that it can't be a trillion years old.
My view is that the CMB, together with the wealth of other information we have about the Universe as a whole, paints a pretty coherent picture. And this picture - that the Universe began around 15 billion years ago and has been expanding and cooling ever since - sounds crazy enough! The reason it is believed is that it is a very simple idea, and that it works astonishingly well.
If the CMB was created by the Big Bang and the proceeding few moments of the
early universe, then shouldn't it have had plenty of time to whizz off into
outer space, way faster than the non-light-speed matter (e.g. us) have been
moving out from the Big Bang site, in which case we shouldn't be able to
observe it, I'd have thought?
Submitted by vince.bowdren"AT"jobstream.co.uk 3/99
This is similar to other question which I have received many times. The answer is very simple - your mental picture is incorrect! Since the Big Bang model, the expanding Universe, the speed of light etc., are all far from everyday experience, there are many ways in which people can get the wrong image in their heads.
The first thing to get straight is that there's nothing outside the Universe! By definition the Universe is everything there is: we live inside it; and it isn't expanding into anything.
The next thing you have to get clear is that the Big Bang happened everywhere at once, and shortly afterwards all of the CMB photons were created and suffered their last interactions with matter. So those photons are indeed shooting off into space in all directions at the speed of light.
The CMB photons we see today are coming to us from way across the Universe (about 13 billion light years away, if for example the Universe is 13 billion years old). That's true no matter what direction in the sky we look.
It might help to think what happened to the photons that were made right here, all that time ago. Those particular CMB photons have been whizzing off at the speed of light in all directions, and are now being detected by distant observers (say 13 billion light years away) as part of their Cosmic Microwave Background.
I'm an undergraduate student who is taking an Astronomy course
(abridged) ... can you check these CMBR questions for my last assignment?
Submitted by mmar"AT"sprynet.com 3/99
Q: Explain why the cosmic microwave backround is considered good
evidence for the big bang theory?
A: Cosmic microwave background radiation is found everywhere,
filling all
space. It had been predicted in 1948 by George Gamow's collaborators as
the cooled remnant of the hot fireball created in the Big Bang. The
Universe therefore appears to be changing with time. This conclusion is
supported by the observed evolution of galaxies and quasars as we look to
distant and therefore younger reaches of the Universe. We therefore
exclude the Perfect Cosmological Principle and the Steady State theory, and
accept the Cosmological Principle and the Big Band as basic premises. The
2.73K CMB is residual radiation left over from the Big Bang.
Q: Why is the cosmic backround such a low temperature today?
A:
The stretching of wavelength with expansion
can account for its lower temperature.
Photons do not slow down. Only wavelengths and
frequencies are affected by the expansion. Redshifting what would have
been on average visible light photons to that of microwave region by the
continued expansion of the universe is the important effect.
Pretty good answers I'd say, which I only edited a little. Pity it was too late to help you with the assignment! Did you pass I wonder?
What physics would fractal analysis of the CMB reveal or constrain, if
any?
Submitted by tr211"AT"hermes.cam.ac.uk 4/99
The short answer is none!
The slightly longer answer is that, to the extent that the fluctuations in the CMB behave approximately like a power-law in scale, at least at the largest angular separations, then this analysis has already been done extensively for the COBE data. Whether or not people referred to it as a fractal is a matter of personal preference, but you can certainly read all about these analyses in the original papers in the Astrophysical Journal. However, there's now pretty good evidence that this power law (or fractal) behaviour doesn't continue to smaller scales. Moreover, the simplest theories predict excess fluctuations on degree scales, and a rich structure at scales below that. Far more interesting and informative than a boring old fractal!
Assuming the universe to be fourteen billion years old - how much energy
(in ergs) would the CMB contain (in the universe)?
Submitted by sclufer"AT"pop.erols.com 6/99
Since we know the temperature of the CMB quite accurately, and we know that it is very well described as equilibrium (blackbody) radiation, then we know pretty much everything about it. In particular it's energy density is just a standard physical constant times the temperature to the fourth power, which comes out to be about 0.262 eV cm-3, or about 4.21 × 10-13 ergs cm-3. (I could ask why you are using an antiquated unit like the erg, but let's not quibble!).
There's always a problem with what you mean by "the Universe", since in principle it could be infinite. However, in practice, we only know about the region that light can have reached us from in the age of the Universe. If the Universe is 14 billion years old, then the observable universe is about 14 billion light years in radius (actually a little different from this, since it's been expanding, but we've already agreed not to quibble!).
So the total energy of the CMB in the observable Universe is just the energy density multiplied by the volume of a sphere with radius 14 billion light years. This turns out to be about 4 × 1072 ergs. In whatever units you use, that's a lot of energy!
Assuming the critical density is 5 × 10-30 g/cm3
(about equal to
one hydrogen atom in a container 130 centimeters on a side averaged
averaged over the entire observable universe),
how many ergs are we talking about? How does this compare with the CMB?
Submitted by sclufer"AT"pop.erols.com 7/99
What you want to compare is the energy equivalent of that mass density. So you multiply the mass density by c2 to get energy density (using the famous equation!). What you'll find is that the CMB energy density is about 10,000 times smaller than the current mass density. The precise value depends on: (a) how much dark matter there is; and (b) how fast the Universe is expanding.
In more detail, the critical density is given by the formula
critical=3 H02/(8 
G),
and the actual density is a factor of times that
(where lies between 0 and 1 in all likelihood, and
tells us what fraction of this critical density the observed Universe has).
If you write the Hubble constant as
H0=100×h km/s/Mpc, with "h" parameterising our uncertainty in
the Hubble constant (probably 0.5 < h < 0.8),
then the density of the Universe is
a numerical factor times h2.
You can then multiply by the speed of
light squared to get the equivalent energy density. Then taking
the ratio of this with the energy density of the CMB, gives you a value like
matter/radiation = 40,000 h2.
For all reasonable values of and h this lies close
to 10,000.
Because this number is so big, we talk about the Universe today being
"matter dominated". However, we know that when you squeeze a box full of
radiation and matter this ratio goes down. So in the expanding Universe we
expect that if we go back early enough it is the radiation energy density
which dominates. Hence we talk about the early Universe
(before about 100,000 years after the Big Bang) being "radiation dominated".
When light travels through a gravitational field it bends and can
also redshift. When light travels for 40 billion years through space and
gravitational fields it must redshift. Is this taken into account when
considering the Hubble constant and the age of the universe?
Submitted by DNoga"AT"stpaulshosp.bc.ca 8/99
The short answer is "yes", and I'm tempted to just leave it at that!
But habit forces me to be more long-winded.
First let's make clear that estimates for the age of the Universe come in around 14 billion years (40 is way on the long side). These estimates involve figuring out the age of the oldest things (e.g. globular clusters) and also determining how fast the Universe is expanding together with how much the expansion rate has been changing. Explicit calculations are done in the context of models of simple space-times which are consistent with Einstein's General Theory of Relativity, and thus properly contain all the effects of gravitational fields.
It may also help to clarify the redshifting effect. The effects of gravity are usually described by the "gravitational potential". In empty space this potential is unchanging, and you can think of a clump of matter as being a "potential well", i.e. a region where the potential is lower (actually the sign of the effect is really unimportant, but the convention is to think of matter as "holes" in the potential). When light leaves an object at the bottom of a potential well it gets gravitationally redshifted on its way out. Hence light coming from the surface of a white dwarf or neutron star (which are quite deep potential wells) can be significantly redshifted, and light from near a black hole would be hugely redshifted. Similarly, if you lived inside a potential well, then you'd see all the light from outside being blueshifted. BUT light that travels straight through a potential well gets blueshifted on its way in, and redshifted on its way out, by an amount that cancels. So you get no effect.
In detail, the bending of light and redshifting around clumps of mass in the Universe can have exciting observational consequences. All of the rich phenomena of gravitational lenses for example. There's also direct relevance for the CMB, because on small scales the CMB photons get affected by clumps of matter in the relatively nearby Universe, creating small-scale hot and cold spots ("anisotropies") on the CMB sky. Gravitational lensing affects the CMB anisotropies at levels which are measurable. And another possible signal comes from the effects of potential wells which were changing during the time the CMB photons crossed them. These give very weak effects which may one day be detectable on the angular scale of galaxies or clusters of galaxies, perhaps giving us more information about how such objects formed.
Has the redshift of a practically ideal blackbody like the CMB actually been
measured or calculated from a model? If measured what absorption line was
used as a reference?
Submitted by LABELE"AT"aol.com 9/99
It sounds like you have already answered that question! Since there is no reference line, you can't measure the redshift. In fact there's no way to tell the difference between a blackbody at lower temperature and a blackbody which has been redshifted - the spectrum retains exactly the same shape as the Universe expands. If you think about it, if that were not the case, then the CMB spectrum would change shape as the Universe expanded, and hence it would be pretty unlikely that we'd observe it to be such a precise blackbody shape today.
To put it another way, you can think of the CMB as being roughly 3 Kelvin blackbody radiation existing today, or as 30 Kelvin blackbody radiation which has been redshifted by a factor of 10, or as 300 Kelvin radiation which has been redshifted by a factor of 100, etc. They are all equivalent.
As far as the theoretical picture is concerned, the CMB photons last interacted with matter when the temperature of the Universe was about 3000 Kelvin, and the photons have redshifted about a factor of 1000 since then. So you can think of them as being a view from that redshift. But the photons themselves were produced much earlier, in a much hotter phase. And so it's probably best to picture them as the glow from period in the early Universe when the temperature was billions of Kelvin, and since the Universe has expanded by factors billions since then, we observe the CMB today at 3K.
Although there are general arguments (involving synthesis of the light elements in the early Universe for example) for predicting that the CMB should have roughly the order of magnitude of temperature that's observed (and these arguments were discussed as early as the 1940s), I've no doubt other arguments would be put forward if it had turned out to be a quite different temperature. There's no fundamental reason known which can explain why the current temperature is 3K rather than, say, 5K or 0.2K.
Does the time of decoupling have anything to do with the
mean free path of photons when the universe expanded and cooled to
approximately 3000 K?
Submitted by LABELE"AT"aol.com 9/99
Yes!
At around 3000K the radiation was cool enough to allow the electrons to combine with the nuclei (mainly hydrogen and helium), and so the Universe became neutral.
Now photons interact much more strongly with charged particles than they do with neutral ones (not surprising, since photons are the carrier particles for the electromagnetic force). In particular, interactions with free electrons were frequent when the Universe was ionized. But once the Universe became neutral (when the temperature dropped below about 3000K) the photons no longer "saw" the matter, and just travelled freely through space. The average distance between photon encounters with matter (otherwise known as the "mean free path") went from being very short, at earlier times, to extremely long at later times. The era that marks this change is the "last scattering epoch" when the Universe became neutral.
We see the CMB anisotropiesfrom mainly around this time - it's impossible to see much further back than that, since it's like looking through a dense fog. The CMB sky can be thought of as a distant sphere surrounding us, with the anisotropies that we see being places on that sphere where the "fog" was a little thicker or a little less thick.
I was wondering why CBR is reaching us only now; was the initial
expansion of the universe at such a great
rate the light could not match its pace and is hence reaching us only now?
Submitted by hbchai"AT"hotmail.com 10/99
As I have said when answering similar questions before, I think you need to get a better mental picture of the CMB, in order not to confuse yourself. For this purpose it's much better to think of ourselves at the centre of a spherical region of the Universe, with CMB photons coming at us from all directions (people at other locations in the Universe can also think of themselves at the centre of a spherical region, so there is no implication of a true centre of the Universe!). We will see CMB photons tomorrow and the day after, but they have come from slightly different places, since the CMB originated everywhere in the early Universe.
The Universe is full of CMB photons, travelling at the speed of light. When we look in a particular direction, we see the photons that were produced the light travel time ago in that direction. Let's assume for simplicity that they were made at the first instant, t=0 (in fact they last interacted with matter about 300,000 years after the Big Bang). If the Universe is say 13 billion years old, then we see the CMB photons which have been travelling towards us for 13 billion years. In a different direction we see photons arriving from a different place, which have also been travelling for 13 billion years.
Someone who is living on a planet which is about 13 billion light years away from us will be seeing CMB photons coming from where we are, if they look in our direction. Hence they can learn about what our little corner of the Universe was like 13 billion years ago. We can't see our own region at any time other than right now, but we can see a whole sphere around us that is 13 billion light years away. When we study the CMB sky we are learning about the bits of the Universe that are 13 billion light years from us, as they were 13 billion years ago. Hence a study of the CMB gives us statistical information about the very early Universe.
Does that make more sense?
I'm thinking about a potato in a microwave oven ... What does the CMB come
from? In other words, what or where is the potato that is radiating the energy?
Submitted by skelley"AT"fc.fcps.k12.va.us 11/99
The thing that is radiating the microwaves is the whole potato! That is, it's the entire Universe. If you like, you can think of us being inside an oven, which maybe was once very hot, but at the moment has a temperature of about 2.7 Kelvin. So the Universe is full of radiation that is 2.7 degrees above absolute zero - and that radiation was given off from matter when it was hot, very early in the history of the Universe.
Maybe it helps to think like this: in a regular oven the radiation comes from the hot element. Infrared radiation is emitted and travels at the speed of light, being absorbed by the walls of the oven. Now think of an oven with walls which are very far away, and which is filled with radiating hot elements. If all those were switched on and off at once then at some time later (say 1 hour) we'd see radiation coming from those parts which are (say) 1 light hour away. Everyone else in this oven will also see radiation, coming from different radiators, but basically the same thing. Now imagine this oven expanding, so that the radiation coming from distant sources of heat gets stretched on its way to you, so that it has longer wavelength, and so appears colder. If you can picture that, then you're pretty close to a full understanding of the "hot Big Bang" model of the Universe!
I want to know if it“s
possible to get the "sound" of the cosmic microwave
background radiation. Does exist some kind of
recording? How can I get it?
Submitted by manoribas"AT"yahoo.com 11/99
Someone else asked a similar question earlier. But I sense here a slightly different source of confusion.
Popular accounts of the Cosmic Microwave Background often use phrases such as "echo of the Big Bang". This is meant as an analogy, and it is unfortunate that it could be taken to imply that the CMB is somehow associated with sound waves. So let me be quite clear: the CMB is a sort of radiation which bathes the Universe and can be picked up with radio-type telescopes. These microwaves are a form of electro-magnetic radiation, like any other microwaves, and have nothing to do with sound (which are compression waves, travelling through air for example). You certainly cannot hear the CMB - unless of course you have radio antennae instead of ears!
[Response added later]
I think could be another answer like
that: "CMB peaks at about 150 GHz. Our limit for the sound is about 20
kHz. So if you divide the frequency by factor of (say) 10^7 and direct
the signal to the speaker you could hear rather noisy sound at about
15 kHz. It has something to do with the CMB."
Submitted by mvg@credit.biysk.ru
Good point. The factor by which you have to scale the CMB redio frequencies to get into the range of human perception for sound waves is about 107. Has anyone tried to simulate this and made a recording of what it sounds like?
Is the discovery of CMB a strong piece of
evidence that the universe was once much hotter than it is now?
Submitted by croswell"AT"email.unc.edu 12/99
Yes!
See the answer to some of the questions above. The simple answer is that the spectrum of the CMB indicates that it comes from something that had relaxed into very precise thermal equilibrium. The only source that we know of is the early Universe, when it was much hotter and denser. There have been other suggested origins for the CMB, proposed when the spectrum was much more crudely measured. But since the spectrum is now known very accurately, none of these alternative suggestions are even vaguely possible.
I understand that the CMB came from the "last scattering surface", which
was extremely hot. The CMB temperature (2.76K) thus cannot correspond to the
temperature of THAT surface. So what is it?
Submitted by debouche"AT"kings.edu 1/00
In an expanding Universe light gets redshifted and thereby loses energy. This is effectively the same as the "Joule-Kelvin" expansion of gases that keeps your refrigerator cool. A better way to think about it in the cosmological context is that the photons get stretched on their way to you, and are observed with a longer wavelength than they has when they were emitted. It turns out that this expansion preserves the shape of the "blackbody" radiation curve, and so the CMB appears as a blackbody with a temperature which is lower by the redshift factor between the "last scattering surface" and today, which corresponds to about 1000.
I'm looking for any pictures about noise
effects in CMB images obtained by various sky scanning methods.
Could you explain me simply what's "destriping
methods" in image processing?
Submitted by polmar"AT"pt.tizeta.it 1/00
Ah, a technical question!
The point here is that CMB maps are made by scanning over the sky in some way. So any long timescale drifts in the detectors can lead to the appearance of stripes on the sky. These reflect the geometry of the scanning strategy, and would clearly not be interpretted as being real. Nevertheless they would complicate the extraction of the cosmological signal, and hence ideally one would like to minimize such striping effects.
There are several approaches here. First of all, and most importantly, is to design the experiment to keep such long term drifts as small as possible. Secondly is to design the scanning strategy so that it is relatively easy to extract the stripes. And thirdly is to develop methods for removing the stripes from the maps - here there are many lessons to be learned from image analysis in other fields.
A great deal of work has been done recently in developing approaches to the removal of such instrumental artefacts. And a great more effort will be expended before the Planck satellite (for example) returns data. I suspect therefore, that the best methods, which are both fast and accurate, have yet to be developed. In the meantime a recent summary of some of the best work (in particular for the LFI part of Planck) can be found in the paper by Carlo Burigana and collaborators here
What about CMB polarization?
Has it been measured already?
Submitted by yurickk"AT"hotmail.com 3/00
Good question!
As well as having variations in temperature across the sky, the CMB photons are also weakly polarized. The variation in the direction of the polarization can in principle be measured, and since it depends sensitively on details of the cosmological model, then precise measurement of the polarization can lead to additional constraints on parameters of the theories.
The short answer to this question is "no". But it is expected that such measurements are not far off. There are several experiments, either existing or being built, which have the sensitivity to detect polarization on the CMB sky.
The first thing to be done will likely be a crude measurement which establishes the general strength of the polarized signal. If this turns out to conform to the standard theoretical predictions, then this will be a strong piece of evidence in support of conventional wisdom. If the results are very different, then of course that will be more exciting!
After that there will follow attempts to measure the variation of the polarization with angular scale, or in other words the polarization power spectrum (actually there is more than one power spectrum for polarization). Such measurement can help narrow down the cosmological parameters in ways beyond what is possible with just the temperature. One example if that polarization is more sensitive to a small contribution of gravity waves affecting the CMB, which may provide information about the early epoch when the initial seeds for galaxy formation were laid down. Another example is that polarization can also give a direct measurement of the epoch at which the Universe re-ionized (something that happened in the relatively recent history of the Universe due to ionizing radiation from stars and quasars). Thus polarization measurements may help unravel further details of the processes which formed structure in the Universe.
I was hoping to find a way to use the CMB in a paper I'm writing for calculus.
How is calculus used in analyzing CMB? What sort of formulas or
calculations involving calculus are used?
Submitted by blaine_5"AT"hotmail.com 4/00
This is certainly a question I've not been asked before!
Calculus is used so widely in theoretical cosmology, and in data analysis, that I've never really stopped to think of examples. Here's one for starters though: theoretical predictions for the CMB involve solving a system of equations for density variations, temperature vairations, etc. These equations are derived by considering small differences away from the average densities etc. - and so certainly this involves differentiation. To make detailed predictions for particular cosmological models you start with some simple initial conditions and then evolve to the present time - and this certainly involves integration.
These are just a couple of examples on the theoretical modelling side. The mathematical treatment of the anisotropies, and discussion of analysis methods for dealing with them, is also full to bursting with integrals and derivatives! Calculus is a way cool invention. How anyone could get through a whole day without using it is beyond me!
...if magnetic imprints of radition still exsist in the universe than can
we develope a system to harness these impulses to ...
move objects through space... (abridged)"
Submitted by goomba"AT"ismi.net 3/00
The fact that there is a Cosmic Microwave Background permeating the Universe does indeed mean that the energy density of "empty" space is not zero. There are oscillating electric and magnetic fields threading the Universe, with an equivalent temperature of about 2.73 degrees above absolute zero.
So there is certainly energy out there. But the laws of thermodynamics tell you that this energy is essentially impossible to extract as useful work. You would need to make a heat engine which was connected to something of even lower temperature! And if you tried to make something of lower temperature it would take more energy to do that than you would be able to extract. So the answer is that there's no obvious way of using the energy in the CMB to do anything like power spaceships.
But if you think of a way to do this, then you should patent it right away, since you would effectively have the perpetual motion machine that inventors have dreamed about for millenia!
How does a billion photons relate to the CMBR of 2.73K?
Submitted by Saturn1001"AT"aol.com 3/00
The CMB seems to be very well described as a "blackbody" - an ideal arrangement of photons in thermal equilibrium. The spectrum is what you get if you take a box of photons and allow them to relax, or what you get if you heat up an ideal emitter/absorber of radiation.
The point about a blackbody distribution of photons is that when you know the temperature you know everything else too: energy density, number of photons per unit volume, peak wavelength, etc. For a blackbody of temperature 2.73K, the number density is 412.77 photons per cubic centimetre.
In the Universe around us we can also try to estimate the average density of regular sorts of matter, otherwise known as baryons (meaning protons and neurons, as opposed to more exotic forms of matter). The current estimates are much less certain than for the CMB photon density, but come out around 2 × 10-7 baryons per cubic centimetre. If you take the ratio of these two densities you find that on average the Universe contains about 2 billion photons for every proton or neutron. I think this is the number you were asking about.
The photons that are observed in CMB observations have presumably been
traveling through space undisturbed since sometime after the big bang. But
thay have apparently lost energy in the process, as their wavelength has
changed. Where did this energy go? Did it go into the gravitational field?
Submitted by jamont"AT"visto.com 4/00
I'm surprised I haven't been asked this before, since it's such a good question!
There are several different ways of looking at this. One answer would be that the energy goes into the gravitational potential energy of the whole Universe. Another answer is that it takes energy to expand the Universe, and that this "work" comes out of the contents which are expanding.
Yet another answer is that the cooling of the CMB as the Universe expands is part of a simple relativisitic solution which describes the entire Universe, using the theoretical basis of General Relativity, which is a well tested theory of gravity. And if part of that solution appears to violate energy conservation, then tough!
I think that the CMR redshift z=1000 is caused by two effects.
The doppler effect because the universe is expanding.
The other effect is caused by gravity.
The matter density is decreasing because the universe is expanding so the
gravity well is decreasing so
the wavelength is increasing.
I'd like to know which effect contribute more to the redshift.
Submitted by r_lichtensh"AT"yahoo.com 4/00
The first effect is correct. Although it's better to think of an expanding Universe, rather than in terms of speeds - I'll explain. We see the CMB photons on the "last scattering surface" when they interacted with matter at z=1000 or so. What this means is that distances in the Universe were smaller by a factor of 1000 when the photons were last scattered, and they've been redshifting as they travelled through space towards us. This is more correct than thinking about a Doppler effect caused by a velocity.
The second explanation that you describe sounds like one of the important effects for generating anisotropies on the CMB sky. Variations in the gravitational potential wells on the last scattering surface (caused by variations in the density of matter at that time) lead to different redshifts and blueshifts of CMB photons, as they climb out of, or fall into, these wells. These shifts are going to be small in amplitude, since the Universe was pretty smooth back then. In fact they are about 1 part in 100,000, and this is what leads to the temperature anisotropies at large angular scales on the CMB sky.
You've just re-discovered the "Sachs-Wolfe effect"! If you'd written this before 1967, maybe it would have been named after you instead!
I am confused by calculations of the average y-distortion due to
clusters of galaxies ... (heavily abridged)
Submitted by sum"AT"rri.res.in 4/00
This is a rather technical question in detail, but let me answer it here in general terms.
The so-called "y"-parameter is a number which tells you about deviations away from a pure blackbody shape of the CMB spectrum. At some level we eventually expect to find that the CMB isn't quite a blackbody. And certainly we know that the CMB photons can have their energies changed a little as they pass through the hot electrons whizzing around in clusters of galaxies. So the total effect of this, averaged over all the sky will give a small (but eventually measurable) deviation from blackbody.
This energy-exchange through hot cluster gas is usually referred to as the Sunyaev-Zel'dovich effect, after the two Soviet theorists who first described the effect around 1970. The idea is that as the photons pass thorugh the hot cluster gas, some fraction of them scatter off the electrons, gaining some energy in the process. So when you look at the CMB through a rich cluster of galaxies you should see a small change in the spectrum, in the sense that there are slightly fewer low energy photons and slightly more higher energy photons than there should be for a blackbody spectral shape. The deviation is measured by a parameter, called "y" by Sunyaev and Zel'dovich (who derived the particular functional form of the distortion). This Sunyaev-Zel'dovich effect has now been detected for tens of individual clusters, and the study of the S-Z effect provides valuable information about the clusters themselves.
The point is that we know there are lots of clusters out there, and so we could imagine adding up all the y-distortions and averaging over the whole sky. This gives a value which is typically at least a factor of 10 below the limits which the COBE FIRAS experiment placed on such deviations from a blackbody.
Rather than measuring the average y-parameter itself, we're likely to learn more about it through deeper and deeper studies of the S-Z effect through individual cluster of galaxies, as well as through smaller groups, filaments and other structures filled with hot gas.
I just read an article that gives a
theory alternative to the Standard Model to explain the CMBR, the observable
redshift proportional to distance and the "missing matter"... the author
claims that there is diatomic hydrogen that was previously unobserved ...
(abridged)
Submitted by gazic_miles"AT"si.com 5/00
I'm all for challenging conventional wisdom. But people who do so have to do a lot of research and get their facts right before they'll be taken seriously. There are plenty of examples of changes in scientific paradigms which came about through careful study by several independent experts. I'm not aware of any major change in scientific thinking that's happened as a result of a comparative lay-person coming along with some vague idea. That's not to say that it's impossible, but just that there's an awful lot of stuff to know before you can really argue persuasively. Just as I wouldn't expect to be taken too seriously if I started arguing detailed points of law in a courtroom, someone who hasn't spent years studying physics, mathematics and astronomy, shouldn't expect to be taken too seriously when they argue about cosmology.
OK, so now that I've gotten that off my chest, let me answer your question!
It has been suggested many times that there could be more molecular hydrogen out there than expected. It is genuinely difficult to detect (although by no means entirely invisible), particularly if it is clumpy. There has been some serious study of the possibility that such clumps of cold molecular hydrogen could contribute some fraction of the dark matter in the outer parts of galaxies. However, although you can hide a lot of hydrogen this way, it's hard to make up a large fraction of the dark matter, or of the entire mass of the Universe for example. There are a number of quite firm constraints on this. So the reasonable view is that there could be more molecular hydrogen out there, but not so much that it substantially contributes to the total mass of the Universe.
You go on in your full question to discuss how you don't follow the details of this H2-explains-everything idea. And I completely agree! I don't understand at all the idea that molecular hydrogen could lead to an apparent redshifting of distant galaxies. The velocity of light would change almost imperceptibly when passing through a cloud of molecular hydrogen, so if there were any effect at all it would be incredibly tiny. Certainly there's no way to produce the redshifting factors of as high as 5 that are now seen. I also am at a loss to understand when molecular hydrogen would give a blackbody spectrum, rather than a spectrum with either emission or absorption lines of molecular hydrogen, at levels which would have been easily detectable.
In the standard model, the Universe becomes neutral at about 300,000 years after the big bang. Subsequently there are small amounts of molecular hydrogen formed (through reactions involving rare H- ions). When blobs of material gather themselves together under their own gravity and become denser (on their way to forming stars and galaxies) then 3-body reactions and more complicated processes lead to the formation of lrager amounts of H2. This molecular hydrogen is probably important for the release of internal energy from within these blobs as they cool down further and get dense enough to turn into stars etc. In this way molecular hydrogen is believed to have played an important role in the formation of the first objects that lit up the Universe many billions of years ago.
That's an exciting enough story for H2, without the need to try to get it to explain the dark matter, the CMB, and, presumably, Gamma-ray bursts, the origin of life, UFOs, the Face on Mars and the Loch Ness Monster!
I would love to know what is contained on your web page. unfortunately, the
background makes the reading of the text nearly impossible.
Submitted by DRidge"AT"aol.com 6/00
That's a problem on some combinations of computer and browser. To be honest, it's the least interesting pages that are hardest to read! You can always turn off graphics in your preferences or force an easier to read font. In other words you can try turning off the background on the internet, although you can't do that with the Cosmic Microwave Background!
When the room is empty, and everything is silent, there is a high pitch ringing
in the ears. Is this from the Big Bang? If not, what causes this?
Submitted by theodorus"AT"mad.scientist.com 7/00
Your ears are certainly not capable of detecting the Cosmic Microwave Background. Sometimes sound is used as an analogy for this radiation left over from the Big Bang - but the CMB is like radio waves rather than sound waves.
So no, that's not what you're hearing. Presumably it's something physiological to do with hearing your own circulatory system, or something like that. Although the static that you see when you tune your TV between channels is partly coming from the CMB. But you couldn't pick up TV or radio just with your ears either. Or at least most of us can't!
If you start hearing voices too, don't let me know!
Can energy be derived from cosmic radiation, much like a photovoltaic cell
does from visible light?
Submitted by enriqueromero"AT"infovia.hn 7/00
Unfortunately not. At one level this would be against the laws of thermodynamics, since is like a "thermal bath" filling the whole Universe with a temperature of about 2.73 Kelvin - so you'd need a "heat engine" colder than that to extract energy! In any case, from a practical point of view, the energy per unit volume in the CMB is minuscule by everyday standards - that's why it can pervade everything while having essentially no effects.
Could
you explain why Wien's law can be used to argue that CMBR radiation was
hotter in the past? what was this argument.
Submitted by ksawano"AT"indiana.edu 5/00
Wien's law is the property of blackbodies (i.e. ideal absorbers and emitters of radiation) which states that the wavelength of the peak of the radiation is inversely proportional to the temperature. The CMB peaks near 1 millimetre wavelength and has a temperature near 3 Kelvin. Something which is 10 times hotter than the CMB would peak at a wavelength 10 times smaller. We are at about 300 Kelvin (approximately room temperature) and our personal glow peaks at wavelengths about 100 times shorter than for the CMB, which is in the middle of the infra-red band.
There's no way of arguing from Wien's law that the Universe used to be hotter. The roughly 3 Kelvin radiation that we see could have been formed nearby at this temperature, or it could have come from an earlier time, redshifted on its way to us through the Universe, and be observed to be proportionally cooler than when it was emitted. That's what we think happened, since there's no other way of making so much energy, in every direction, and with such a good blackbody shape to its spectrum.
In the standard cosmological picture the CMB photons last scattered with matter in the Universe at about half a million years after the Big Bang. At that time the radiation was about 3000 Kelvin. The photons have been redshifted by a factor of about 1000 since then. By applying Wien's law you can then see that the CMB photons, with their wavelengths stretched by 1000, will appear as a blackbody with a temperature about 1000 times smaller, or about 3 Kelvin. It's the fact that the Universe is expanding (with freely-travelling photons having their wavelengths expanded in the process) which argues that the Universe must have been hotter in the past.
If we assume that the big bang started in a single point and was the start
of all matter in the Universe, then how do you account for CMB seeming to be
uniformly everywhere in the Universe?
There are more detailed answers to this question above. The short answer is that the Universe may indeed have started at a point, but that point was everywhere! If you could see back to the very beginning of the history of the Universe (and you almost can using the CMB), then you'd see that beginning in every direction. In other words, we're seeing the CMB radiation coming from all around us, and it all originates in the very early Universe.
As CMB can only travel at the speed of light are there parts of this
infinite Universe that are still empty of CMB and even Galaxies? If so, over
time will CMB be spread so thinly as not to be measureable?
The answer is similar to the last question. The Universe is filled with CMB radiation. The photons are moving in every direction at the speed of light, and each region of the Universe continues to be full of these photons. As the Universe expands the radiation cools, and will continue to be detectable essentially for ever, provided that you can imagine building more and more sensitive detectors.
I saw a news report about "a cloud so cold it absorbed the 3K CMB".
Could this be a misunderstanding of the Sunyaev-Zeldovich effect ?
That sounds like a good guess.
It's pretty hard to get much colder than the CMB, since it pervades everything. You can build a fridge which is one of the coldest places in the Universe for a short while (for example the bolometers which are used as CMB detectors are often cooled to about 0.1 Kelvin). But it takes energy to do this, and you can't keep those CMB photons out for ever! In other words an object won't get colder than its surroundings on its own, which is a statement of the second law of thermodynamics.
There are a couple of exceptions to this. One is in the expanding Universe at early (but not too early) times. After the matter and the radiation decoupled, but before matter was reheated by the effects of the first stars etc., it cooled more rapidly than the radiation. So there was probably a time in the history of the Universe when the matter (particularly the less dense bits) fell to below even 1 Kelvin. However, once stars, quasars and galaxies turned on the matter pretty quickly heated up to temperatures well above that of the CMB.
Another exception is in particularly dense molecular clouds. The ratio of populations of energy levels is governed by what is usually called an "excitation temperature". This can be different from the "actual" temperature of the material (usually referred to as the "kinetic temperature"). For some transitions (e.g. in formaldehyde) it's possible for this excitation temperature to be below that of the CMB. Hence one can occasionally see the line in absorption against the CMB.
In the so-called Sunyaev-Zel'dovich effect, photons travelling through the hot gas in clusters of galaxies gain energy. This means that effectively photons are taken from the low energy end of the spectrum and shifted to the high energy end. So if you started with a blackbody spectrum, then you end up with relatively fewer low energy photons and relatively more high energy photons. Since this effect is most easily studied at low energies (with radio telescopes), then what is detected when you look through a galaxy cluster is a "decrement" in the CMB. If you were speaking sloppily, then you might describe this as colder CMB. So this might explain what you read.
The way I read Plank's equations, the CMB would correspond to atoms of
hydrogen moving through intergalactic space at an average speed of just
under 250 Meters per second.... [abridged]
I do not follow your suggestion in detail. But one thing I can say is that the particles which comprise the CMB, and which have been detected in the billions by countless experiments, are certainly photons and not atoms of any sort. You can think of these photons as having been emitted by some matter which was in extraordinarily good thermal equilibrium at the time. It's extremely hard to arrange for this at recent times, for matter at close to 3 Kelvin, and quite easy to arrange for this from matter at very early times at much higher temperatures. The photons have then redshifted on their way to us, preserving their near-ideal "blackbody" spectrum, but with a much reduced temperature.
Imagining that it is a recent source of hydrogen atoms (for example) which somehow emitted the CMB photons, is an easily discounted idea. Hydrogen atoms would lead to detectable spectral lines if they existed in the required abundance, and there's also the source of the energy to explain (which is very large). More feasible would be particles of dust absorbing starlight, being heated to about 3 Kelvin, and and re-radiating the energy at microwave wavelengths. Attempts were made to model such partciles, but there are several objections. One is that there would have to be so much dust that you wouldn't be able to see distant galaxies! The second is that it seems to be impossible to invent a particular composition for the dust grains that can give nearly as good a blackbody spectrum as is observed. Hence we are left with the only viable solution: the CMB photons have a genuinely cosmic origin, and are a remnant of a hot early phase of the Universe.
1) The sun is known to produce 'solar wind' in significant amounts with
enough radial velocity for the particles (mostly hydrogen atoms) to
escape the gravity well of the sun and have some velocity left over.
2) Since the sun is not all that unusual as stars go, we can reasonably
expect that
lots of other stars are doing the same thing.
3) Adding 1) & 2) together and integrating over cosmologically long
periods of time equates to a significant amount of 'stuff' in
interstellar/intergalactic space.
4) Each of these particles is what Planck would refer to in his equations
as an 'ideal radiator.'
5) These particles readily exchange quanta of energy back and forth
between photons of microwave radiation and kinetic energy.
6) This pretty much defines the possibility of equilibrium; lots of time
and ready exchange.
Where are
there observations which disprove this development? [abridged]
This is a follow-up to the previous e-mail. I think I now understand the thrust of the question: could interstellar hydrogen be the source of the CMB?
Such ideas were certainly considered in the early history of the CMB. But it became clear fairly early on that it's impossible to have such a local source. Let me just mention a few things. One immediate problem is the isotropy: there's lots of gas in the disk of our Galaxy (and other spiral galaxies), and so we would expect to see a lot more radiation coming from the Galactic plane, and from nearby galaxies if that was the source. The second basic problem is getting the spectrum to be so close to a blackbody, and this applies for almost any material you can think of. There are several difficulties with the idea of hydrogen gas in particular. There's no reason for the gas within our Galaxy to be all at the same temperature, since the sources of heating (presumably stars etc.) vary dramatically from one part of the Galaxy to another. And hydrogen will always give a line spectrum, either absorption lines or emission lines, and that's true whether the hydrogen is mainly atomic or mainly molecular. You can look, for example, for absorption of local hydrogen in the spectrum of a distant object (like a quasar) to estimate how much hydrogen there might be in the outer regions of galaxies, for example, and you find that the answer is not very much. And it's hard to imagine that hydrogen gas distributed around our Galaxy would give anything like a blackbody spectrum.
The two basic facts about the CMB, that it is very close to isotropic, and that it is a very precise blackbody, are the things which make it very hard to explain through any local process. Hence we are led to look for a source which exists uniformly in all directions and which was in extraordinarily good thermal equilibrium. The hot Big Bang idea for the early history of the Universe makes it easy to produce the Cosmic Microwave Background which we observe.
For more discussion of this and related topics you could try to track down "The case for the relativistic hot big bang cosmology", by Peebles, Schramm, Kron & Turner, 1991, Nature, volume 532, pages 769-776. One of the most detailed studies of the failure to fit the CMB with re-radiated starlight is "Needling the Universe", by Hawkins & Wright, 1988, Astrophysical Journal, volume 324, pages 46-59.
I have recently been told about something called a "monoblock" which is
supposed to have something to do with CMB and the big bang. In all my readings
of the big bang and CMB I have never heard of a "monoblock" before.
Can you help?
Let me not pretend that I am an expert on experimental hardware! However, I believe a monoblock is a particular sort of amplifier. It is entirely possible that Penzias & Wilson used such a thing as part of their equipment which was used to detect the CMB in 1965. Can anyone else verify this?
Assumptions:
From most points in the universe, one will measure a CMBR dipole. Thus, one
would have to accelerate to attain a frame of reference "at rest" relative
to the CMBR.
Questions:
Having attained that "rest frame", would one not have to accelerate constantly
to stay at rest (to counter attraction of all the mass scattered around the
universe)? [abridged]
I think the assumption is wrong, and therefore the question doesn't need to be asked.
The fact that there's a CMB dipole (one side of the sky hotter and the other side colder than the average) tells us that we are moving at a certain speed in a certain direction with respect to the "preferred" reference frame (i.e. the one in which there is no observed dipole). To get ourselves into this dipole-free frame we just have to move with a velocity which cancels out the dipole-producing velocity. There's no need to accelerate (accept the rapid acceleration you'd need to do to change velocity of course).
Our local motion (which makes us move relative to the "CMB frame" and hence gives us a dipole to observe) is caused by nearby clusters and superclusters of galaxies pulling us around. It's true that over cosmological timescales these objects are also moving. And so if we wanted to keep ourselves always in the dipole-free frame we'd have to make small adjustments to our velocity as we moved and got pulled around by different objects. But these changes would be on roughly billion year timescales. And so to get into the frame with no CMB dipole basically just requires the following 3 steps: (1) observe today's dipole; (2) move towards the coldest direction at just the right speed to cancel the dipole; and (3) maintain basically that same velocity forever.
Why is the peak of the spectrum in the Microwave part of the
spectrum? Is it because the univers was "Hot" when created, so
radiated mostly in the IR, and has now cooled, so radiates mostly in
the microwave.
In the very early Universe things were so hot that the average photon was in the gamma-ray part of the spectrum - so much, much hotter than the infrared! It has been getting cooler and cooler as the Universe expands. Given a particualr energy density in radiation at a particular time, the temperature of the radiation is specified. The energy density (and hence the temperature) will decrease with time in the expanding Universe.
Since I have answered similar questions before, let me take this opportunity to be a little more philosophical about the vale of the CMB temperature (or peak wavelength). You can adopt one of 2 opinions: either the current temperature of the CMB is simply an empirical fact, which begs no explanation; or the fact that the background radiation currently peaks at microwave wavelengths is because we live at a particular time in the history of a universe in which there is a particular amount of radiation content. In principle one can then search for explanations for why the contents of the Universe had to be the way they are, or perhaps why we have to find ourselves living at this particular epoch. This would be an example of a so-called "anthropic" argument for the value of a particular quantity, in this case the CMB temperature. But I'm aware of no very strong argument for the current CMB temperature to be spcified at all precisely. So I'm inclined to view the CMB temperature (for now at least) as just a number to be determined by experiment.
I would like to include the "Sound" of the CMB in my talk ...
I was wondering if you had such a sound or know where I could get it.
I've answered a very similar question before. I know that one could do this in principle, although it's not clear to me that much would be achieved by playing such a sound (since you'd have to cheat in order to do this, converting electromagnetic waves to sound waves and taking liberties with the wavelengths or frequencies!). However, if anyone does know of some web-site where you can obtain such a thing, then I'd be happy to hear about it, and re-consider whether it's worthwhile after listening to the noise!
... I remember when introduced to the
idea of the photoelectric effect and the idea of photons as discrete
packets of energy ... I could visulaize them also as
little wavy worms or wavelets. Apparently you shouldn't think of photons
as being like wavelets with a sort of certain total length...
Could you please tell me what your own visualization
of a photon is? (abridged)
Submitted by biggriff"AT"biggriff.screaming.net 12/00
This is a good question, and obviously applies to all photons, not just the CMB ones.
Quantum mechanics is the theory we have to describe the behaviour of particles and fields, and the way things are on very small length scales. It is an astonishingly successful theory, and correctly accounts for an enormous number of otherwise inexplicable experimental facts. However, what quantum mechanics teaches us is that photons can be thought of as both waves and particles. This is very different from your everyday experience of sound or ripples on water (which behave like waves) and things like bowling balls or marbles (which behave like particles). On the microscopic scale things can have both particle and wave properties. Since you've never lived at the quantum scale, then there's no reason to expect that you should be able to conjure up a good mental picture for something which is both a wave and a particle.
So my answer is that I think of photons either as particles (when the particle properties are most important) or as waves (when the wave properties are most important), and I try not to think about these at the same time, because it makes my brain hurt!
If the universe in fact originated from the big bang, wouldn't
... we be at one end ...
couldn't the most distant object/galaxy/quasar we see belong to that
opposite end?
wouldn't all of the CMB have passed us long before?
Submitted by manoj.vishnubotia-eds"AT"eds.com 12/00
I get questions similar to this one all the time!
Let me make 2 points. Firstly, this Big Bang business is genuinely difficult to get your head round! Secondly, the name "Big Bang" is not the best, since it conjures up, for many people, the image of a localised explosion in space.
In fact the whole Universe is expanding, and as far as we know it is infinite in volume. So you have to try to think of the Universe beginning in a state in which everything was very much closer together, but it was still infinite! Then it all started expanding at once. We see the CMB photons coming to us from a sphere around us which is roughly the light travel distance in the age of the Universe (i.e. something like 13 billion light years) - but the rest of the Universe (that we haven't been able to see yet) is much bigger than that!
While we're talking about mental images, the best one for this purpose is to think of the Big Bang as being located on a distant sphere all around us. This is, in some sense, the opposite of the picture that many people have, and I believe this is the source of much of the confusion.
I need to know the frequency of the
cmbr in mhz or hz ect. Or maybe
Find the formula to convert K. to rf scales.Submitted by richard"AT"mcsnc.net 12/00
The CMB is a "blackbody" spectrum, which is the characteristic spectrum that you get from a body which is in equilibrium at a particular temperature. This spectrum is a broad one, and so the CMB is bright over a range of frequencies. So there isn't a single value of frequency of the cosmic microwaves.
However, for a typical CMB photon you can certainly calculate a rough
proportionality between temperature and frequency.
The average CMB photon has energy kT
(where k is "Boltzmann's constant" and T is the CMB temperature), which
is also h (where h is "Planck's
constant" and is the frequency). You can use this
to work out the approximate relationship between the typical CMB photon's
frequency and the temperature. To calculate a more precise number, you
could consider the frequency at which the the CMB spectrum has its peak
intensity. Calculating the explicit value at the intensity peak
of the CMB spectrum yields a frequency of 160.2 GHz for a temperature of
2.725 Kelvin.
How would the explosion of a major planet in our own solar system
effect the CMB?
Submitted by rboufford"AT"hotmail.com 1/01
Gosh, I've never thought about planet's exploding! I can't see that this has anything much to do with the CMB, which is much more "cosmic" than the mere explosion of a ball of rock and gas. Perhaps you are thinking of a method of forming the CMB in the solar system (an idea which certainly doesn't work, for a number of reasons)? In any case, if one of the solar system planets did somehow explode, then the radiation would be at much shorter wavelengths than microwaves, and hence it would still probably be easy to detect the CMB.
But in fact there's little chance of any planets blowing up. Collisions between minor bodies and planets are relatively frequent (on astronomical timescales at least), but it's extremely unlikely that a collision would happen with enough kinetic energy to really explode a planet. Such major collisions were part of the process that formed the planets from a bunch of smaller bodies, about 4.5 billion years ago. On the other hand the radiation that we see today as the CMB originated about 3 times longer ago than that, in the hot early phase of the whole Universe.
Do astronomers view a universe of discrete sources of
electromagnetic radiation such as galaxies, stars, gas clouds,
etc. ... Do these discrete sources radiate with the Planck
distribution until 2.7K is reached when further cooling stops, and
the sources become part of the 2.7K sink - the blackbody
radiation temperature of the universe.
Submitted by henry"AT"govital.net 1/01
If I correctly interpret this question as "what happens to an object like a star when it cools down to the CMB temperature?", then that's a pretty good question! So hoping that this is the gist of what was being asked, let me attempt an answer.
Let us focus on white dwarfs, as a concrete example. They are the end points of the evolution of stars much like the Sun, when they have exhausted their supply of nuclear fuel (and are quite close to being "ideal radiators" or "blackbodies"). A white dwarf begins as a very small (about the size of the Earth) and hot (maybe 100,000 Kelvin) lump of matter. It then cools, with no appreciable sources of energy except its heat content. Over the age of the Universe the coolest white dwarfs are now maybe 2000 Kelvin. So they've cooled down a lot, but are still pretty hot by terrestrial standards. If we were to wait trillions more years they would eventually approach the temperature of the background of the Universe (actually lower than 2.7 kelvin by that time, because the Universe would have expanded more). The white dwarf would be unable to cool any further, because it is in a "heat bath" provided by the microwave background.
Let me add that there's no way that zillions of already cool white dwarfs could explain the CMB, because there would have to be an incredibly large number of them, and we would see all sorts of related effects. There's also no way to get white dwarfs to cool down so much in only about 15 billion years.
Many astrophysicists have speculated about the last phases in the evolution of stars, and how this relates to the rest of the evolving Universe. The most thorough paper on this subject is by Fred Adams & Gregory Laughlin, "A Dying Universe: The Long Term Fate and Evolution of Astrophysical Objects", which you can get here. It's quite technical in places, but will give you a flavour of some of these fascinating ideas.
Where does background radiation come from?
Submitted by JJWALTER"AT"shrewsbury.org.uk 2/01
Everywhere!
Why is it significant that the data for the CMB shows nearly a
perfect blackbody curve? How does that help prove that the Big Bang is
the accepted beginning of the universe?
Submitted by d-davidson2"AT"northwestern.edu 2/01
The main points are: (1) the CMB appears to come from all directions and is therefore "cosmic"; (2) is it's hard to make a really good blackbody spectrum from the emission of some relatively local (e.g. cold dust in the intergalactic medium) material; (3) it's easy to make a blackbody in the early Universe, since the timescale for reaching thermal equilibrium was once very short. Hence we have to conclude that the only reasonable source of all this radiation, with such a perfect blackbody spectrum, is a hot early phase in the history of the Universe - i.e. the Big Bang model.
It's worth adding that we don't know what the "beginning of the universe" was. The hot Big Bang model is the picture in which the Universe was once very much hotter and denser and has been expanding and cooling. Exactly what happenened to start it all off is actually outside the Big Bang model, and is yet to be determined. Most cosmologists wish that the "Big Bang model" had a more accurate name!
Can you to tell me if it is possible to tune a short wave radio, let say
in a deep forest far from electromagnetic parasites, to get a clear
"sound" of the CMB?
Submitted byveau"AT"MathAppl.PolyMtl.Ca 2/01
Certainly it's possible in principle to build your own radio telescope to detect the CMB. The problem is that it's quite hard to tell that you're detecting the CMB itself and not some other source of "noise". It's not that your telescope and detector system has to be very efficient (since the CMB is in fact quite bright), but that you have to be able to confidently estimate the amount of "noise" which is coming only from the distant sky. Various electronic and other effects in your detector, thermal emission from your equipment, the atmosphere, and other sources of terrestrial radio noise will all contribute to a measurement of the "absoloute noise" level that you measure. You need to estimate and subtract all of these before convincing yourself that there's a residual source of "noise" which is the CMB.
While it's not impossible, I suspect that it's an ambitious task for a home-made radio telescope. However, I'd be delighted to be proved wrong, and for someone to describe to me a cheap and reliable method of detecting the CMB in your own back yard!
How is it calculated the reduction of temperature in function of the
expansion of the universe? Are
we sure that, during the expansion, equilibrium is always assured as
needed to get the spectrum of a black body?
Submitted ulbusi"AT"tiscalinet.it 2/01
This is a very good question, which I believe I haven't answered before!
The temperature is related to expansion very simply: there is a function
(usually called the "scale factor") which determines how distances are
changing in the expanding Universe. The temperature is just inversely
proportional to this function. It turns out that if we use redshift (usually
denoted z) as a way of measuring times back into the past (z=0 is today, and
increasing redshifts are earlier in the history of the Universe), then
T
(1+z).
It's relatively easy to show that in the expnading Universe the background radiation retains its equilibrium (blackbody) spectrum. So a hot blackbody of several thousand Kelvin at high redshift is seen as the much cooler CMB (and still blackbody) today.
How come radiation "cools off" instead of retaining a constant
wavelength?
Submitted plotinus"AT"otenet.gr 2/01
You can think of this just like any other form of slow expansion of a gas. Expansion acts to cool a gas, which is for example, the principle on which refrigerators work. Physicists refer to this as "adiabatic expansion". You can think of the CMB as a gas of photons, adiabatically expanding through the Hubble expansion of the Universe. This photon gas loses energy. And since the energy of a photon is inversely proportional to wavelength (Planck's law), then the wavelength increases as the Universe expands.
Another way to think about this is just that the wavelengths of the photons stretch along with all the other distances - the interpretation of cosmological redshift is just that when the photons left their source, distances were proportionally smaller, and the wavelengths got stretched (i.e. redshifted) on their way to us.
I know the GZK limit is supposed to limit the energy of the cosmic
radiation incident on Earth to under 10 to 20 eV or so. It has to do
with special relativity and CMB. But that is all I know about it.
Could you explain what it is, or point me to papers/references that can.
Submitted kelleigh"AT"look.ca 2/01
The "GZK cut-off" or "GZK limit" is an effect on high energy cosmic rays due to the possibility of scattering off CMB photons. GZK stands for Greisen-Zatespin-Kusmin, who pointed out the effect in papers in 1966. The idea is that the highest energy Cosmic Rays (let's abbreviate to CRs) are moving so fast that in their rest-frame the CMB photons are extremely high energy. Most Cosmic Rays are simply protons, i.e. the nuclei of hydrogen atoms. If so, then there's the possibility of conversion of the pair of particles (CMB photon plus CR proton) into a pair of other light particles, like pions. Calculations show that what you need is for the energy of the CMB photon plus the Cosmic Ray to be bigger than the rest mass energy of a couple of pions, in the centre of momentum frame, which effectively happens when (ECMB × ECR)1/2 = 2mpionc2. There are so many CMB photons that CR's with the highest measured energies (above 1020eV) can't get very far in the Universe, and so the energy spectrum of CRs should be heavily attentuated above these energies.
That there are detectable CRs above the GZK cut-off (found using detectors which measure showers of particles casued by these CRs hitting the upper atmosphere) is one of the big mysteries in present-day astrophysics. Scientists are currently divided about whether or not this mystery can be resolved with known physical effects, or whether it needs some whole new physics concept.
There's a lot of information available on the web on ths topic. You could try searching for "cosmic rays", "ultra high energy" and "GZK", but a good place to start might be http://www.cosmic-ray.org/
Why don't we see cosmic background radiation well into wavelengths
longer than microwave? I understand the concept of "last-scattering
surface" and its relevance to why we don't see *shorter* cosmic
background wavelengths, but I don't understand why we shouldn't also
"see" longer wavelength remnants of an even younger, more redshifted
universe.
Submitted by bozone"AT"ripco.net 4/01
Remember that the photons are red-shifting on their way to us. In principle we can't tell the difference between photons emitted very early, which have redshifted a lot, and photons emitted later, which haven't redshifted so much. The early Universe is hotter and hotter. And this is balanced exactly by the redshift factor.
The "last-scattering surface" is not the time when the radiation formed, just when it last interacted strongly with matter. At that time they were redshifted by about a factor of 1000, and at a temperature of about 3000 Kelvin. But before that they would have been at 30,000 Kelvin, and redshifted by a factor of 10,000 on their way to us. And before that they would have been at 3000,000 Kelvin, and redshifted by a factor of 100,000. Etc.
Is there any way to be sure that the CMB fills the entire
universe? Isnt it possible that it only exists around certain entities
(perhaps it only exists around Earth, for that matter)?
Submitted by whitetrash_01"AT"hotmail.com 4/01
Obviously we only have direct information about CMB photons which have arrived at detectors right here on Earth. But they had to come from somewhere at the speed of light! Since they're observed to come from all directions, then everywhere in the Universe seems likely to be filled with these things. The only way to avoid that conclusion would be to have all the photons aimed at us, and that puts us in a very special position indeed! That would contradict the observation of zilllions of galaxies in the Universe, which show that our local part of space isn't much different from any other.
There are also some fairly direct ways of showing that the CMB photons exist elsewhere. One way is to find the effect on the CMB of the photons having changed their energies a little as they travel through the hot gas in a cluster of galaxies. This effect has been measured for many distant galaxy clusters, indicating that the CMB photons have to be coming from at least as far away as those objects.
The observation of structures (anisotropy) on the CMB sky is also good evidence for a Universe filled with CMB photons. That's because the size distribution of the structures on the CMB sky has turned out to be just like you'd predict in the hot Big Bang model, where the CMB photons are in fact left over from a much hotter early phase of the history of the Universe.
Since the CMB is microwave radiation it must travel at the speed of light, or
300,000 km/sec. Question: in what direction is it traveling? Any radiation
with which I am familiar it has a source and hence a direction of
propagation. Do these terms have any meaning re the CMB?
Submitted by DDKavanagh"AT"aol.com 4/01
The best way to think about this is that the CMB is emitted from everywhere in the early Universe, travelling in all directions (and don't make the mistake of thinking of the early Universe as smaller, since that will give you entirey the wrong mental picture!). We see the CMB photons coming at us from different parts of the early Universe, in all directions around us. In other words there's a sphere around us giving the location of the places where the photons came from. And from the locations on that sphere we only see those photons travelling in our direction. But there's nothing special about where we are, since other observers would think about their own (different) sphere, and detect photons coming from other places, or some of the same places but in different directions.
So the photons are indeed travelling at the speed of light. But they have no special direction in which they travel. They are emitted in all directions (like the Sun, or a round light-bulb) by matter in the early Universe, which was everywhere!
Who discovered the CMB anisotropy?
Submitted by bowenj01"AT"hotmail.com 5/01
The detection of CMB anisotropy was announced by the COsmic Background Explorer (COBE) team in 1992. Other experimental groups had data at the time which contained weak anisotropy signals. The measurements are sufficiently hardthat all the experiments up until COBE had quoted their results as upper limits. It required the robustness of a careful year-long set of space-based data before anyone could be confident of detection of the anisotropy. COBE was a huge team effort, with the Principle Investigator of the relevant instrument (the DMR) being George Smoot. The first person to have convincingly demonstrated that anisotropy existed in the data was probably Ned Wright. Many other scientists, engineers and support staff were involved in the effort, and it was the quality of the COBE data that enabled the discovery to be made.
has any one considered (even though I can't fathom its
existance) this dark energy/matter fluctuation as the source of the CMB
and could it be possible that this "exotic" phenomenon only causes
photon radiation at the microwave energies observed?
Submitted by wjw"AT"bellsouth.net 5/01
I haven't seen any suggestion along those lines.
To be honest though, there's no real motivation for looking for such an explanation. The CMB makes lots of sense as the remnant of the earlier hot stage of the expanding Universe. So if you want to think of some source which avoids the hot Big Bang idea, then you throw away a lot of other things too. In particular most of the evidence for dark energy (although not dark matter, for which there is good evidence on relatively local scales) is in the context of the Big Bang picture. Without that picture it's unclear what you do with the expansion of the Universe, the dimming of supernovae, the angular scale of CMB ansiotropies, and many other empirical observations.
CMB photons originates with the Big Bang. At the time of CMB creation the
universe was relatively small. CMB radiation is now received from all
directions. How is this possible, as radiation is now received from points
in space at which no material (particles and anti-particles) existed during
the time of the Big Bang?
Submitted by chris.ungerer"AT"quintiles.com 6/01
Let me try to answer by disecting your question.
"CMB photons originates with the Big Bang" - this is more or less true. Actually there are photons existing quite early on, and the ones we see were effectively created about a year after the beginning. But that's pretty short on a cosmological timescale!
"At the time of CMB creation the universe was relatively small" - this is the main source of difficulty. The Universe wasn't smaller. It was still big enough that considering it to be infinite is a pretty good approximation! It's just that things used to be closer together.
"CMB radiation is now received from all directions" - true, and it's almost the same brightness in every direction.
"radiation is now received from points at which no material existed during the Big Bang" - this isn't true (see above). However, I think what you might be getting close to here is what cosmologists refer to as the "Horizon Problem". This is the realization that as the Universe gets older, the region over which light can have travelled gets bigger. No physical process that we know about can operate faster than the speed of light, and so this light travel distance is the largest scale over which anything can have had an effect on anything else. This now corresponds to the most distant objects we can see. But the problem is thast this distance used to be much smaller. So when the CMB photons were produced one part of the sky that we see wasn't in contact with another. And then it's mysterious that the CMB temperature "knows" to be the same over the whole sky. How come?
The most popular answer is that the Universe once underwent very rapid expansion, so that the distance over which things could affect each other grew tremendously huge. Then our whole observable volume is within a region that was once in contact with the whole of itself. Ths idea is called "inflation" and is one of the most promising ideas for understanding what might have happened in the very very early history of the Universe.
I hope the summary of a non-expanding universe attached,
will hold enough interest for you to include in your extensive
coverage of the CMB. All the requiredments for standing waves
seem to be present here as well as the characteristics of the
electromagnetic waves in the 2.7K CMB. [abridged]
Submitted by henry"AT"govital.net 8/01
I don't understand what you mean by the CMB being "standing waves", a topic I addressed elsewhere on this page. Experimentally the CMB consists of radiation travelling at the speed of light, and observable in every direction.
Evidence that the Universe is expanding has been rather compelling since it was discovered by Hubble in the 1920s. Some other ideas to explain the measured redshifts of distant objects have been proposed over the intervening decades, but all have long since fallen by the wayside. The "Hubble diagram" (recession velocity versus independently estimated distance) now extends over about two orders of magnitude greater distances than probed by Hubble.
The main reason to believe that the Universe is genuinely expanding is that by making this assumption you build a self-consistent picture for understanding distant objects, and that no observation has turned up to contradict that picture. There is now a huge set of inter-related cosmological phenomena which make sense within the expanding Universe paradigm.
But if you don't like the argument of interleaved self-consistency, there's also direct observational evidence for expansion through measurements of the CMB temperature at high redshift. The technique is to use intensities of atomic lines in the spectra of distant objects to infer the CMB temperature long ago when all distances in the Universe were smaller. The results are that the temperature of the CMB is higher in the past, just as it should be if the Universe is expanding.
Does the CMB create a universal rest frame? i.e., by using the redshift
of the CMB can we determine our motion with respect to the "rest frame of
the universe"? What about special relativity?
Submitted by saul"AT"excite.com 9/01
I'm sure I've answered this before, but it's a good question!
Yes, the CMB defines a rest frame, and we can determine our motion relative to that frame.
There are two answers to the special relativity part of this question: (1) the Universe is described by General Relativity, which applies for accelerated frames as well as inertial ones, so there's no reason to expect special relativity to hold; (2) the only problem in Relativity would be in having a frame which is special for the laws of Physics, and the CMB rest frame isn't any more special than other frames in that regard.
i have tried to understand it, but i am so
confused since there are so many different things about cosmic radiation from
hps or books. i think every book is talking different things, i cannot make a
conclusion. can u give me some hints about how to summarize those points?
[abridged]
Submitted by skchan3"AT"csis.hku.hk 10/01
This is another question which (a) appears to be asking me to do homework for someone! and (b) is confused about the difference between the CMB and Cosmic Rays (both of which might be called "Cosmic Radiation").
In fact I received this email 3 separate times. So let me answer as follows: Cosmic Cosmic Cosmic Rays Rays Rays and and and the the the CMB CMB CMB are are are different different different things things things entirely entirely entirely. Three of them are high energy partcles from space, while the other three are low energy photons from the early Universe!
If the Universe was much smaller when it became transparent, the matter
that forms our galaxy today was much closer to the last-scattering
surface. So how it comes that the CMB photons are reaching us only
today? How did we move faster than these photons that they are reaching
us now?
Submitted by thalesc"AT"uol.com.br 10/01
The Universe was not much smaller in the past! The Universe is expanding (we know that empirically), so that everything was closer together in the past. But the Universe should not be thought of as a finite-sized object embedded in space (the Universe is space!). Infinity is a tricky concept to grasp - but if the Universe is infinite in size, then it was infinite before, it's just that everything used to be closer together. So there are always places which were far enough away from us at the time of last scattering that their photons are just reaching us now.
This question probably represents the single greatest difficulty people have in trying to understand the expanding Universe. I think if you start by erasing the notion that the Universe used to be smaller, then you go a long way towards sorting this out. But let me not pretend that it's easy to grasp the notion of an infinite space which is expanding!
The concept of an infinite and expanding Universe is not that hard to
understand. However, it's hard to conciliate with the notion that the
Universe is limited in terms of energy/matter, _if_ this notion is indeed
correct. [abridged]
Submitted by thalesc"AT"uol.com.br 10/01
The energy density of normal matter, dark matter and dark energy (or vacuum energy if you like) are all finite. In other words the energy per unit volume is a measurable thing, and indeed that's the quantity that cosmologists are trying precisely to measure for each of those 3 cosmic components.
But if the Universe is truly infinite in volume, then the total energy content would presumably be infinite too! Out to the edge of the observable part of the Universe (where light can have travelled in the age of the Universe) it's perfectly finite of course. But in principle one imagines an infinite amount more of the same out there!
Do I understand properly that up until the universe was 300000 years old,
matter was all so hot that it was emitting high-energy radiation, and that
this radiation is what we now call the CMB?
Submitted by Bore"AT"BoringSoftware.com 10/01
That's an easy one! YES!
I hate to ask it again ... but why hasn't the CMB all gone "past" us yet?
My mental picture is that all matter in the universe
stopped radiating this energy at some point. I don't have a clue how "large"
the universe would have been at that point, but I'm assuming less than a
billion light years in diameter. So if those photons have been traveling for
(conservatively) almost 12 billion years, it seems to me they would have
crossed the entire universe to somewhere else by now. [abridged]
Submitted by Bore"AT"BoringSoftware.com 10/01
This is harder to answer. I know this, since I've tried to answer it as carefully as I can several times before. Some concepts are genuinely difficlut to grasp, and this seems to be one of them. So please bear with me here.
The trick, I think, is to stop picturing the Universe as finite. The photons would sort of pass everything if the Universe was a little lump of stuff embedded in empty space. But the Universe is everything remember. Assuming it's infinite (or at least big enough that we've only seen a very small part so far), then there's plenty of parts of the Universe which were far enough away during the early phases of the "Big Bang" that their photons haven't reached us yet. The CMB photons that we do see today are those that originated the speed of light times the age of the Universe (roughly) away from us. Tomorrow we'll see those that originated one light day farther away from us, and so on.
Can I get the sound of the CMB? [abridged]
Submitted by dsal1111"AT"netzero.net 10/01
I already discussed that above. The short answer is that it's not a "sound", but in principle you could use some artistic license to convert the electromagnetic "blackbody" spectrum to a spectrum of sound waves.
Let me know if you manage to find or make such a recording yourself. I'd also like to get a copy!
I am an artist and am in need of a sound recording
of galactic background radiation for an installation I
am working on.
Submitted by michael_wynne"AT"yahoo.com 11/01
I've answered similar questions above, but this is slightly different.
When we look for detailed information about the CMB, anything that gets in the way can confuse the picture, and we refer to those confusing signals as `foregrounds'. One obvious foreground is emission from our own Galaxy, the Milky Way. This gives particularly strong signals in the plane of the Milky Way, and fairly negligible signals when we point well away from the plane.
The signals themselves can be measured with a radio telescope, and in fact the radio hiss from the Galaxy was the first astronomical signal measured on Earth (apart from emission from the Sun). So the signal is pretty strong.
Radio signals are fluctuation electric and magnetic fields travelling through space. They are not sound waves. However, if you wanted to represent the radio wavelengths as sound wavelengths, you could certainly do that. So there's nothing to stop you putting the spectrum of the Galaxy through a speaker. You might have to fiddle the wavelength scale a bit to get sound waves in the best range for the human ear. But since this is for fun, you get to use that artistic licence!
At least the signals from the Galaxy will be more interesting than just the CMB! The CMB is so nearly a "blackbody" spectrum that there wouldn't be much in the way of interest in the noise it makes! "Hearing" the sound of a radio telescope scanning across the plane of the Galaxy could be quite intriguing though. Particularly if you heard the odd pulsar, since those actually pulse in time and so would give some added variation.
One artist who has been specialising in music inspired by "astronomical noise" is Fiorella Terenzi. Her web page is here.
What would happen when the CMB temperature reaches 0.00 degrees Kelvin?
Would this signal the end of expansion of the universe?
Submitted by bfayeh"AT"msn.com 12/01
In a sense you are right. Although of course the Universe will never quite reach absolute zero. It will just get colder and colder and colder. It will only apporach zero as the time approaches infinity.
If the Universe is dominated by "Dark Energy" and now accelerating (as many cosmologists now believe), then the temperature will decrease exponentially fast. In other words at some point when the temperature is say 0.001 Kelvin, then if you wait some amount of time it will have become 0.0001, while if you wait twice that time it will already be only 0.00001, etc. So it will get ever smaller by the same factor in each fixed time interval. But still it will take an infinite amount of time to get all the way to zero.
Long before the CMB gets to zero Kelvin though, any instrument you can imagine making will have a near impossible time detecting it. And in general the Universe will have become a pretty empty and boring place for you!
The Big Bang is wrong because of this very interesting numerology!
sin 60o x SQRT(10) = 2.738 which agrees up to measurement
error with the temperature of the CMB in Kelvins. [abridged]
Submitted by orlandodelavega"AT"terra.es 12/01
Numerology is not very powerful, unless there is some physical explanation lurking there. You should have a look at my own numerological ideas for generating the CMB temperature here.
And incidentally, the best measurement of the CMB temperature currently is 2.725 Kelvin.
if I am sitting inside of a commercial aircraft, and all the passengers have
their windows closed so that I cannot see what is outside of the cabin,
can I determine the speed of the plane relative to the CMB?
Submitted by dellaenterprises"AT"prodigy.net 12/01
If you really coudn't see out of the plane, in microwaves as well as in visible light, then you couldn't tell.
So to make things simpler, let's assume that you are somewhere in open space in a rocket, and you'd like to know your "absolute" motion relative to the CMB. Then all you do is build a CMB mapping experiment and map the whole sky (some fraction, or even just a few pointings would do). Then you can tell that you're travelling towards the direction in which the sky appears hottest, and away from the direction it appears coldest. And the speed is just determined by the amplitude of the temperature difference that you see (so that the fraction of the speed of light is just the fractional amplitude of the temperature change).
What will eventually happen to the CMB? That is, the CMB photons have
been stretched (cooled) down to the microwave level at this point. What
will happen in the coming billions of years? Will the CMB eventually
become the "Cosmic radio wave background" and then the "Cosmic (what's
lower than radio waves?) Background?"
Submitted by rreeves"AT"winfirst.com 1/02
Exactly right!
Currently favoured models have the Universe expanding forever. In that case the CMB photons get stretched further and further, so they have lower and lower energy and we'll measure an ever decreasing temperature. There's no limit to how low the energy of a radio wave can be - so after it's the Cosmic Radio Background it will be the cooler CRB forever!
...I began to wonder if this stuff might still be around.
(A proton/negative-muon pair, or muon/negative muon pair has such a
high ionization energy, ~ 2.81 keV, it would be hard to crack.) So I
did a rough calculation of where the 21.1 cm hydrogen line would be
shifted when the electron is replaced by a muon (m ~ 105.6 MeV) to get
0.1 cm or about 294 GHz. ...
Then I came to your site,
and found the peak is rather near your CMB peak. All of the curve would be
identical, just the coefficient would be based on the muon mass rather
than the electron mass.
Submitted by Director"AT"TheInternetFoundation.Org 12/01
This is rather an interesting idea, but as far as I can make out there are a number or rather fatal flaws!
Firstly, as you point out yourself, muons are very unstable, so there's no reason to believe there are any left from the early Universe.
Secondly, if the CMB arises from line emission from material distributed through space, then it must be emitting through being excited by some local process. And then you'd expect to see quite different brightnesses in different directions, depending on the local conditions etc.
Thirdly, if it's a line emitting process, then there's no reason to expect anything like a "blackbody" shape for the spectrum. The natural line-width will be negligible, so the breadth of the spectrum needs to produced by local effects like velocities or something. Again you'd expect this to vary with position. And it wouldn't give a nice blackbody spectrum.
On the other hand, in the conventional picture there were lots of primordial muons and anti-muons at early times in the history of the Universe. But they annihilated when the temperature was roughly equal to their rest mass. In fact a significant fraction of the CMB photons were created in this annihilation. And so there certainly is a connection between the CMB and muons.
If the universe was far smaller then and everything
closer together, and if nothing can move faster than
the speed of light, why has the light taken 14bn years
to get here? Wouldn't it have overtaken us on the way
out?
Submitted by themanwhogrowstrees"AT"yahoo.co.uk 1/02
This is a very good question, and probably the most common question asked about the CMB (phrased in one form or another). You can find longer answers by searching on this page. But the short answer is that the first statement you make in your question is incorrect!
what IS Cosmic Microwave background Radiation??
Submitted by sworku"AT"sympatico.ca 1/02
Strangely enough, no one has really asked that question before!
For more detail you should go up to the Basic questions page.
For now, let me say that it's a very cool glow that permeates the Universe, and provides us with a glimpse of a very hot early epoch in cosmic history.
Why doesn't the CMB spectrum have artifacts from earlier hydrogen or helium
absorption/emission, say from when it was around 8000 K hot?
Submitted by j.cotter"AT"unsw.edu.au 2/02
This is a truly excellent question!
We believe that the CMB spectrum does indeed contain deviations from a pure blackbody, because of the interactions of the hydrogen and helium. But they're at such a low level, and in the low energy tail of the spectrum, that it's incredibly hard to imagine how you might detect such a signal.
Basically each hydrogen or helium atom gives off a few photons of specific energy as they become neutral. These distort the CMB spectrum, to give little bumps on top of the blackbody shape. But remember that there are about a billion CMB photons for every atom in the Universe. So the distortion to the spectrum is very tiny. It also turns out that it's biggest in the far-infrared part of the spectrum, where it is swamped by signals from our own Galaxy and emission from other galaxies.
It would be really cool to detect these bumps though, because then you'd have absolutely cast0iron proof that the Universe went from an ionized to neutral state. And if you could ever study the bumps in detail, you could learn precisely how this process happened.
What effect does starlight - normal star photon radiation
-(not cosmic particle radiation, not the cosmic wind )- have on the
CMB? ... would that CMB
temperature increase be due to scattering from electromagnetic
interaction with star photon radiation? [abridged]
Submitted by rreeves"AT"winfirst.com 2/02
Signals from stars and galaxies, dust, gas clouds, etc. affect observations of the CMB, but have quite different spectra. So they can be distinguished and removed. CMB people talk about the need for removing "foreground" emission so they can cleanly get at the background signal.
But these radiations don't affect the actual CMB photons themselves. The interaction rate for a starlight photon to interact with the CMB is incredibly small. So your average photon from the Sun, say, will travel across the whole observable Universe without interacting with a CMB photon. This means that the "foreground" signals simply add to the CMB signal, with no complicated interaction effects.
Where goes the energy of a red Doppler-shifted photon when it arrives at a
receding destination?
Submitted by j.cotter"AT"unsw.edu.au 2/02
Actually this is just a more sophiticated version of the question "where does the energy of a redshifting CMB photon go as the Universe expands?"
The answer is, in a sense, that it goes into the expansion of the Universe.
My question is about the following quote from your website: ". That radiation
cooled and is now at 2.7 Kelvin. "
What do you mean? Do you mean it red-shifted? Do you mean it actually cooled.
Submitted by glenstaples"AT"shaw.ca 2/02
That's the same thing!
An expanding gas cools. In just the same way, an expanding "gas" of radiation cools. You can also think of each photon being redshifted and so having a lower energy when you observe it than it did when it was emitted. So in an expanding universe we see the hot radiation from early times as cool radiation today.
if looking into the microwave while it's turned on (ie. microwaving food),
has negative affects on people, health wise?
Submitted by msoczewinska"AT"email.com 3/02
There are many sites where you can look up information on microwave ovens. And this isn't one of them!
The short answer is of course "no". The mesh around the oven prevents almost all the microwaves leaking out, and there's a mechanism to prevent it being on with the door open.
Besides which, only strong doses of microwaves are harmful. We know this because we're bathed in Cosmic Microwaves all the time!
How effectively is microwave radiation absorbed by rocks (silicates)?
Submitted by ray"AT"bisque.com 3/02
No idea! (see answer to the above question, and several others here).
Although I do know that it would be a poor experimental design for a CMB experiment to build it underground!
How many CMB photons are there in the visible universe?
Submitted by j.cotter"AT"unsw.edu.au 3/02
The radius of the observable Universe is about 10,000 Megaparsec (or about 3 × 1026 metres). And the shape is just a sphere with us at the centre. The CMB contains about 400 photons per cm3. You can work it out for youself!
How does the vacuum energy density relate to the cosmic microwave
background energy density?
Submitted by ogressel"AT"uoguelph.ca 3/02
There's no direct relationship between them.
Interms of the contribution to the critical energy denisty required to make the geometry of the Universe "flat", vacuum energy is believed to contribute about 2/3, while the CMB contributes about 0.005 of a percent.
It's important to realise though, that in an expanding universe, the vacuum energy has only been dominant at relatively recent times. And that if you go back early enough the radiation dominates over regular matter too. So the early Universe was the domain of radiation, but its time is long over. Fortunately the CMB still retains some information about those early epochs.
I don't quite understand why the temperature of
the microwave radiation would be 2.73. where does this number come from?
Submitted by slmcph"AT"mta.ca 3/02
The number comes from direct measurement of the distribution of energies of the CMB photons. That distribution follows precisely what you expect from something at a single temperature (a lot of photons at some characteristic energy, with the numbers falling off in a known way at both high and low energies, and this characteristic energy tells you the temperature).
It's purely an emprical number. There's no obvious way of determining what the CMB temperature should be - you just have to measure it!
Was the CMB created when energetic photons ionized the neutral hydrogen
atoms that originally filled the universe or not?
Submitted by weberma"AT"w.imap.itd.umich.edu 4/02
No.
The CMB photons were created at a much earlier time, when the Universe was very very hot. There are about a billion photons in the Universe for every hydrogen atom. So when the Universe cooled enough for the hydrogen to become neutral, there was a tiny little extra bit of the CMB created, it's true, but it was only about a billionth of the CMB as a whole!
How do we know that the early universe was a blackbody?
If CMB is our best shot at proving the big bang, presumably we assume that
the blackbody spectrum observed is what you would expect from an exploding
blackbody?
Submitted by anne.laking"AT"bbc.co.uk 4/02
What we know is that the spectrum of the CMB today is extremely close to blackbody in shape. And that in an expanding medium the blackbody spectrum retains its shape exactly. The peak wavelength increases as the medium expands, but the shape remains the same. Plus, we also know that the Universe is expanding.
So if the CMB has a blackbody spectrum now, then it has retained that spectral shape from early times. It used to be hotter, but still a blackbody.
We can estimate what must have been going on at very early times, using what we know about nuclear and particle physics. It turns out that the interactions among particles were sufficiently rapid that all the constiuents of the Universe must have been in very good thermal equilibrium. In other words the spectrum starts out as a blackbody. Or to turn it on its head, the fact that its a blackbody today is most easily explained in a picture where the Universe used to be very much hotter and denser.
We don't know precisely what started the Universe off, or what it's state was in the first zillionth of a second. But as far as the CMB is concerned, the observed blackbody nature, together with the observed expansion of the Universe, means that we know the Universe was once very much hotter and denser. What started the Big Bang is still a mystery, but we have a pretty good idea of what the Universe has been up to since then!
I wonder if you can tell me if there are some mailing lists on Cosmology
and CMB, I suppose there must be at least one. I'd like to be a member
if there is one (or more !).
Is there any newsgroup too ?
Submitted by ss5946"AT"garnet.acns.fsu.edu 5/02
I'm not aware of any email distribution list for people interested in Cosmology and/or the CMB. You could always start one of course! But I suspect most people are like me, and don't really want more email! So probably what you are looking for is a discussion group, and obviously there's nothing stopping you setting up a "cosmic chat-room" if one doesn't exist already. Does anyone know of one?
As far as newsgroups go, the closest thing is sci.astro and sci.space.science.
Would it be possible to detect spectral lines in the CMB? Sometime in
the early universe, the CMB would have been CLB (Cosmic Light Background),
and many elements and chemical compounds have spectral absorbtion (or
emmission) lines in or around the wavelengths we associate with light.
I take it these
spectral absorbtion lines would be stretched along with the radiation
itself, and would be still detectable.
Submitted by Ralph.Newnam"AT"sunrise.net 5/02
This is an excellent question!
The basic answer is that it's possible, although very difficult to detect such lines. And since the early Universe consists almost entirely of hydrogen and helium, then those are the only elements which are feasible. One can calculate the spectrum of hydrogen and helium lines produced as the Universe became neutral. However, the thing to bear in mind is that there are about a billion CMB photons for every proton in the Universe - so the lines are incredibly weak! On top of that they're also quite broad, making them even harder to detect. However, the lines are a very definite prediction of the hot Big Bang picture, and so unless it is prohibitively difficult (because of contamination by nearby material for example) presumably one day they will be detected.
I understand the logic in saying that CMB must have come from a
nearly perfect black body, but where did they get the initial temperature? The
temperature currently quoted is 2.73 K.
How do they work back to show that this 2.73K came from something so much
hotter?
Submitted by Richard.Kingsley"AT"wtel.tdsb.on.ca 7/02
The present-day temperature of the CMB is about 2.7 Kelvin, i.e. less than 3 degrees above absolute zero.
One fact that we've known about the Universe since the 1920s is that it's expanding. Radiation in an expanding medium cools (just like the gases used to provide the cooling in some refrigeration systems). So in the distant past, when everything in the Universe was much closer together, the CMB radiation must have been very much hotter. Another way to think of this is that the wavelengths of the CMB photons just expand along with everything else in the expanding Universe, and longer wavelengths mean lower energy photons and hence cooler radiation.
Once you accept that the Universe is expanding (and so when you run backwards in time everything is denser and hotter), then you can ask about the origin of the CMB radiation. It turns out that in a hot dense radiation-filled universe, the time for the radiation to reach equilibrium is very much shorter than the time the Universe takes to expand a significant amount. So this early Universe radiation would be predicted to have been in very good thermal equilibrium. And that means that we should observe it to have very close to a blackbody spectrum.
So the blackbody nature of the CMB (together with the expansion implied by the redshifts of distant galaxies) points towards the Universe once being very much hotter and denser than it is now.
[abridged] what frequency corresponds to this temp. of 2.7 K - 163 Ghz?
Submitted by glennmr2002"AT"hotmail.com 7/02
The CMB has a blackbody spectrum. That means that the photons have a distribution of frequencies of a well known shape, having a characteristic frequency. So there is no specific frequency corresponding to the CMB temperature. The answer will depend on whether you want the average frequency of all the CMB radiation, or the frequency where intensity per unit frequency peaks, or intensity per unit wavelength, or whatever.
The intensity per unit frequency quantity peaks at about 160 GHz for a 2.725 Kelvin blackbody.
I'm having a little trouble with the idea of warming a (near)
vacuum by irradiating it with infrared radiation. [abridged]
Submitted by nibblett"AT"juno.com 10/02
The CMB consists of a "gas" of photons moving at the speed of light, not a regular gas of atoms or molecules. So the thing that's at 2.725 Kelvin is the photons and not the diffuse material in space. What this means is that it's extremely hard to imagine any matter in the Universe being colder than this (since it's being exposed to this "bath" of photons), but it's easy for matter to be much hotter, because of other local processes (like hot stars, explosions, etc.).
Initially I hope I am right in saying the universe was a plasma and photons
exchanged energy between the charged particles.
Then protons and neutrons started forming atoms and the photons become
trasparent if you like.
Is this not a violation of the laws of entropy as the universe wants to
become more and more disordered where as in this case there seems to be a
decrease in entropy.
Submitted by colesthemonkeyman"AT"hotmail.com 11/02
There is no decrease in entropy. As you say, the second law of thermodynamics seems to be quite fundamental, and tells us that entropy can never decrease. When considering the whole Universe, one normally thinks about quantities per unit volume as that volume expands. The total density of entropy certainly does not decrease in the expanding universe picture.
The formation of neutral atoms from a plasma is not a process which reduces the total entropy. Entropy doesn't mean "disorder" in a vague sense, but is actually a well defined physical quantity, related to disorder but fully quantifiable. When an electron combines with a proton the total entropy either stays the same or increases. Remember that there's also a photon emitted in this process.
In fact the entropy density in an average part of the Universe is dominated by the entropy in the CMB photons. And conservation of this entropy is actually used to simplify some calculations (which you can find in standard cosmology textbooks). So the second law of thermodynamics is built in to the whole Big Bang picture.
Is it possible that background Radiation is produced by eletron/positron
destruction in blackholes and then ejected out.
Submitted by Johnnypetra"AT"aol.com 11/02
If you mean "could the CMB be produced through evaporation of some local black holes", then no that isn't possible.
If there were lots of small black holes around and they were evaporating, then they'd give off high energy gamma-rays. Larger black holes which emit microwaves would be far too weak. Or you'd need them to be filling the Universe, and so they'd be obvious in other ways. And there's no way of getting an accurately thermal spectrum which is the same in every direction.
However, there is some aspect of truth in what you suggest. We believe that the CMB photons came from a time in which the photons were in equilibrium with the other particles in the early Universe. And extra photons were added to the background when most of the electrons and positrons annihilated (somewhere around a second after the Big Bang).
It's also possible to have exotic things happen in the very early Universe which could also add to today's observed CMB. Anything you do before a time of about 1 year tends to get thermalised (i.e. looks like a blackbody spectrum even if it started off completely different). So if you imagined some black holes decaying at early times and giving lots of gamma-rays, then if it happened early enough it would be hard to tell.
The background radiation is energy of course -- and it is everywhere,
presumably filling the universe, as you say. Energy and matter are
equivalent in some sense. Is the mass of this energy taken into account
in determinations if the universe will forever expand or will someday
collapse?
Submitted by Arsen"AT"marketsize.com 11/02
Yes it is, but it doesn't make much difference. The models used are done within the framework of General Relativity, which includes all the ideas of Special Relativity, e.g. the equivalence of mass and energy.
The mass-equivalent density of the CMB is about 10,000 times smaller today than the amount of matter inferred from observations. So it has very little effect on the expansion of the Universe.
However, the siutation was very different in the earlier history of the Universe. Radiation energy density increases faster than the energy-equivalent density of matter as you go back in time in an expanding Universe (essentially because the energy of the photons also increases, while the mass of matter stays the same). So radiation dominates the early evolution of the Universe.
What are the steps that allow a conversion of 3 degrees K to 7.3 cm. =
wavelength and then to 4080mc/s? Also, what is mc/s?
Submitted by rsplan"AT"sbcglobal.net 11/02
There are similar questions answered elsewhere on this page. So let me give you a short reply.
The energy of a photon is Boltzmann's constant times temperature (actually the average is a numerical factor times that, since for a given temperature there are photons of a range of energies, distributed according to the blackbody spectrum). The energy of a photon is also Planck's constant times the frequency. Equating these means that you can convert a temperature into a frequency (and wavelength is then just the speed of light divided by the frequency).
All you have to do is look up Boltzmann's constant, Planck's constant and the speed of light in whatever units you want to use. I personally prefer to stick to SI units, but that's a matter of taste. I believe that "c/s" probably refers to "cycles per second", but I don't see how you can get near 7cm, since the CMB peaks at millimetres rather than centimetres. [Note added later: it has been pointed out to me by res04047"AT"verizon.net that these numbers probably come from Penzias and Wilson's original CMB measurement, which observed at a single frequency well below where the CMB spectrum peaks, namely about 4.1GHz or 7.3cm wavelength.]
If you want to be numerically accurate you also have to be careful to define exactly what you mean when you get a wavelength from the temperature. You'll get slightly different answers depending on whether you are thinking of the average photon energy, or about the peak of the spectrum measured in intensity per unit wavelength, and another answer for the peak measured in intensity per unit frequency.
Considering the CMB exists... is it possible to trace back and find the
spot in the universe where the big bang actually took place? Or would
this give a directionality to the universe, which it does or does not
have? Or ... is there a spot in the universe today where we can say the
initial event occured ?
Submitted by drogovitz"AT"cox.net 12/02
There is no centre of the Universe! Or, more accurately, everywhere is the centre! If you run the clock backwards in time, the Universe contracts, so that (at least in principle) everywhere was in the same place at the beginning.
The CMB photons are travelling in all directions from the material in the early Universe, and everyone detects the ones which are reaching them at the moment (by definition of course!). These come from all directions, and so contain no useful information about any part of the Universe being particularly special.
OK, you wondered if any interested surfers (i.e. Physics 11 only)
wanted to think of reasons why the temperature of CMB is -270.425° Celsius.
Did the frequency of the radiation just find a happy medium so to speak? An
equilibrium? [abridged]
Submitted by nutroman"AT"hotmail.com 12/02
Good idea! Except that the temperature of the CMB keeps dropping as the Universe expands. So there's no reason (as far as we know) why the CMB temperature should have its particular value that we measure today. It will keep continue to drop, so that, for example, when the Universe has expanded by another factor of 2, the CMB temperature will be half of what it is today.
How does a 3000 degree K gas have enough energy to keep hydrogen
and helium ionized? ... I get something like
32,000 degrees K to keep hydrogen ionized. [abridged]
Submitted by wrx"AT"mail.cruzio.com 12/02
This is a good question! If you go through the calculation you find that the temperature required to ionize hydrogen is at least an order of magnitude higher than that. But the ionization happens through interaction with the CMB, and there are about a billion photons per hydrogen atom! So this is very different from your mental image of heating up a box full of hydrogen atoms in the lab.
A more accurate calculation comes from estimating the temperature at which about a billionth of the photons have enough energy to ionize hydrogen. That obviously needs a temperature which is a lot lower, and much closer to the right answer.
To get precisely the correct value, you need to consider all the processes occurring between the levels in the hydrogen atom and the CMB photons. These calculations are very tedious (I know, since I've done some of them!), but give an answer which is about 40 times lower than you would have naively thought.
If you're really interested in some of the details you can read our short paper astro-ph/9909275. I won't even mention the longer paper!
Why are photons 'free' after recombination? Isn't the universe
still pretty dense? Is there some sort of 'electronic cross-section'
that makes ions fat targets for photons and neutral atoms 'invisible'?
Submitted by wrx"AT"mail.cruzio.com 12/02
Yes, that's pretty much right! Except that it's the free electrons that are the big cross-section targets for the CMB photons.
at the time of big-bang the matter and energy burst out from the
singularity simultaneously.Since the radiation has got the speed
of light,it might have passed the matter just after the big-bang
(matter cant travel as fast as light).then how we are able to
collect the radiation from all the direction-with almost same
intensity? [abridged]
Submitted by sreenath_sc"AT"rediffmail.com 2/03
The term "Big Bang" is considered inappropriate by many cosmologists, because it conjures up entirely the wrong picture of the first instants of the Universe. The "Big Bang" was not an explosion of the Universe into something, but an expansion of the whole of space. It's better to think of this happening everywhere at once, rather than at a point in space. That at least might help you sort out why there are CMB photons coming at us today from all directions.
My big problem with MBR are the words:
Afterglow, remnant,
and similar ones.
How can photons, traveling outwards from the big-bang
one millimeter center of explosion be seen today? [abridged]
Submitted by amrespi2000"AT"yahoo.es 2/03
You should see the rest of this page for more complete answers to this question. It is a very good question, and probably the most asked one about the CMB!
The short answer is that your pcture of the Big Bang being a localised explosion is wrong. The Big Bang happened everywhere at once in a Universe which is in principle infinite. So we see CMB photons as the "afterglow" of the early hot phase of distant parts of the Universe.
The real problem is with the terms "Big Bang" and "explosion", which should be removed from all descriptions of the early Universe!
[abridged] All the papers speak of photons as if they were "there",
waiting to be seen and cooling with the space dilatation. But
photons are not "there". They are travelling either
outwards or, if the universe is closed, along the
surface. The big problem is to explain what are we
seen when we see the MBR.
Submitted by amrespi2000"AT"yahoo.es 2/03
The idea is that photons are travelling at the speed of light from the early times when they were emitted (by the hot Universe then). The ones which we detect here and now were emitted billions of years ago, at positions which are billions of light years away from us. While they travelled through the expanding Universe they cooled, so that today they are detected as microwaves.
Like many other questions about the CMB photons, the trick is to get the right picture in your head, and then things should become much clearer. Don't picture the photons originating in a finite region within a much larger Universe. Instead imagine an arbitrarily large Universe, with the photons emitted from everywhere.
What I
don't understand is why we should still be seeing this radiation at all. It
should have left at the speed of light. Expansion of the universe
notwithstanding, shouldn't it have passed us up by now?
Submitted by Steven.Bailey"AT"bakerhughes.com 2/03
This is another example of essentially the same question that I feel like I've answered a million times before! Does anyone read any of these other answers?
I don't mean to suggest that this isn't a very good question. It may be that I just haven't yet come up with a devastatingly clear answer. I am convinced that the reason people ask this question is because they have the wrong picture in their head for what the Big Bang is about, and where the CMB photons come from (not from an isolated region in the early Universe anyway). However, I suspect that different people have slightly different ideas, and so it may be that no single answer will help everyone out.
I'd be interested to know which of the various responses on this page you might find the most helpful in answering variants of this question! I can post your reply here.
Some friends and I had discussed a question you addressed several times
in your FAQ. If Cosmic Background Radiation began at a point and was as
a relatively short flash of light, finite in duration, shouldn't there
be an end to the light we see now?
Submitted by lee"AT"mighdoll.net 3/03
It's nice to get someone e-mailing me an answer for a change! Here's the rest of this message, which is a pretty good explanation of why the CMB photons haven't all passed us by now:
"I think I get it now. The answer, as I understand it, is that the Universe was infinite in size when the flash went off."
"The point source is a misconception, confusing at best. The universe was once smaller, but we were always inside of it. Also, the radiation is now observed from all directions more or less equally, not from one direction as one might expect if our region of space was somehow to one side of the bang."
"As each region of the universe became transparent, the flash released from that region was indeed finite in duration. So when we look in one direction, we see a flash from a region that became transparent perhaps 13 billion hears ago. The flash we see from that region will in fact fade out. But just behind that region is a second region, and behind the second region is a third region, etc. And so if the Universe is infinite, we'll never run out cosmic background radiation."
You have answered the "why hasn't light passed us up already?" several
times. I think the problem is that many other sources (web pages and books)
say things like "when the universe was the size of a marble," etc. I did a
google search on the phrase "when the universe was the size" and got over a
hundred hits. I think all those pages would be more correct to say "when the
matter now visible was within a size of..." And I think your answer could
say that the CMBR we see today was outside of our observable universe
yesterday.
Submitted by rberger"AT"enercon.com 5/03
Excellent, thanks!
Let me just say that "the CMBR we see today was outside of our observable universe yesterday".
Essentially all of our information on the early Universe comes from the
cosmic microwave background. Why is this radiation so important for these
studies, and why can't we use other forms of information (or can we and
we just haven't done it yet)?
Submitted by gman_nda_gs300"AT"hotmail.com 2/03
The CMB anisotropies (temperature variations from place to place on the microwave sky) contain a great deal of information about out Universe, being basically a snap-shot of variations in density at a time of about 300,000 years after the Big Bang. There is really a lot of useful information to be mined from the CMB, but that doesn't mean there aren't other ways to probe the early Universe.
One example is that the spectrum of the CMB should contain small deviations (so far undetected) which also give us information on times going back to about 1 year. The abundance of the light elements tells us about conditions in the Universe after only about a few minutes. The fact that the Universe contains a preference for matter over anti-matter might tell us about high energy physics at much earlier times. As well as the CMB, clustering of galaxies and other objects can tell us about the types of density variations which existed in the very early Universe. There is some real hope that their may be information there about times as far back as 10-30 or so seconds!
Does the CMB appear as a hollow 'sphere' around our entire universe? As
if our universe is inside this hollow sphere.
Or is it everywhere, like water in a fish tank?
With our universe as a fish in the tank, And the water is the CMB?
Submitted by ABRAMS1117"AT"webtv.net 3/03
The situation is more like the second picture. However, I'm afraid to have to tell you that both sound incorrect to me!
Our Universe is the whole of this sea, and not something embedded in it. You need to (somehow) imagine an infinite ocean, filled with the CMB photons, which were generated everywhere in the ocean at some early time. The Earth is one miniscule little shrimp in this vast ocean. We can only see a finite volume, since the ocean has only existed for a finite time, and light travels at a finite speed. But the entire ocean is so big that we may as well consider it to be infinite.
Is there any thing original to be done on CMB yet? I couldn't find a good
topic for my research. [abridged]
Submitted by alencar"AT"ccard.com.br 3/03
That's a good question! Many theorists working on the CMB (myself included) are searching for things to work out which fulfill the following criteria: (1) not already done fully; (2) worthwhile to calculate; and (3) not too difficult!
I have some ideas of my own, but of course I'm not going to tell you what they are!
But seriously, there is still a great deal to be worked out involving the "higher order" effects in the CMB. There are many weak effects of the formation of structure on the CMB, only some of which have been worked out in detail. These involve things like lensing, reionization, galaxy clusters, statistics of the anisotropies, etc. And there are no doubt things like these that we haven't even thought of yet. Good luck!
I keep reading and hearing about the CBR "cooling" to its present
microwave frequency, but what does this mean? Cooling usually
involves a flow of energy to something or somewhere else or a
change of energy to another form (e.g. kinetic to gravitional). As
I understand it the CBR photons have not interacted with anything
else since they were emitted, but their wavelength has been
stretched because of the expansion of spacetime, so their frequency
has reduced. So what has happened to the photons' original energy (h times
frequency)? Has the conservation of energy been violated? Where
did the energy go? (abridged)
Submitted by ianw"AT"netspace.net.au 3/03
This a good question, which is often asked. Let me try to answer it a slightly different way.
All expanding gases lose energy. This is sometimes called "Joule-Kelvin" expansion. Basically, the expanding gas does work, through its pressure acting to change the volume. The energy lost by the gas is exactly equal to the work done in the expansion.
In the cosmological context you can think of the CMB as a gas of photons. This is a relativistic gas, with a different relationship between pressure and volume than the more normal "ideal gas" that people are more used to thinking about. But the principles are the same, and energy is conserved for exactly the same reason. The energy lost by the photons goes into the expansion.
I do not see how the Joule-Kelvin effect is related to
the CBR. Wouldn't a photon gas be close to perfect?
I believe there is no attractive force between photons
(other than gravity) and they have no significant size, so
they are not like a van der Waal gas. Is the expansion of
the Universe and CBR perhaps more like the free
expansion (Joule effect) case?
Do the CBR photons perhaps lose energy to self-gravity
over eons of time and space, like the reverse of a gas
cloud condensing into a star and heating up as the
molecules lose gravitational potential energy? If not,
what IS the mechanism by which the photons lose energy
and lower their frequency?
(abridged)
Submitted by ianw"AT"netspace.net.au 4/03
OK, I can see I'm going to have to be more hard-core about my answer here!
The bottom-line is that within the context of the solutions of General Relativity which apply to a smooth expanding Universe, there is local conservation of energy. For any finite volume the energy lost by the photon gas is equal to the "PdV" work of the expansion. Stating that this is like Joule-Kelvin expansion is just an analogy.
There are of course many ways to think about this. In one picture you just imagine that the photons are redshifted by the expansion velocity. So if that helps you, then there's the explanation! However, this isn't really the case in an expanding Universe in General (rather than Special) Relativity. It's better to think of redshift as being caused by the changing scale factor, i.e. photons have longer observed wavelengths than they had when they were emitted because the Universe has been expanding while they travelled towards us.
It's true that normally you would like to think about the physical interactions which change the photon energies. But this isn't a normal everyday situation we're talking about here! The decrease in energy of the photons has nothing to do with interactions with matter, or virtual particles, or even the Doppler effect. It is really a consequence of General Relativity. Photons travelling through an expanding Universe lose energy on their journey between emitter and receiver, without interacting with anything.
(3^3*2^8*pi^3/a^7)^1/2*Qo*([1N]*G/c^4)^1/2*[mol]*NA/N^3)=2.7111K
The formula is only a conjecture! But:
There is a contribution from the free electron to the CMB and so to the
temperature. Tb=2.73K
Possible?
As well as from other "particles" (universe, electron, hydrogen atom
...)
(Qo is the Planck temperature and NA Avogadro's number. N=integer about
1e22 reflects the restmass of the electron.)
Submitted by manfred.geilhaupt"AT"fh-niederrhein.de 4/03
Good effort at CMB temperature numerology! However, I'm not sure this is dimensionally correct! And I think that dividing by 10^22 arbitrarily is against the rules! You can see my own attempts at this on my "Who Chose the Temperature?" page.
Since no one knows what the emission spectrum of the spontaneous creation of
sub-atomic particles in space would be, isn't it somewhat possible that
that's where the CMB comes from?
Submitted by Beanstalkr"AT"aol.com 3/03
I'm not sure what exactly you mean by "the spontaneous creation of sub-atomic particles in space", but if you're talking about a "Steady State/ Continuous Creation" sort of picture, then there are many reasons why this doesn't work. Not least of which is the need to explain the power spectrum of CMB anisotropies, which is a stunning confirmation of the standard hot early Universe picture.
But just as there's some truth in the Steady State model (since it's quite like inflation in many ways), there's something in what you say. In the standard cosmological picture the Universe used to be very much hotter and denser, and at early enough times was filled with particle/anti-particle pairs in equilibrium with the radiation. As the Universe cooled most of these annihilated, adding to the photon background. So you can say that most of the CMB photons had their origin in the spontaneous destruction of matter!
if the CMB is cooled by the expansion of the
universe, would it not be possible to measure it's temperature very
accurately and determine the rate of cooling? [abridged]
Submitted by thxgoon"AT"hotmail.com 4/03
In principle you could certainly do this. Let's imagine you measured the temperature of the CMB today and then waited 10 years and measured it again. Then of course it will be a bit cooler. But by how much? Well the CMB is redshifted by the expansion of the Universe, and the expansion rate is given by the Hubble constant. The Hubble constant is measured to be about 70 km/s/Mpc (kilometres per second per Megaparsec). The Hubble time (one over H0 in time units) is about 14 billion years. So in 10 years the CMB temperature will have changed by only about one part in a billion! That's pretty hard to measure, and so this method isn't going to be a way to estimate the expansion rate of the Universe.
If the CMB really exists, and
therefore the big bang is right and there is no god, why can't I just hold
my food (e.g. burritos) up in the air to cook them? Why aren't I getting
cooked as we speak.
Submitted by ilovecosmology"AT"hotmail.com 4/03
Cosmic microwaves fill the entire Universe. However they're extremely dilute by terrestrial standards. CMB radiation at 2.725 Kelvin corresponds to a flux of approximately 3 × 10-6 Watts per square metre. So the energy passing through a burrito (depending on how big the burrito is) must be around 10-7 Watts. On the other hand your microwave oven is an efficient way of converting hundreds of Watts of power into microwaves and directing them at your food. In addition microwave ovens tune their radiation to zap the molecules in your food, whereas the CMB covers a wide range of frequencies.
So you can't cook your food by holding it up in the CMB I'm afraid. I have no opinion on whether or not this proves the non-existence of God though!
Here's a good site which is like the microwave ovens version of this one: Microwave Ovens
Does CMB affect us today at all? If yes, how so?
Submitted by orem2"AT"TCNJ.EDU 4/03
No, it doesn't.
The CMB is completely useless. There are no direct applications that I know about, either beneficial or harmful.
Except of course that the anisotropies give us a way of determining exactly what sort of Universe we live in. One of the definitions of Civilisation is a society which places a value on certain things simply for their interest, rather than simply valuing utility. The CMB fits firmly into that category. We stufy it "merely" because understanding the entire Universe is pretty interesting!
If you could please tell me the frequency of the CMB.
I think there is a frequency give or take.
I'm not concerned with 1/100,000 deviations from average. I just want
the average.
Submitted by nikrubenstein"AT"hotmail.com 4/03
The CMB is "blackbody radiation" meaning that it is the sort of radition you get from a hot body which has settled down to be at a particular temperature. The energy is spread over a wide range of wavelengths, and the shape of this energy spectrum is described by a specific mathematical formula, which rises as a power of frequency and falls of exponentially at higher frequencies. The spectrum peaks around a frequency of 300GHz, but it's important to realise that the photons have a spread of frequencies around this value, and CMB photons can be detected from hundreds of MHz up to hundreds of GHz.
I got to wondering -- how bright would the night sky be if we could see the micr
owave background? Compared, say, to moonlight?
Submitted by res04047"AT"verizon.net 5/03
The answer depends on what units you use.
In terms of energy flux, the CMB is fairly similar to starlight within our Galaxy. So if you could see it, and your eyes detected energy flux, then it would be a uniform glow, with individual stars visible on top of it, and the Sun of course very much brighter.
If, on the other hand, you detected number of photons per unit time, then there are a lot more of those than from starlight, and so the CMB would dominate.
You could also easily imagine other sorts of detectors, which effectively work with different units, and would give different answers.
Who discovered the Cosmic Microwave background?
Submitted by asim_48"AT"yahoo.com 7/03
The Cosmic Microwave Background was discovered by Arno Penzias and Robert Wilson sometime in 1964 during studies they had been making using a sensitive microwave receiving system at Bell Labs. The results were published in 1965 and the pair received the Nobel Prize in Physics for their discovery in 1978.
Given that all of space expands at the
same rate...then spaces that are farther apart from one another expand away
from each other faster than spaces that are closer together ...
At what distance between points would space expand faster than the speed of
light? ...
Given the tiny amount of radiation energy that can theoretically emit from
black holes as virtual pairs of matter/antimatter as one part of the pair is
ejected past the event Horizon, and the other out into space ...
Could this show up as a constant background radiation that could be detected
from any portion of the sky? [abridged]
Submitted by jjrittenhouse"AT"hotmail.com 7/03
This is a very nice question! And a proper answer would, I fear, be quite long!
Let me say a few short things in response. Firstly, this is not a good explanation for the origin of the CMB in any model that I'm aware of. Seondly, parts of the Universe certainly can expand away from each other faster than the speed of light. And thirdly, your description <>does bear some striking similarities to a picture of how fluctuations are made during a period of inflation - you can kind of think of this as like a temperature that empty space has as a consequence of accelerating, then that temperature is manifested in fluctuations in the relevant fields at the inflationary epoch, which later become the fluctuations in density, which even later get imprinted at last scattering as the CMB anisotropies.
someone is asking me the order of magnitude of the number of real photons there
are per meter cube in the CMB. do you have the answer ready or can i work it
out from N = sigma T**4/chv where hv is the photon energy at the max of the
black body law for T about 3K ?
Submitted by 7lucas"AT"fundp.ac.be 8/03
You can get a rough answer from the formula that you wrote down, which will be off by some small factor (depending what you use for the average frequency). The correct answer simply comes from integrating over the blackbody function. The value obtained is about 411 CMB photons per cubic centimetre.
I am confused about the time and distance scales. The wavelength of the
CMD is about 1000 times longer than that emitted by a black body at
3,000 K. That part is clear. When I divide the age of the universe, 15
billion years (to use a round number) by the age at last-scattering
(300,000 years, the oft quoted number), I get a factor of 50,000. How
can the universe be 50,000 times older than it was at time of last
scattering, but only 1,000 times larger?
Submitted by pbwilson"AT"cox.net 8/03
I'm always happy to get questions which show that people have been thinking!
The answer to your quandary is that the stretching of lengths in the expanding Universe is not at the speed of light. In other words the evolution of the "scale factor" is not proportional to time. This is because of the matter in the Universe which causes deceleration (at least until fairly recent times when it appears the Dark Energy causes it to accelerate). It turns out that for most of the history between now and last scattering (when the CMB photons had their anisotropies imprinted) the scale factor evolved as roughly the 2/3 power of time. So the age ratio is expected to be about the scale factor ratio to the 3/2 power. That's much nearer to the numbers that you were stating. Although to do it properly one needs to solve the detailed equations, taking into account the accelerating phase etc.
The recent results from the WMAP probe included a very precise estimate of
the age of the Universe of 13.7 +/- 0.2 billion years. I have yet to find a
good explanation for how the age of the Universe is derived from the cosmic
background radiation patterns that were recorded by WMAP.
Submitted by djalbright"AT"generatortech.com 10/03
The WMAP satellite precisely measured theanisotropies over a wide range of angular scales. The way that the amplitudes of the anisotropies vary with angular scale can be fitted using cosmological models. The anisotropies are accurately fit using a model which has come to be known as the "standard cosmological model", with well determined values of its parameters. This model has close to flat geometry, and is dominated by dark energy, with about one third of the Universe's energy density in dark matter and about 4 per cent in ordinary matter. It is expanding at a particular rate, measured to an accuracy of better than 10 per cent, and it contains fluctuations of the sort produced in "inflationary" models of the very early Universe. The age of the Universe is a derived parameter within these models, but it turns out to be quite robust. If you restrict the range of models to just those which have exactly flat geometry, then you get the number you quoted. If you allow more freedom in the models, then you always get pretty much the same number, but with larger undertainty.
You write we see microwave radiation from the last scattering. Let me suppose
we are in the Milky Way and we are looking into the last scattering
surface A across the sky. "A" was the scattering object farther away 1 million
years after BB from the precursor of the Milky Way. 1 million years after the
BigBang Milky Way and "A" were X light years apart. At that moment radiation
came out of A towards Milky Way. Should it not have gone by by now? How can we
still see it?
Submitted by tierrasana33"AT"netscape.net 10/03
This is another example of the most asked question about the CMB! "How come the photons haven't already passed us?" has been answered on these pages several times before.
Let me add to this question that if we see "A" in CMB photons today, then it must be the right distance from us so that light would take basically the age of the Universe to travel between us. So the distance "X" is whatever it has to be for us to be receiving the early Universe photons today. For objects closer than this, we are seeing the CMB photons coming from behind them, also distance "X" away.
Right now it is at 2.73 Kelvin. Is it getting cooler?
Why do I ask? Because a Stanford PHD student described some work they
were doing with extremely low temperatures in the range of .5 Kelvin. He
claimed
that light sent into chambers at this temperature actually slowed down ...
can we look forward to a time in the (far distant) future when the CMB reaches
.5 Kelvin and the light between the galaxies actually slows down? [abridged]
Submitted by Michael.bechler"AT"Eclipsys.com 11/03
Yes, the CMB temperature is decreasing, but it takes a cosmologically long time for it to change by much. Basically it takes about the Hubble time to change by order unity, in other words you'd have to wait millions of years to see a change which is very noticeable.
I'm not familiar wth the slow-down effects you describe, but it surely must involve matter at fairly high densities. The speed of light in a vacuum is always, well, the speed of light! It's slower through material, and presumably these people have been investigating some effect where it slows further in matter approaching zero Kelvin. But in the average part of the Universe the density is so extremely low that all such effects will be negligible.
I am just wondering if CMB radiation could be used to determine the outline
of a blackhole? Does CMB radiation accumulate in huge quantities within the
vicinity of steep gravitational wells? In short, What is the effect of
Blackholes on the CMB radiation?
Submitted by Michael.bechler"AT"Eclipsys.com 11/03
This is a nice question!
The answer depends on whether you are asking a practical question, or a point of principle.
Practically speaking, the size of a black hole is so small, and they are so far away, that it will surely be next to impossible ever to measure any effects. It is made even harder by the fact that black holes are typically surrounded by accretion disks and other hot matter, which will have a big effect at the wavelengths you'd observe the CMB. In any case, there are way easier ways of detecting black holes!
The question of principle is slightly complicated. The effect will be mainly the gravitational lensing of the CMB by the black hole. But, since this just moves light around, and the CMB is pretty darn uniform, you wouldn't really see any difference! However, you would have an effect on the CMB anisotropies, and this is potentially measureable, just as the lensing effects of galaxy clusters are measureable in the near future (e.g. by the Planck satellite).
I was wondering if you knew how the spectrum Cosmic Microwave Background
supported the idea that the Universe expanded from it's inital hot state.
Submitted by sogus"AT"email.arizona.edu 11/03
Yes, I do know that!
But presumably you also would like me to tell you the answer, right?
The spectrum is such a good blackbody, that no one has a good idea how to make it except from a hot early phase of the Universe. The "Big Bang" model naturally makes a hot blackbody spectrum early on, and then it just gets redshifted to the present, retaining its shape, but cooling.
Somebody wrote that "most of the entropy of the universe is found in the
CMBR"..please explain further. What is Entropy by the way? Is it something
tangible, something that could be felt? A force? or a form of Energy?. Could
it counter Gravity and Time Reversal?
Submitted by fidel_mc"AT"hotmail.com 12/03
Good question!
The answer to the last part is "no". Entropy isn't nearly so mysterious. It's a quantity which describes the degree of disorder in a system. It is a quantity which cannot decrease for a closed system, and which is often constant if physical processes are very slow. For much of the history of the Universe the entropy per unit volume has been constant (provided you take account of the expansion of the Universe).
For a uniform "gas" of particles the amount of entropy can be calculated, and depends on the energy, pressure and temperature. For a gas of normal matter particles the entropy is proportional to the number of particles, N, multiplied by (m c2/kT), where m is the particle mass, c is the speed of light, k is Boltzmann's constant and T is the temperature. However, for a "gas" of photons (or other highly relativistic particles) the entropy is just proportional to N. Since there are a lot of CMB photons, then their entropy density dominates the entropy of the Universe.
When will the photons in this frequency range stop coming? In other
words will there ever be a time when no more CMB will be detected?
Submitted by retep29805"AT"netzero.com 12/03
As far as we know the CMB photons will keep coming forever!
However, they will continue to redshift, so that billions of years in the future most of them will be at longer wavelengths. Our far-distant descendents will presumably talk about the "Cosmic Radio Background"!
... I am having some
difficulty coming to terms with the notion that if the CMB is 15 billion
years out, that the big bang is only +300,000 years beyond it, in a
universe that is spacially very small...
So how is it that after 15 billion years we are being bombarded by big bang
radiation on all directions that should have by now accelerated past us,
since we are matter originating frrom the same event? [abridged]
Submitted by gterry"AT"au1.ibm.com 12/03
This is, I believe, another example of the "why haven't the CMB photons past us" question. I suggest reading the answers to other similar questions on this page, to see if that helps.
A short answer might be that you should think of the early Universe being very large. The CMB is simply the photons we see coming from a sphere around us of size equal to the light travel time in the age of the Universe (or minus that 300,000 years if you like). It might help to think about sound waves coming at you from a huge crowd of people who all shouted something at once - at any given time you hear the sound from a circle of people around you, with the radius being the sound travel distance since the time of the Big Shout.
Suppose I had the technical means to manuever myself so I was in the CMB
reference frame: Would I then be unable to decect any photons from the CMB?
Submitted by rich"AT"concordma.com 12/03
No. You would still detect the CMB, but it wouldn't have a dipole pattern. In other words, you'd get the same flux of photons in every direction. Since we're actually not in this frame, but moving at a few hundred kilometres per second with respect to it, we see about 0.1% more photons coming from one side of the sky (the side we're moving towards) and the same amplitude of deficit from the opposite side. This is what we call the "CMB dipole", and we can use it to measure our speed through space.
In case there's a confusion here, being "in the CMB reference frame" means nothing more nor less than being in the frame of reference in which there is no dipole. It doesn't mean moving at the speed of light along with the CMB photons. They are coming at us from all directions, and hence there is no frame of reference in which they are all at rest, even if we could travel at the speed of light.
If this radiation is mainly photons, whay is it still visible, rather than
having radiated out of sight, range and mind about 12 billion years ago?
Why is it still visible equally (with local variations) in all directions?
Submitted by Anjodon"AT"aol.com 01/04
Because it comes from the whole Universe! You can read several different answers to this (or essentially this) question elsewhere on this page.
Here's an analogy which might help. Imagine you are at a very important sports event, with an enormous crowd. So enormous in fact, that it takes a significant amount of time for sound to travel from one side of the crowd to the other. Let us say that you are somewhere in the middle of the crowd, so we don't have to worry about any edges etc. And imagine that at some particular instant there is a crucial point scored in the game. Everyone in the crowd lets out a loud shout at the same time (they are all watching the game with basically no time delay, since the speed of light is so fast, in case you were worried about that complication!). Now the question is, if you wait until, say, 13.5 seconds later, which people are you hearing the shout from?
The answer is that there is a circle of people around you, each located 13.5 "sound seconds" away from you (i.e. the distance sound travels in 13.5 seconds). The shouts from the closer people have passed you already. The shouts from the more distant people haven't reached you yet (this requires that the size of the crowd is several kilometres across, but never mind that!). If you wait a little bit longer you will be hearing the shout from the people who are 13.6 "sound seconds" away, etc. And if the crowd is big enough, then there will always be people sufficiently far away from you that you will be hearing their shouts at later and later times, from more and more distant circles centred on you. And some friend of yours located at some other position in the crowd will also hear the shout from a 13.5 "sound second" radius circle, but their circle will be different from yours.
The arrival of the CMB photons is exactly like this! Except that we live in a 3 dimensional Universe, rather than a 2 dimensional ceowd of people, and that the CMB photons travel at the speed of light, rather than the speed of sound (about 1,000,000 times slower). At some epoch in the early Universe, everywhere emitted CMB photons. About 13.5 billion years later we are now seeing the photons arriving at us from a sphere of radius 13.5 billion light years. 100 million years from now we'll be seeing the photons from parts of the Universe which are 13.6 billion light years away, etc. So the photons did not pass us all already, because they come from everywhere - just like shouts from a huge crowd of people.
Although CMB which we observe _locally_ have T=2.7,
CMB which was generated here 13 billon years ago
does _not_ have T=2.7 _in our frame of reference_.
It is flying some 13 billion light years away
from us now with v=c but it is still gamma rays!
So, E = (energy density) x (volume of sphere)
isn't ok - energy density is not constant
(again, _in our frame of reference_)
Do I miss something here?
[abridged]
Submitted by vda"AT"port.imtp.ilyichevsk.odessa.ua 01/04
The CMB photons are travelling at the speed of light in all directions, and hence the ones we see in some direction today came from over there a long time ago! But an observer in that location today will be seeing our CMB photons now. There has been a factor of 1000 in redshift between the CMB "last scattering surface" and today, and hence the photons have had their wavelengths shifted by that factor. That means that the photons leaving our last scattering surface were roughly 1 micron in wavelength at that epoch, and redshift to roughly 1 mm wavelength on their way to us. At that same original epoch our region of the Universe was full of roughly 1 micron photons too, and those photons propagated in every direction, and today are reaching distant observers (at the location of our last-scattering surface, but at today's epoch), who would detect them with a wavelength of about 1 mm too.
According to the expanding Universe picture, the Universe really was hotter in the past, with temperature simply inversely proportional to (1+redshift) [here I'm using redshift as a label for the time coordinate, higher redshifts mean earlier times].
If we want to estimate the total energy in the CMB in some volume at the present epoch, then we just take today's CMB density and multiply by volume. There's no redshifting in this calculation, since we're assuming we can do the calculation at a single time. Of course you could also ask about the integrated density along a coordinate where time changes proportional to distance (technically called the "light-cone"), but that would be a different question.
What is responsible for the "background" aspect of CMB. We speak of
"background noise" and understand it as composed of emitted and
reflected noise in an echoic environment. Since all the original
radiation that makes up the CMB was emitted over the relatively short
period of time of plasma coalescence about 380,000 years after the big
bang and streamed away in all directions at the speed of light, with all
the matter formed during the coalescence following behind at less than
the speed of light, how can we be bathed in this radiation today? What
is the mechanism by which it is reflected or refracted or absorbed and
re-emitted back to us today? What causes the echo?
Submitted by ogdennr"AT"laplaza.org 01/04
This is a somewhat more sophisticated version of the "why haven't the CMB photons passed us already" question, which I've answered many times before!
One thing to understand is that everywhere in the early Universe was emitting photons, not just some finite region. So there is no reflection, refraction or absorption - the photons simply stream straight towards us from wherever they originated. For us this is a spherical shell the distance away that light has travelled since the Universe went neutral about 400,000 years after the Big Bang.
[abridged]
I am wondering if you might be able to steer me in the direction
of some other very easily read information regarding the CMB, other than
your wonderful site of course.
Submitted by christopherrobincox"AT"peoplepc.com 02/04
Although obviously nothing compares to this site, you might want to check out one of these: Wayne Hu's introdution to the CMB; WMAP's overview of the CMB (and you could look at other things there too).
Is the CMB red-shifted, and if so, how much?
Submitted by JCEvans"AT"hntb.com 03/04
Yes, the CMB is very much redshifted!
The photons which make up the CMB were created in the hot early Universe, and have been decreasing in energy as the Universe expands and cools. We see the imprint of the anisotropies caused when they last interacted with matter at a redshift of around 1000. But the photons themselves were created when the Universe was hot enough to make lots of particle/anti-particle pairs, at redshifts of billions.
So you can think of the CMB photons being created as high energy gamma-rays when the Universe was billions of Kelvin. They've had their wavelengths stretched by a factor of something like a billion so that today they represent radiation with a temperature of only a few Kelvins.
hey, you guys dont no if thats the edge of the universe or that
the cosmic backround just isnt some big line around a huge group of galaxies
that we are inside kind of like the thing that gois around the solar
system , i bet there are a shit load of bubbled in mass amounts of
galaxies and star things that were in, i cant stand that you guyts
actually think that the universe is finte thats rediciluous. First of
all you can only say wat you telescopes can see and defently you guys
cant see infinte space so dont bother with that cbm stuff its ridicilous.
Submitted by blizzard710"AT"hotmail.com 03/04
That's an interesting point of view, which I thought I'd share with everyone in its entirety.
Actually my own hunch is that the Universe is infinite in size. Or at least half that big anyway.
I read somewhere that at the point of last
scattering, the CMB (although not in microwave form) was some 3000
degrees Kelvin. If this is so and it is now cooled to 2.73 degrees
Kelvin by the effects of universal expansion as the universe becomes
less dense, by my estimates of averages, in about 12 to 13 million years
the average temperature will reach zero Kelvin, or absolute zero and
will no longer be detectable. Is this a correct assumption?
Submitted by grojo"AT"elpn.com 04/04
The CMB is getting cooler all the time, in direct proportion to the expansion of the Universe. The more the Universe expands, the colder it will get. But it won't ever get to zero, although you can imagine it being arbitrarily small by going further and further into the future.
In a Universe dominated by Dark Energy (currently the best guess picture), the expansion is becoming exponential. The expansion rate (otherwise known as the Hubble parameter) becomes constant, and we're almost in that situation already. Assuming exponential expansion with the exponent measured in Hubble times (about 1010 years), then the sums are relatively easy to do. You need to wait about 10 billion years before the temperature has fallen to about 1 Kelvin. And in 12 million years the temperature has only changed by only about a thousandth of a Kelvin.
You state in some of your answers that we can only observe CMB photons
which could have traveled this far at the speed of light since the last
scattering; implying, to me anyway, that there would be something beyond
that horizon. If there is indeed something beyond that horizon
(possibly a lot), doesn't that mean that the universe is older than the
estimates, since those estimates must be based on what is observable?
If I'm not way off base with my reasoning and the models have not
already accounted for this, what are the implications of an older and
larger universe?
Submitted by JCEvans"AT"hntb.com 05/04
There is certainly stuff beyond the "last scattering surface", although we can't directly see back then using CMB anisotropies. But we can probe those distances (and times, since looking out further and further means looking back further and further in time) using other probes of the early Universe (such as details of the formation of the light elements).
However, the best picture we have of the Universe puts the present age at about 13.5 billion years, but the age at "last scattering" at only about 300,000 years. So there really isn't very much further back that you can go! In other words the photons reaching us from the "last scattering surface" left on their journey not very long after the Big Bang (at least by cosmological timescale standards!).
The actual "particle horizon" is the distance beyond which we can't see because there hasn't been enough time for light signals to reach us from there. That's about 13.5 billion light years away (or actually a bit further because of the expansion of the Universe in that time). And of course there's lots more Universe beyond that, it's just that we won't be able (even in principle) to find out about it till later!
I enjoyed your tutorial in ISM and the mechanics of how the CMB may have
been emitted. Could you give me a flavor (or a good reference) that goes
into the theory of how the photons of the CMB were cooled, from the
presumably very high temperatures at which their emissions occurred, down to
the 2.7 deg Kelvin level. I would like to be able to develop a notion for
the energy loss/frequency shifting mechanism such as adiabatic expansion,
tired light, or whatever caused the centroid of the CMB spectrum to decline
(or if my conception is wrong, to start off so cool at the time of
emission).
Submitted by WHeller"AT"fdic.gov 05/04
It's really very simple to understand the idea. It's just that the Universe is expanding, and as the photons travel through space they have their wavelengths stretched along with the expansion. So all the photons produced in the early Universe get shifted to longer wavelengths as time goes on. This means that their energies are reduced (since the energy of a photon is inversely proportional to its wavelength). And hence an initially hot spectrum of photons looks just like a lower and lower temperature spectrum as the expanding Universe gets older. If the Universe was contracting, then the CMB would be heating up and would eventually cook us (but don't worry, that won't happen!).
the wave-length of CBR is constantly changing as the universe expands because
the CBR is expanding along with the universe. If we could note a change in the
wave-length of CBR, it would give insight into the rate of the expansion of
the universe. Is this right? If the rate of the expansion of the universe
is known based on Hubble's constant, how long should it be before we notice
a change in the wave-length of CBR?
Submitted by timothy.patry"AT"us.army.mil 07/04
Good question!
You're right that the CMB photons are being stretched all the time. Or another way to think about it is that the blackbody radiation is cooling, and so its temperature is decreasing. Unfortunately it takes a timescale of order the age of the Universe for the temperature to change by a substantial amount! You'd need to measure the temperature to about 1 part in 108 in order to see the difference over a human lifetime!
I would like to know if anyone has
looked for coherent electromagnetic radiation in
the radio frequency range - could
there be another type of CMB out there?
Submitted by alistair"AT"goforit64.fsnet.co.uk 07/04
I confess that I'm not really sure what this question means, sorry!
By "coherent electromagnetic radiation" I normally think about lasers and masers. Well you can certainly get such radiation in the microwave part of the spectrum, and there are even astronomical sources known to be masing. However, the CMB radiation comes from everywhere, and I'm not even sure how to talk about the idea of "coherent" waves coming from all around me. But I think the answer is that if the radiation was "laser-like" we would have known about it in about 1965. There could of course be "masing spots" coming at us from the microwave sky, but the only such sources we know about are very much local and not "cosmic background".
Our detectors see a steady stream of CMB photons raining down from all
directions. As I understand
it, these photons originated at a specific time (BB + 380kyr) at every point
in the expanding
universe. Like most people, I am tempted to ask where the photons we see NOW
have been all this
time. However, am I right in saying that we are seeing the event run over and
over, like a
continuous film loop in the sky? Did the photons we see at each second start
off in a shell one light-second further away?
Submitted by Sturandall"AT"btinternet.com 07/04
I think you're more or less on the right track here (although I'm not sure about the continuous film loop analogy).
Try thinking about the photons arriving here and now from all directions. A second ago they formed a spherical shell around us, one light second in radius, and all moving radially inwards. A second before that, they were in a shell 2 light seconds in radius, etc. So the photons arriving at us now used to be somewhere else and on their way here.
Once particular photons have passed us they'll never be back! But there's no need to worry, because there are always plenty more photons out there!
Last night I read ALL of your email answers to CMB questions.
I admire your skill and find your patience wonderful (even not minding
answering the same question more than once!)
The bits I have the most trouble with are
(1) "Space" - what IS it. How can it "expand". How can an "infinite"
thing expand?
(2) The radiation is downshifted "because the space it is in has got
bigger" - by which you seem to mean "THINGS in it have now got more
space between them"
Submitted by johnfree"AT"uk2.net 09/04
Firstly, let me congratulate you on reading all these FAQs - that must have taken quite a while! Then let me try to give some kind of answer to your questions.
For the first question, "What is Space?", that's really a very good one! There are probably several joke answers to this, and several clever-sounding philosophical answers that don't really help. The dictionary definition "that which makes extended objects conceivable and possible" is probably about the best you can do for an answer. Within General Relativity (our currently best idea for a theory of gravity and motion) you can think of space as a "field", in a similar way (although with different properties) to the electromagnetic field.
In a sense the reason that it can expand is because there are solutions of General Relativity in which it can expand! But that's really not a very satisfying answer. Some people like to think of the expansion of the Universe as the space between galaxies expanding, while other people like to think of it simply as the distances between galaxies getting bigger, and a third set of people can't see why the first two ideas are different!
Having failed to give you much of an answer for your first question, let me do an equally poor job on the second one! The radiation in the Universe is in a sense doing the "work" which makes the Universe expand. And objects in the Universe don't absorb all the radiation because there hasn't been enough time for that to happen. In other words the typical CMB photon can travel all the way to us from the early Universe without much chance of interacting with anything along the way. So the CMB photons continue to stretch as the Universe expands, and they don't get absorbed by the matter.
You wrote:
"The CMB photons we see today are coming to us from way across the Universe
(about 13 billion light years away, if for example the Universe is 13 billion
years old)."
Why do 13 billion yr old photons designate points in space 13 bly away?
Submitted by dreeves"AT"brandeis.edu 09/04
The photons don't choose us, we choose the photons!
The photons that we are observing right now must have come from a sphere around us which is (say) 13 billion light years in radius. There's nothing otherwise special about those photons. They "chose" to be observed by us now only in the sense that their neighbouring photons back then "chose" to be observed today by cosmic neighbours of ours.
Is it true then that the CMB wasn't always microwave? That it is microwave
now based on the degree of cooling the universe is at right now? That
previously the cosmic background (CB) was on the higher-energy end of the
spectrum, and that as time goes on and the universe continues to cool, the
CB will continue to migrate toward lower and lower energy regions of the EM
spectrum?
Submitted by hkc1"AT"tof-one.org 09/04
Yes! Yes! And yes!
(Those sure were easy questions to answer!)
Isn't it possible then that the CMB could essentially be a
manifestation of the "fabric" of space-time...the thing for which Einstein
mathematically "nails down" its behavior in general relativity? And if so,
is it not possible that certain fundamental "constants" that govern the
physics of our day could actually be slowly changing as the fabric of the
universe expands and cools?
Submitted by hkc1"AT"tof-one.org 11/04
No!
But actually these are harder questions to answer!
For the first question, I think you are asking whether the CMB can be the Dark Energy, or cosmological constant introduced by Einstein. That's not possible, since they behave in fundamentally different ways. The CMB is radiation, while the Dark Energy is "vacuum energy" (or something only subtly different from that in some models). These two substances have quite different relationships between their energy density and their pressure, which leads them to evolve quite differently in an expanding universe.
As for fundamental constants changing in time, this is certainly possible in principle. However, you have to be careful about what you mean by a constant, which is related to measurement and units. It's only really meaningful to talk about the variation of "dimensionless" quantities, or in other words ratios of fundamental constants. There are some claims that such a variation may have been seen by looking at spectral lines over a wide range of redshifts, but most cosmologists remain skeptical of these results, since the systematic uncertainties are hard to assess. There are also models in which the constants might vary in an observable way, but again, most cosmologists don't regard any of these models as particularly appealing right now.
I do not quite understand why the CMB is a prefect blackbody. How could
the... freeze out at recombination/decoupling not release a... distorted
signal since the Last Scattering Surface would still have a distribution
of photon energies even at 3000 K? Also, wouldn't this also result in H
absorption lines?
Submitted by lucero"AT"chem.ucla.edu 11/04
A "blackbody" is just the equilibrium distribution of photon energies for a given temperature. Back at the time of recombination the CMB was in vey good thermal equilbrium and therefore was very close to having a blackbody spectrum. This means that there was indeed a distribution of photon energies - just the right distribution to be a blackbody spectrum.
As for the second part of your question: there wouldn't be hydrogen absorption lines, since the amount of absorption is quite negligible. However, there will be a set of emission lines coming from all the hydrogen that went from ionized to neutral at the recombination epoch. These lines are incredibly weak however - remember that there's about a billion CMB photons for every atom in the Universe, and so there's about a billion times less of these line photons than general CMB photons.
I guess I just don't understand why it is still all around when there is =
no source for the radiation. Doesn't it travel?
Submitted by lucero"AT"chem.ucla.edu 11/04
There certainly is a source for the CMB. It's the entire early Universe! And the photons travel at the speed of light from where they were made until they reach us, about 13 billion years later. The Universe is much bigger than 13 billion light years, and all of it made these photons.
"According to the Big Bang theory, the cosmic microwave background was =
created when energetic photons ionized the neutral hydrogen atoms that =
originally filled the universe." Is this statement sensible? Why or why not
Submitted by claudias"AT"rice.edu 11/04
This sounds like a homework exercise in some class! I think I've waited long enough to answer it though, that it's unlikely to help get anyone a better grade!
My answer would be that the statement is quite confused. The CMB photons were created in the very early Universe, and in fact you can think of their origin as being from the annihilation of particle/anti-particle pairs at very high energy as the Universe cooled. I don't know what the "energetic photons" in the statement are, if they aren't the CMB photons themselves. And in the Big Bang picture, nothing ionized the neutral hydrogen atoms to be begin with, since the Universe started off hot and ionized. So neutral hydrogen atoms did not fill the Universe at early times.
Please help.
I am doing a project on the big bang thoery and i would like to get
some information out of you.
Please tell me some cool information about cosmic microwave
background; i'll take anything. Information that you did not put on
your site.
Submitted by inherprime"AT"gmail.com 11/04
How do I respond to such a request? Isn't the information on this page more than enough?
The CMB is one of the genuinely coolest things in the entire Universe, being only about 2.735 degrees above absolute zero!
I am an amateur,so sorry for these silly questions: how can we be so sure
that CMB cames from 13 billions years ? And how do we know that CMB has not
changed since then for some unknown reason?
Submitted by FeliciL"AT"ANCE.IT 10/04
Don't apologise for being an amateur - I'm an amateur at many things myself!
We can be pretty sure that the CMB photons come from the early Universe for several reasons. The strongest ones are probably: (1) the "blackbody" spectrum, which can only have come from something in extremely good thermal equilibrium, and the only candidate is a hot early phase in the Universe; (2) the near isotropy of the radiation, pointing to an extremely distant origin; (3) the pattern of anisotropies, which can only be understood as arising from density variations at the time the Universe cooled enough to go from being a plasma to being neutral.
The CMB certainly has changed since it was formed in the early Universe - it's much colder today than it was then!
I have commenced a research paper on dark matter. The cosmic microwave
background seems to be intertwined into dark matter's existence. Can
you brief me on this? Also, can the inferred invisible particles in the
universe exert the enormous gravitational force necessary to keep
stellar galaxies together and in rotation? Or do you think another
force is necessary, and gravity is perhaps a "subordinate" in this
picture (or perhaps even negligible since it is a weak force)?
Submitted by tyro7djm"AT"vfemail.net 10/04
I have to remind readers of this page that I always delay when answering questions which seem to be asking me to do homework for someone!
There are many connections between the CMB and dark matter. Certainly one cannot understand the pattern of anisotropies properly without invoking dark matter. So a recent piece of evidence in favour of cold dark (i.e. "non-baryonic") matter is that there's a direct constraint on the amounts of baryonic and non-baryonic matter coming from the CMB anisotropies. You need about 5 or 6 times as much "cold dark matter" as regular matter.
Although there are several ideas for non-gravitational forces which might be operating on large scales, there is currently no theory which can explain a large suite of different sorts of cosmological information without having this dark matter.
Where does it come from? What are the sources? What are the Proportions in
our atmosphere of alpha, beta and gamma radiation. A pie chart would be
ideal.
Submitted by putnamrm"AT"shrewsbury.org.uk 11/04
This looks like another Cosmic Ray question to me!
The CMB is composed entirely of photons, i.e. very, very low energy gamma-rays, with no alpha or beta radiation at all. So the pie chart would be a complete pie labelled "photons".
was there any one study that
stands out in particular as the "discovery" of the microwave background? Who
has doen the most influential work in this field, and where could I find
articles they have published on this topic?Submitted by Zachary.A.Mayer"AT"Dartmouth.EDU 11/04
Although the early history of the CMB has several twists and turns, it was definitively discovered by Penzias and Wilson, whose discovery paper was published in 1965. You can find out a lot more about them by doing an internet search on "penzias and wilson" for example.
Lots of people have worked on the CMB since those days, both on the experimental and theoretical sides. Robert Dicke did some of the most important early experimental work, with Dave Wilkinson carrying this on for the rest of his career, and eventually having the WMAP satellite named in his honour. There have been lots of other great experimentalists, but since they're mostly still alive, I don't want to get into a prioritisation contest! Modesty also forbids me to name the most important theorists!
I would like to know if
the CMB could be responsible for keeping the electron spinning around the
nucleus of all atoms in living organisms, a little bit as microwave
radiations accelerate electrons in food.
Submitted by slemaire"AT"uottawa.ca 01/05
No. The intensity of the CMB is so low (compared to pretty much anything you can think of that you could generate yourself, microwave or otherwise) that it must have negligible effect on human beings and other living organisms.
Electrons orbit atomic nuclei essentially through the electrostatic force between opposite charges, although you need to throw in some quantum mechanics to understand what's going on in any detail.
If background radiation comes from all directions, why would you need a
horn and waveguide to detect the signal ? Wouldn't a super high gain
transistor detector circuit receive a more accurate signal if it were
direct coupled to a tuned full wave antennae ?
Submitted by king_james_1"AT"verizon.net 01/05
You obviously know a lot more about radio technology than I do!
However, I think you may have things a little confused here. The CMB is very "broadband", since it has a blackbody spectrum, and can be detected over about 4 decades in frequency. So you don't want to tune to any particular frequency (and in fact will lose detection efficiency in a very narrow band detector). The second thing is that the CMB comes from all directions, and so you can point your antenna anywhere, except that you'd like to collect a lot of radiation, and so your antenna has a dish or horn of some sort. But you're certainly right that a high gain transistor might make a good detector.
In your answer to the "How come we can tell what motion we have with respect
to the CMB?" question, there is one more point that could be mentioned. In an
expanding universe, two distant objects that are each at rest with respect to
the CMB will typically be in motion relative to each other, right?
Submitted by danieldarre"AT"yahoo.com.ar 01/05
Correct! The 2 observers will be in the "Hubble flow", i.e. they will be receding from each other along with the expanding Universe. If they're at rest with respect to the CMB, then they won't have any "peculiar velocity" relative to this "Hubble flow".
How would it be seen an object (mass) staying at rest respect to that absolute
(expanding) rest frame, from a reference system that is traveling with respect
to it at the speed of the earth, from the point of view of time dilation due
to relativistic effects.?
Submitted by danieldarre"AT"yahoo.com.ar 01/05
A distant object which is in the "Hubble flow" will be seen to have a redshift as a result of the expansion. You can think of this as simply due to the expansion of the wavelengths of the photons as they travel to you from the distant object, or you can think of it as like the Doppler effect of a velocity (although this really only works for fairly modest speeds, otherwise you have to be a lot more careful).
An object observed at some high redshift will also be observed to have a "time dilation", i.e. the timescales in the frame of an observer there will appear to be longer than your time by a factor of (1+z), where z is the usual sumbol for redshift. The light curves of distant supernovae show just such an effect.
An object which has its own "peculiar velocity" relative to the expansion will be seen to have redshifted wavelengths in its spectrum and a time dilation on its clocks which is an appropriate combination of the 2 effects. Explicitly you have (1+ztotal) = (1+zHubble)×(1+zpeculiar).
I am curious why nobody is curious about the spread of the photons
frequensys when thay where created in the big bang. Why this background
radiation, 1mm to one tenth of a meter?
[abridged]
When energy make photons in the big bang it has all of the spectrum to
chose in why this narrow band?
Submitted by sim"AT"telia.com 01/05
The early Universe is in very good "thermal equilibrium", which means that there are photons with a wide range of energies, with the spread dependent on the temperature that the Universe has. This spread in frequencies is referred to as a "blackbody spectrum", which is a particular mathematical expression determining the precise number of photons at each frequency. This blackbody radiation was once very hot, but cooled as the Universe expanded. All of the frequencies of the photons get redshifted, so that the blackbody shape stays the same, except that it changes to be a blackbody of lower and lower temperature.
So the CMB photons are not from a narrow range of frequencies at all. You may have got that impression from reading about specific measurements of the CMB, which use an instrument which only covers a narrow range of frequencies. But the CMB itself is quite a broad frequency function. Given that the temperature today is about 3 Kelvin, the peak frequency is somewhere around 100GHz. The number of photons has fallen off so much at frequencies corresponding to very long wavelength radio or very short wavelength visible photons that you couldn't hope to detect the CMB in those wavebands. However, you can detect the CMB over a pretty wide range, from a little below 1GHz to almost 1000GHz.
I am also at a loss at understanding the concept of photons cooling after
they have been generated. Is there any URL you can refer me to
explaining/describing how emitted radiation might cool in an expanding
universe (ie. to the current level of around 2.7K). I'm comfortable with
the kinetic cooling of gasses as they expand, but the cooling or reducing
frequency of propagating electromagnetic radiation is has challenged my
knowledge of physics somewhat.
Submitted by jcoldrey"AT"bigpond.net.au 01/05
OK, here's an attempt to answer this question from the point of view of someone who has some basic grounding in physics (say at the introductory college level).
You can think of the CMB as being a "radiation gas", i.e. a collection of photons behaves like a gas of atoms, with the big difference being the relative amount of pressure per unit density. Another way of saying this is that radiation behaves like a gas with a "gamma" (ratio of specific heats factor) which is 4/3, rather than the 5/3 that you have for an "ideal gas" of regular atoms.
This radiation gas will cool if you expand it, just like any other gas, except that the amount of cooling is different because of the different "gamma" factor. So if you want to think of the CMB in this way, then you can understand why it cools, just like a regular gas would. And the amount of cooling explained in this way turns out to be just the same as saying that the energy of the photons goes down as the wavelength of the photons gets stretched along with the expansion.
You have said:
"BTW, a fun fact: the coordinate system defined by the CMB is not inertial!"
May I know why?
Submitted by bhalerao"AT"nuclth.theory.tifr.res.in 01/05
I don't recall ever saying that!
But it's true that you can't fully consistently deal with frames in an expanding Universe within the context of special relativity, but need the full General Relativity theory. Although space appears to be close to flat, it's still not "Euclidean", because it's expanding. So one needs to be careful to set up a framework in which you can talk about a "peculiar velocity" relative to a set of hypothetical observers who are moving along with the expansion.
If the CMB was emitted some billions of years ago and travels at the
speed of light and we have "traveled" from that same point in space time
at lots less than the speed light, how is it that we can see it?
Wouldn't it be long "past" us? And how come we can see it in all
directions? [FOLLOW UP]: I found the answer in your email answers section.
I think I understand it but ... It is not an easy concept.
Submitted by tjax"AT"comcast.net 02/05
I like it when people answer their own questions before I get to it!
You're correct that this is not an easy concept. It may be that different people have different ways of picturing this. But my feeling is that for most people the main difficulty is that they are starting off with the wrong image, and then it's extremely hard to get it right after that. So it might help to totally erase the idea that the Universe used to be smaller (or that it started with a singularity or very small region embedded in some larger space) - instead start with the idea that it has always been infinite in size (even if everything used to be closer together in the past) and take it from there.
Is it logical to maintain (as we appear to do) that the space-time
expansion can affect minute entities like photons by stretching their
wavelengths and yet the same space-time, the curvature of which is
gravity, also holds together galaxies and galactic clusters stretching
only voids between them? Whence this discrimination?
Submitted by adarnay"AT"sbcglobal.net 03/05
This is a very good question! The answer is "yes, it is logical to maintain this", and the best way to think about the distinction is to consider what the space is doing.
As you point out, galaxies aren't expnading along with the distances between galaxies, because they are self-gravitating objects which no longer care about what the rest of the Universe is doing. Photons within galaxies don't have their wavelengths stretched either, since the space within the galaxies isn't expanding (although there could of course be effects arising from velocity or gravitational differences between emitter and observer).
The wavelengths of the photons get stretched because the space is expanding as the photons travel through that space. So it's really only because the photons are propagating through the expanding space that their wavelengths are stretched - or at least that's one way to think about it which I think is helpful to resolve your dilemma.
[abridged]
Specifically, as a photon travels through inflating space and its
wavelength gets stretched, where does the lost energy and momentum go?
Submitted by WHeller"AT"fdic.gov 03/05
The energy goes into the expansion of the Universe!
Could you be a bit more specific? What is the coupling mechanism by
which the universe (or some of its mass or energy) picks up the lost
energy of the cooled photons?
Submitted by WHeller"AT"fdic.gov 04/05
There's no "coupling mechanism". It's simply thermodynamics. Per unit volume of the Universe, the energy lost due to the expansion is equal to the "work done". This has to be true, since all cosmological models are calculated within the context of General Relativity, a theory which manifestly conserves energy.
My major is French Litterature so I am not so familiar with science. But I
guess I understood the relation between CMB, Big Bang and current
universe's temperature being 3K.
However, I don't exactly get the cosmologic? concept or the mental picture
I should be having while thinking about the whole universe. If CMB and our
universe is like our microwave oven, how could we ever measure the CMB from
the past universe - temperature of which should be way higher?
Submitted by doyon1004"AT"hotmail.com 03/05
Thanks for your question. I'm trying to guess what picture you might have in your head in order to try to figure out what sort of answer would be most helpful to you. So I apologise in advance if I get this totally wrong!
You should really s start by removing any idea that the Universe is like a microwave oven! I see nothing very useful in that analogy. But nevertheless we could continue with it, provided you're prepared to think of it as an immense and expanding oven, whose walls are extremely far away! You have to imagine that at some early time the oven was turned on and filled with photons everywhere (actually photons of such high energy that they were gamma-rays rather than microwaves). Then the oven is made to expand (but was always huge in size remember!), so that the distances between the photons get stretched, and the wavelengths of the photons also get stretched along with the space.
Today we live at the time when the wavelengths have stretched so much (compared with the time the oven was turned on) that they have become microwave photons. So we observe microwave photons reaching us from the very distant Universe, and emitted at a time when the Universe was very hot. We can see these photons coming to us from all directions, defining a sphere around us which has radius equal to the light travel distance in the time since the oven was turned on. And provided the walls of this oven are considerably further away than this distance of the "observable part of the oven", then to all intents and purposes we can consider the oven to be infinite in size.
And provided you can picture such a microwave oven, then you can do away with the oven entirely, and you're left with a pretty accurate picture of the Universe!
[abridged]
Im stuck on the problem of light aberration, associated both with
the measurements of positions of the distant star (object) you give as an
example, and the measurement of earths speed with respect to the CMB. It all
comes in the end to the resolution of a right triangle, one of its normal sides been v and one of the others being c. But Ive found few explanations for free
on the web, seemingly at odds between each other.
I was trying to elaborate a consistent word document with a concrete question.
But I think it better, as Cosmology is your field, if you are kind to put me in
c ontact with some information where I can figure out the problem by myself
first.
Submitted by danieldarre"AT"yahoo.com.ar 03/05
I find that "Eric Weisstein's World of Physics" is a good place to start for basic physics principles. You can find a description of stellar aberration here
I noticed that in your response to an earlier question (11/98) to the
wavelength and frequency of CBR your state that CBR is approx 150GHz @ 3K.
How does this relate to Smoot's statement in his book that CBM had an approx
wavelength of 7.35cm IE approx 4080 MHz? Is not this the frequency detected
by Penzias and Wilson?
Submitted by ed.hudson"AT"equilinx.com.au 04/05
The CMB radiation is "blackbody", meaning it has a broad spectrum of wavelengths with a precise mathematical shape consistent with a single temperature (2.725 Kelvin in this case).
The peak of the CMB spectrum (in appropriate units) is at about 150 GHz. However, you can detect it over a wide range of frequencies (or wavelengths). Penzias & Wilson first detected it at quite low frequency, namely 4080 MHz, which corresponds to 7.35 cm wavelength. But that is in fact nowhere near where the spectrum peaks. Anisotropy experiments typically work at frequencies much closer to the peak, since the "foregounds" are much less of a problem there.
In your list of FAQs about CMB, you state that the ratio of photons to
ordinary matter in the primordial universe was fixed when the energy of
those photons fell below that required for pair production. Presumably
this would also have determined the initial temperature of the photon
"gas" in thermal equilibrium during subsequent cosmic expansion. I
would assume that the lightest (stable?) particles that could be created
were electron/ positron pairs, giving an initial temperature for the CMB
of about 1.04 MeV or 1.2 x 10[10] K. But, if recent findings of neutrino
oscillations are correct, these particles also have a (very small) rest
mass. If neutrino/ antineutrino pair production were the threshold
process, it would correspond to a much lower initial CMB temperature.
Am I way off base or is there a grain of substance in my reasoning?
Submitted by skylar"AT"nednet.net 05/05
There is some grain of truth in what you are saying, but also some confusion I think.
There's no "initial" temperature in the expanding Universe. The CMB temperature continues to climb as you push back to earlier and earlier times (until eventually you reach energies where you don't know how to do physics any more!).
Neutrinos interact through the weak nuclear force, and this stops ("freezes out") just about the same time as the electron-positron pairs annihilate - actually a little before. So the neutrinos were already evolving separately from the photons by the time of the last particle annihilations. There should be a background of cosmic neutrinos, with roughly the same density today as the photons. However, because they decoupled from the photons before electron-positron annihilation, then it is predicted that the neutrino background today should have a slightly lower temperature (since the CMB got boosted by the annihilation photons, while the neutrinos carried on having the "old" temperature).
The predicted neutrino temperature is about 1.9 Kelvin. So if anyone can figure out how to measure the background of incredibly low energy neutrinos then we could test this! But remember that high energy neutrinos from the Sun are pretty darn hard to detect, and they have about a billion times the energy.
Redshift has three causes: the rapid recession of an object emitting light,
an extremely massive and dense object emitting light and the streching of
spacetime due to the expansion of the Universe. How does General Relativity
show that the CMB is about 46 Gly (giga light years) away and recedeing at
about 50c?
Submitted by emission_nebula"AT"excite.com 05/05
I'm not aware that all of these statements are true.
General Relativity tells us, in a sense, that all the causes of redshift that you mention are really the same - it's just a question of point of view. For nearby objects it's easiest to think of the expansion of the Universe in terms of the speed of recession of galaxies relative to each other. But for more distant things (where the speed would approach the speed of light) it's much more helpful to think of the redshift as coming from the stretching of distances between objects.
The best estimates for the age of the Universe are between 13 and 14 billion years. We also know that the Universe is expanding, and that the expansion rate has not been constant - in fact it was decelerating for a while, but is now accelerating. So the Universe is "bigger" now than it was when the photons (that we're observing today) left on their journey. For the current best model, taking into account the decelerating and accelerating phases, we get a distance out to the "CMB photosphere" of around 14 Gpc (Giga-parsecs, in astronomer units), which is indeed close to 46 GLY (billion Light Years).
For the speed of recession, you need to be careful to define precisely what you mean. I'm not sure what the "50c" is supposed to refer to. If the CMB was receeding from us at many times the speed of light, then certainly the photons wouldn't be reaching us at all!
i read "Misconceptions About the Big Bang", Scientific American, March 2005.
the authors state that more distant galaxies do excede the lightspeed, c. the
authors also state that we can see galaxies that are receding faster than c.
http://www.astro.ucla.edu/~wright/cosmolog.htm defines cosmic redshift as z =
e^(v/c) - 1 so the speed of any galaxy whose a redshift is 1.7 equals c, and
the speed of any galaxy whose redshift is greater than 1.7 excedes c.
Submitted by emission_nebula"AT"excite.com 11/05
The problem with speed is that it's the change of distance per unit time, and in relativity we know that both distance and time are observer-dependent quantitities. So you have to be very careful about precisely what distance and time coordinate you are talking about before you discuss the speed. There are several ambiguities, for example whose time coordinate do I use? when do I measure the distance? do I divide out by the expansion of the Universe? am I only thinking about things along the "past light cone"?
I read that article by David and Lineweaver, and I'm a great fan of Ned Wright's web-pages. I know all of these people, have respect for their abilities to explain things clearly, and wouldn't argue with them for an instant!
Nevertheless, I think it's conceptually much easier to just think of expansion in the Universe as a stretching of the space in which everything lives, and to expunge all thoughts of speed from the picture! It is certainly true that in some coordinates there are objects which are currently moving away from us faster than the speed of light. But that's not important! Nothing is actually whizzing past anything else at v>c. The apparent speed of distant objects comes from the overall stretching of distances, a concept which is neither in our every day Euclidean or Galilean view of space-time, nor even in special relativity. So trying to think about this "inside the box" of Galilean or special relativity isn't very useful.
I have a question? what caused cosmic microwave background? Please help!
Submitted by bupshaw"AT"uabmc.edu 06/05
It's the left-over radiation from the hot early Universe. There's a lot more information about it on this page!
[abridged] Matter is easily observed to cool to 2.7K,
and radiate at this temperature in a state of thermodynamic
equilibrium with the 2.7K CMB - proof that matter is a
source of the 2.7K CMB, and of long-sought "cold, dark
matter".
Submitted by johnson"AT"cogeco.ca 07/05
Certainly there's a connection between matter and radiation. The "hot big bang" view is that the CMB was indeed emitted by matter, but this was at very early times, when the temperature was very high (and the timescale for approaching a blackbody spectrum was very short). The CMB is observed today at 2.725 Kelvin because the Universe has been expanding.
Local sources of microwave photons just can't make such a precise blackbody spectrum. Besides that, we know about the temperature state of lots of regions of the local Universe, and typically the luminous matter is much much hotter than the CMB temperature. Although we don't know very much about the "cold dark matter", we do know 2 basic facts about it: (1) how much there is, about 25-30% of the total density of the Universe; and (2) that it has very weak interactions, with essentially no coupling to the electromagnetic force. This second fact means that the CDM does not interact with photons of any wavelength, and hence has no direct relationship to the CMB.
i am a final year electronics and communications
engineering student and will be working on my final
year project soon. I sport interest in the field of
cosmic background radiation and was wondering if there
was any possibility of doing a project related to the
field.i have been searching for material on and off
the internet,but they seem too large scale for a final
year project. Could you please inform me of the
feasibility of a project in this field.
Submitted by thahirshahnaz"AT"yahoo.co.in 09/05
It is certainly possible to carry out a project looking at effects in real CMB data. Data in a fairly raw form are available from the Wilkinson Microwave Anisotropy Probe (WMAP) experiment, as well as several others. Precisely what you do with these data is entirely up to your imagination!
If you want to get involved in something more hardware related, then it ought to be possible to build a CMB detector using off-the-shelf components. The CMB is actually fairly bright at cm wavelengths (unlike the anisotropies, which are about a factor of 100,000 times fainter!). You'd need some directionality (so you can tell the CMB from the foreground signal which peaks in the Galactic Plane), and you have to have some method of measuring an "absolute" temperature.
Good luck!
At
http://www.livejournal.com/users/vsevcosmos/11322498.html?thread=1410#t1410
a person claiming to be high school teacher George Peterson claims that "the
microwave contributions from the moving wall of the Local Bubble,
interacting with the interstellar medium, would produce a low temperature
microwave hiss. This radiation, near blackbody, smooth to approximately one
part in a hundred thousand, would have as its' temperature, a value of the
order of ~ 2.79 Kelvins" and that "The entire CMB can be derived from this
sole local effect."
Submitted by vorleons"AT"hotmail.com 09/05
There are several things wrong with this.
First of all, one has to ask oneself, "why would someone be so desperate to find an alternative explanation, when the Big Bang picture naturally makes the CMB"?
When the CMB was first discovered the simplest explanation was that it was redshifted radiation from the hot early Universe. However, the constraints on the isotropy of the CMB and on its spectral shape were not very tight until well into the 1970s. So for about a decade it was perfectly reasonable to consider local alternatives for making this microwave background.
But now, 30 years later, we know that the CMB is isotropic to about 1 part in 100,000 (other than the dipole) and that it has about the best blackbody spectrum which has ever been measured.
Any idea which anyone has come up with to make the CMB locally has trouble with both of these properties. If you make the CMB from some radiating substance, then it will typically have either absorption or emission features, since complete thermal equilibrium is extremely hard to establish in today's Universe. And to make it isotropic you have to imagine some picture where, for example, there's a perfect sphere of material around us (with no variations at more than about the 1 part in 100,000 level) and that we're exactly in the centre and that whatever it's made of has no significant effect on our ability to see distant objects through it. Passing all of these stringent criteria is a very tall order!
But it's even worse than that, since we've now measured the anisotropies with very high precision, and they're exactly what we expect from the early low amplitude density variations of the sort we need to have grown all of the structure in the Universe.
So if you want to make the CMB through some local source, you also have to contrive for that source to give precisely the power spectrum of anisotropies which matches what the standard cosmological picture gives you!
As you already told, the theory of special relativity is based
on the argument that there is no preferred rest system.
Do you think that Einstein would have developed his
theory if CMB were already discovered before 1905?
For example: G. Smoot regarded the CMB explicitly as a
'new ether'. Would Einstein have denied the 'ether' if he
had know about CMB?
Submitted by helmuthansen"AT"t-online.de 09/05
This is an intriguing question. But I'm not sure I'd like to second-guess Einstein! (especially in this centenary year of Special Relativity etc.)
Since the existence of the CMB "rest frame" does not violate the Principle of Relativity (which was a central idea that Einstein came up with to tackle relativity), one would presume that the existence of the CMB would not have thrown him off track.
Incidentally, did George Smoot really say that the CMB is the new ether?!
I just heard on
the radio that CBR is moving in a direction which is different to what
you mentioned on your site. Which direction is it and why do you think
this is?
Submitted by jimidybobidybo"AT"hotmail.com 09/05
First of all, the CMB isn't moving in any particular direction, since all the photons that make it up are travelling at the speed of light from where they last scattered in the early Universe to wherever they're going, in every direction!
But we are moving through the sea of CMB photons, which we can detect through the dipole (i.e. half of the sky is brighter/hotter than the other).
There is no significance to the particular direction. Specifying the direction of our velocity vector obviously depends on what coordinate system you use - so you may see apparently different quantities which are really just the same thing in a different coordinate system. The actual direction of the vector also depends on what parts of out motion you take out. If you were to instantaneously measure the CMB dipole with an experiment on the Earth you'd have to consider at least the following list of contributors to the total velocity: any motion of your detector relative to the Earth; your rotation about the Earth's axis; the Earth's motion around the Sun; the Sun's motion relative to its neighbours (the "Local Standard of Rest"); the motion of this collection of stars around the centre of our Galaxy; the motion of our Galaxy relative to the centre of mass of the Local Group of galaxies; and the motion of the Local Group relative to larger scale structures.
So depending precisely which velocity is being talked about, it's easy to get apparently different quantities which are really the same thing in different reference frames.
If the universe was opaque until recombination because all the hydrogen
and helium was ionized, and if the universe underwent reionization after
the stars formed, why is the universe not opaque again?
Submitted by fbaer"AT"c2i2.com 10/05
This is an excellent question!
The reason is that the amount of scattering just isn't enough.
The distance out to the epoch of reionization is obviously pretty far, but the Universe is also pretty darn empty. The dominant scattering is between the photons and free electrons (other processes have been considered, and are all negligible in comparison). If you calculate how much scattering you get out to (say) redshift 6, the answer is around 1 per cent. Out to a redshift of around 15 it has grown to about 10 per cent. And that's about as early as we think the Universe may have reionized. So we expect that as photons travelled through the intergalactic medium on their way to us, they found it to be about 10 per cent "optically thick". [The early Universe is so optically thick because the density was so high back then]
You may think that 10 per cent is no big deal, but in fact this effect is observable! In fact the extra scattering at low redshift partly suppresses the primordial anisotropies (because the scattering is isotropic), and it generates a large-angle polarization signal, which was detected by the WMAP satellite.
By measuring this very weak signal in more detail we'll be able to learn more about the epoch of reionization, which is really quite exciting!
After sending my questions, I thought about scattering some more. One
thing that occurred to me, and which I believe is implicit in your
answer, is that with expansion of the Universe, the distance between
galaxies and such increases linearly with the expansion but the density
of scattering electrons drops with the cube of the expansion. As
expansion increases, photons travel further between objects but the
number of electrons encountered decreases. In addition, some of those
electrons that caused scattering prior to recombination are now in stars
and other bodies.
Submitted by fbaer"AT"c2i2.com 10/05
Exactly, you got it!
I have a question about the cosmic microwave background which I was hoping
you could clear up. If we are moving towards the high-frequency-wavelength
regions of the CMB (and are currently passing through the
low-frequency-wavelength infrared region, on the electromagnetic spectrum)
will we eventually pass through a part of the CMB with the same frequency
as visible light? If so, what will the universe look like? Someone told me
once that everything would appear white.
Submitted by jharte"AT"wellesley.edu 10/05
I'm afraid your question is based on a misunderstanding (or alternatively, I don't understand your question, which is always possible!).
The CMB currently has a temperature of 2.725 Kelvin, with a peak wavelength in the microwave part of the electromagnetic spectrum. It has this same temperature everywhere in the Universe at the present time. It is getting colder as the Universe expands, and so the temperature everywhere is decreasing all the time. This decrease is pretty slow though, so you'll never be able to detect any difference in your lifetime.
You may be confusing things with the CMB "dipole", which is the fact that one side of the CMB sky is slightly hotter than the other (by about a few milliKelvin, i.e. about 0.003 Kelvin). That's caused by a Doppler shift because of our motion through space. Different observers elsewhere in the Universe will see different dipole directions and amplitudes, depending on their velocities. But you'd have to move at nearly the speed of light in order to detect close to visible light (in the direction to which you were moving) for the CMB.
I was able to see Stephen Hawking tonight at the San Jose CPA.
Mighty cool.
One thing I noticed, was when he showed a picture of the CMB,
pointing out that it is not uniform, it kind of looked like a picture
of the Earth.
Has anyone investigated the various density patterns and
similarities to continents? Could there be some parallels to
continental drift and how the CMB drifted (expanded) into clusters?
Just a strange thought that popped into my head as I was watching.
Submitted by 11/05
I think any similarity to the Earth is because: (a) the whole sky is a sphere; and (b) certain choices for the colour table (e.g. green/blue) will look reminiscent of our "little blue marble".
In fact there are some important differences. The main one being that the features on the Earth's surface contain lots of sharp edges. In terms of fluctuations, this means that there has to be strong correlations between the phases (in other words if you make an image with lots of waves, you need the waves to have their steepest parts lining up to create sharp edges). The CMB sky, on the other hand, has very diffuse structures, with no sharp edges. And as far as we can tell the phases are pretty close to being as random as they can be. This is actually one of the clues that the CMB anisotropies may have been generated during an early period of cosmic inflation, with little subsequent development of their structure. This randomness of the phases is the main reason why your eye can't pick out much of interest in CMB maps - the useful information is contained in the variation of the temperatures as you change angular scale.
my name is Ann and I'm doing a project in school about cosmic radiation.
therefor i would like to know what the background radiation is on the
moon.
Submitted by ann.shenyang"AT"gmail.com 12/05
That's an odd question!
The Cosmic Microwave Background would be exactly the same on the Moon, except for the effects of the Earth's atmosphere.
But I suspect you're really thinking about cosmic rays (very high energy particles which are a potential hazard for astronauts), rather than the Cosmic Microwave Background (very low energy photons which exist in such abundance that millions of them are passing through your body at every instant of your life!).
What is the microwave wavelength corresponding to 2.73 Kelvin of
the CMB?
Submitted by richard.sackhouse"AT"videotron.ca 12/05
The CMB has a "blackbody" spectrum, i.e. a broad range of wavelengths, rather than a single specific wavelength. The peak of the CMB spectrum occurs at a wavelength of about 1 millimetre (depending precisely what quantity you are defining for the peak, e.g. intensity per unit wavelength peaks at a different wavelength than intensity per unit frequency). But the CMB is fairly easily detectable at wavelengths covering a factor of 10 higher and lower than that.
At this page you wrote the common thing: "in fact the ions
and electrons are combining for the first time, so it should perhaps be
called "combination"". I've added such a comment here: "Not at all!
Epoch of "recombination" took a good deal of time, maybe 50,000 years
or so, during which ions and some electrons of already low energy were
combining still many times but as the time passed it happened more and
more rare. Each atom had its own time to recombine at the last time,
after it was last time ionised by the most energetic photon." Is there
any mistake?
Submitted by mvg"AT"credit.biysk.ru 12/05
I think this may be mainly semantics rather than science!
When astronomers talk about recombination they usually mean that electrons combining with the ions to make neutral atoms once more. It's in that sense that the cosmological case isn't really recombination. But it's certainly true that an individual atom is zapping in and out of being ionized as the cosmic recombination progresses.
You can read more details about the process of recombination than anyone would reasonably expected to want to know in our paper "How exactly did the Universe become neutral?", which you can find in pdf form here.
By the way: "the surface of last scattering" should be
a 4-dimensional space-time surface, shouldn't it? I see it was the whole
Universe some period of time. Lo! That's not a surface one can easily
imagine! But from the other point of view, as you write in E-mail
part, the surface or a shell is that we see now... That's our point of
view however and I think, that the first one is more correct, isn't it?
Submitted by mvg"AT"credit.biysk.ru 12/05
I think of the last-scattering surface as a 3-dimensional shell around us.
You're right that one should perhaps be a little more careful than that. However, it's typical in cosmology to assume that all observations take place along the "past light cone", i.e. we can only see things as they were the light travel time ago. So more explicitly the last-scattering surface is a slice through our past light cone, which is a sphere in 3 dimensions.
I have read the following statement in Zaldarriaga's "An Introduction to
CMB anisotropies",( which is much more than a simple
introduction!Unfortunately, it contains several errors, which made me
skeptical about each ab every formula) :
"The CMB blackbody formed at z~10^7 at t = 70(T/2.7)^(-2) hours old".
Now, as far as I know, T/2.7 = 1+z .Putting this value into the formula
gives t = 2.5*10^(-9) seconds, much earlier than nucleosynthesis, which
is supposed to have occurred at z = 10^10 !
Obviously there is something wrong, and I would be grateful if you could
point out what it is.
Submitted by georges_melki"AT"yahoo.com 01/06
The photons which made the CMB were "created" at a redshift of about 107. What is meant by this is that photon-creating processes were very rapid before this, while after this epoch photons were still scattered, but they rarely were created or destroyed.
Matias Zaldarriaga is a smart cosmologist, and rarely wrong! What he's giving is an approximate age for the Universe corresponding to this "CMB creation" epoch. You need a cosmological model to figure out the age exactly, and so I presume he's done this for something like the standard cosmological parameters.
When he writes "(T/2.7)^(-2)", he's just giving the scaling relative to different possible measurements of the CMB temperature. So the "T" in this formula is the temperature today, i.e. 2.725 Kelvin. This means that the "(T/2.7)^(-2)" factor just changes the age by about 2 per cent. The age of the Universe at this epoch was therefore about 70 hours.
In fact at these early epochs the age varies approximately like (1+z)-2. When I put in the coefficients I get about the same answer, i.e. around 70 hours for (1+z)=107. The beginning of the nucleosynthesis period (say (1+z)=1010) was therefore a factor of 106 earlier, or about a quarter of a second.
Is there an equation that I can use to find the temperature of the CMBR
with respect to time?
Submitted by TmanMrT"AT"cox.net 01/06
The CMB temperature is changing along with redshift (usually denoted z) in the expanding Universe according to T(z)=T(today) × (1+z), where "today" corresponds to z=0.
To figure out how redshift relates to time, you need to specify the precise parameters of a cosmological model.
However, if you're only interested in changes close to today, then things
are easier. The result is almost independent of the cosmological model,
except for 2 basic parameters, the CMB temperature and the Hubble expansion
parameter. The equation relating a small change in temperature to a
small change in time is simply: 
T =
-T0 H0 t.
Hence in a human lifetime (let's be generaous and call this a century),
the CMB temperature decreases by about 0.00000002 Kelvin!
Is the CMB a phenomenon that has a wavelength?
Submitted by tan_dapeng"AT"hotmail.com 01/06
The CMB photons have a range of wavelengths, peaking around 1 millimetre in size.
if the cmb was created in the very early timeframe of the universe,my
idea of this is that of an enlarging sphere of photons travelling at the
speed of light.why doesnt this wavefront if you will pass us by.why are
we continually being bombarded with it.it doesnt seem right that it would
bounce around the universe but would travel in a strait line until it
strikes something and then would release its energy
Submitted by timbo2000"AT"austin.rr.com 02/06
"Why haven't the CMB photons already passed us" is in fact the most frequently asked question about the CMB!
There are several extensive answers on this very page.
The quick reply is that your picture of "an enlarging sphere of photons", while very common among people trying to understand cosmology, is entirely incorrect! The "Big Bang" happened everywhere, not at in an isolated part of the Universe. It's much better to turn the picture inside-out and have us at the centre, with photons arriving at us now coming from the very early Universe from all directions. In other words, the earliest moments of the Universe can be seen a light-travel-time-of-the-age-of-the-Universe-ago, in all directions.
Considering that the CMB radiation travels at the
speed of light, and the fabric of space has been
expanding for about 15 billion years, and the CMB you
write about began its journey about 300,000 years
after the Big Bang (say, 15 billion years ago), and
the atoms of our solar system were among the atoms on
a similar journey, it would seem that two
contradictory statements are "true":
1. The CMB represents radiation from matter that was a
distance of 15 billion light years away from our
present solar system location 15 billion years ago.
2. Through a combination of atom movement within the
fabric of space plus the expansion of the same fabric
of space, the atoms of our solar system have also
"traveled" billions of light years, but somehow
traveled so far ahead of the CMB that it took 15
billion years for it to reach us!
Submitted by timjbal"AT"yahoo.com 02/06
I suspect this is another variant of the "why have the CMB photons not passed us" question. But I don't understand exactly what you have in mind, and hence it's not clear to me where your source of confusion lies.
So let me address each of the statements you make, in the hope that this will be helpful.
"the CMB radiation travels at the speed of light" - true! - "the fabric of space has been expanding for about 15 billion years" - also true - "the CMB you write about began its journey about 300,000 years after the Big Bang" right again - "the atoms of our solar system were among the atoms on a similar journey" - this I don't get.
The atoms of the solar system certainly aren't travelling at the speed of light. They've just been expanding away from distant objects, along with everything else in the Universe. You should think of the atoms of our Universe being pretty much where they were to start with. We're seeing CMB photons which were emitted by atoms near to the Big Bang in time, and hence have travelled about 15 billion light years to get to us (actually a bit more than this, because the Universe was expanding during that journey, but this is an extra complication that doesn't change the picture). Likewise there's a place (actually a set of places in a sphere around us) very very far away which is right now seeing the CMB photons which were emitted by the atoms which later made our Solar System.
Sir,how is CMB related to GZK cut-off?
Submitted by amidiptimoy"AT"gmail.com 02/06
The "GZK cut-off" is the idea that the highest energy cosmic rays don't reach the Earth because they interact with CMB photons as they travel through intergalactic space. The acronym stands for Greisen-Zatsepin-Kuzmin, the names of the people who proposed this effect in 1966. Their idea is that the highest energy particles (above about 1020eV) will interact with CMB photons to produce pions. There is currently a controversy about whether the GZK cut-off has been observed in cosmic ray experiments, and if not whether it points to some new physics.
Like dozens of people on your question page, I am confused regarding the
idea that the radiation has somehow not already passed us. But please don't
close the email here!!!! - I will refine the question so that it is more
precise.
I understand that the universe has no centre and I noticed that in many of
your explanations to the problem you said that the universe was not smaller
in the past. But most theories of the Big Bang I have read state that it
all began as a singularity. Even my course text book (Cambridge, for 16-18
year olds) says this.
Even if the universe did not begin quite as small as a singularity, the
speed of expansion of the universe is still much smaller than the speed of
light. So by now there should be no radiation left to ever pass us.
Submitted by ruby_murray1@hotmail.com 02/06
The idea that the Universe started with a singularity is an old one. It was essentially proved by Hawking and Penrose and was a tour-de-force of classical cosmology (circa 1970). Here "classical" means General Relativity, with no quantum mechanics. For the last 25 years or so (at least) our ideas for the earliests stages in the history of the Universe are rooted in quantum fields. We know that (running the clock backwards in time) as the Universe gets denser and denser, and hotter and hotter, we eventually come to an epoch when we really don't know the physics well enough to know what is going on, because we need a theory of quantum gravity.
So most cosmologists, if they have a mental picture of the first stages of cosmic history, do not have a singularity as part of the picture! Probably it's some fuzzy mess, maybe with strings or branes, or multiple dimensions, or eternally inflating universes within universes, or some other thing entirely. And who knows, one of them may even be right! But please erase that sentence from your textbook, since it's certainly not the consensus view of the world's cosmologists right now, and hasn't been for at least a quarter of a century!
The other problem with the singularity idea (often followed by a phrase like "and then it exploded") is that it sucks people into the fallacious idea that the early Universe was localized. Thinking along those lines is the path towards misery and despair!
Think of the earliest Universe along something like the following lines and things will be much clearer. I can't promise enlightenment, but I can promise that you'll at least have the hope of understanding why the CMB photons haven't already passed us!
So for the very first instant, no one really knows, but it's not localized! Then very shortly after that, you think of the Universe as being big, maybe infinite, and it's all expanding. In other words the "Big Bang" was everywhere. And today's photons simply come from however far away photons get in the age of the Universe.
so do these photons make up a finite universe or do they incase a finite
universe? or are they infinite going where there is no matter?
Submitted by Jacquianne701949"AT"aol.com 02/06
The simplest picture we have of the Universe is that it's infinite in space. So there is matter everywhere, being distributed pretty much the same way as it is in our observable patch. And there are photons everywhere too, going at the speed of light in every direction. Today we're seeing the ones which come from the light travel distance in the age of the Universe.
although wavelength of microwave is not of order of microwave then why it
called microwave?
Submitted by geeta_with_love"AT"yahoo.com 03/06
The precise definition of "microwave" is fairly murky. A common definition is that it covers frequencies of 300 MHz to 3 THz. The CMB blackbody spectrum certainly peaks inside that range. It's true that the spectrum extends into the "radio" and "infra-red" regimes. But it's fair to refer to is at the Cosmic Microwave Background, because that's the waveband in which most of the radiation lies.
i have to given an interview and i give my intrest of field microwave.i
think they asked the first question- although wavelength of microwave is
not of order of micro then
why it called microwave?
Submitted by geeta_with_love"AT"yahoo.com 03/06
Ah, so I think this is a different question! Why are microwaves called microwaves at all, given that you might have expected that to apply to wavelengths measured in microns (rather than millimetres) - so why "microwaves" instead of "milliwaves"?
I don't know the answer to this! It's a bit like one of my favourite mysteries of the Universe: "why is a grapefruit called a grapefruit"?!
As far as I can tell (looking up dictionaries) "microwave" was first coined in 1931, but I'm not sure by who, and whether the definition has changed since then. Apparently "microwave oven" dates from 1965, "microwave" as a shortened form in 1974 and the verb "to microwave" in 1976.
Assume: The Big Bang occurred
(1) At some single place in the universe( the size of a dime) many
billions of years ago
(2) There is no evidence to support the theory that there were other
places where Big Bangs occurred simultaneously
(3) Within about one minute after the Big Bang, matter cooled and began
to emit micro waves
(4) All the matter has been expanding and the microwaves have
been radiating ever since, so the present size taken up by this diffused
matter is immensely larger, relative to the size it took up at the
instant of , or within one minute of, the Big Bang.
Questions:
(1) Why is the "signal strength" of the CMB the same from any direction
in the universe?
(2) Would it not be strongest from the direction of that single , and
much relatively smaller, ancient location of the Big Bang? ( wherever
that precise location may be, being irrelevant)
(3) Would it not be weakest ( or non existent) if we aim the telescope
180 degrees away?
Submitted by sidgold"AT"rogers.com 03/06
This is a variant on the single most asked question about the CMB - "why haven't all the photons passed us already?" - which has been answered on this page many times before. I urge you to scan the page for answers to similar questions, so you can find the answer that resonates the best for you.
The basic answer is that your very first assumption is incorrect. The Big Bang did not happen "at some single place in the universe". It happened everywhere!
How do you derive the age of the universe from the present value of the
CBR wavelength(s)?
Submitted by pauladriaenssens@pandora.be 03/06
You don't!
The age of the Universe is one of the parameters that comes out of fitting the CMB anisotropies to a suite of models. It has nothing to do with the wavelength spectrum of the CMB. The CMB is getting cooler all the time in the expanding Universe, but that can't be used to learn anything useful about how long the Universe has been expanding unless you have a good idea for what the temperature of the Universe was at some early time. In fact there's no theoretical prediction (that I take seriously at least) for the CMB temperature, and hence its measured temperature isn't a useful constraint on cosmological models.
But the variations in temperature on the CMB sky (i.e. the anisotropies) are very useful in pinning down the precise values of the parameters that describe the Universe, including its age.
how did Arnpo Penzias and Robert wilson find CMB?
Submitted by taytaytwinkle"AT"yahoo.com 04/06
Penzias and Wilson were working for Bell Labs, investigating microwave transmission in the atmosphere (the kind of research which led to today's cell phone technology). The "background hiss" that they always measured in their detector turned out to be the Cosmic Microwave Background.
could you tell me why the expected "sea" of relic
neutrinos (from big bang) have lower energy than the cosmic microwave
background? Is it somehow connected to that the neutrinos decuopled before the
photons? I'd really appreciate an answer!
Submitted by Evelina.Olofsson.2152@student.uu.se 04/06
I think you've answered the question!
The basic idea is that the neutrinos "decouple" (i.e. stop interacting with other particles in the Universe) just before the annihilation of the thermal electrons and positrons (which existed in approximately equal numbers and were about as common as photons at temperatures corresponding to energies above their rest mass). So when the e+-e- pairs annihilated, that created more photons, boosting the temperature of the photon background relative to the neutrinos.
A fairly precise calculation can be done by simply considering entropy before and after the annihilation, and some understanding of the statistics of bosons (photons) versus fermions (all the other particles). The answer is that today's CMB photons occur everywhere with around 400 per cm3 and with a measured tempertaure of about 2.7 Kelvin, while the neutrinos have a temperature of about 1.9 Kelvin and there are about 100 of them per cm3 (per neutrino species). Doing the calculation
I know that ionizing radiation is simply a radiation that has enough
energy to break chemical bonds. Also, I have researched and few websites
categorized cosmic radiation as a form of ionizing radiation. But does
`Cosmic Microwave Background Radiation' carry enough energy to break
chemical bonds? I mean, it is only in the range of microwave. To my
understanding x-rays and gamma rays are only two types of rays in the
electromagnetic spectrum that carries enough energy to do so.
[abridged]
Submitted by djseo0913"AT"yahoo.com 05/06
I think you're confusing the CMB with Cosmic Rays. CMB photons don't have enough energy to ionize anything today (although they did in the early Universe). Cosmic Rays, on the other hand, can be very high energy indeed.
Please check other answers on this page!
What if CMB were not
considered an "echo" as such and was looked upon as a carrier wave for
all forms of matter and energy, wouldn't that necessarily lead to an
explanation for the comings and goings of the universe (e.g. matter,
radiation, particl physics etc...) and lead us a damn sight closer to
"fundamentals" of the universe and the origins on which they would all
impinge ? A good example being sound, harmonics, resonance and
fundamentals as encountered in basic wave mechanics ?
Submitted by fhudson"AT"midsouth.rr.com 06/06
I'm not sure what you mean by a "carrier wave". One of the amazing things that was learned about electromagnetic radiation more than a century ago is that it doesn't need a medium to move in, but will just travel through empty space at the speed of light. That's what CMB photons are - just energy from the early Universe which has travelled through space for roughly the age of the Universe. In some kind of desperate analogy we could say that space itself is the "carrier wave" for all of the particles and waves which fill the Universe.
I think we have a pretty good "fundamental" description of the state of the Universe today, and its constituents (ordinary matter, dark matter, dark energy, neutrinos, photons, gravity waves, etc.). We still have only taken small steps to understanding the origin of everything, or in other words the very first instants of the history of the Universe. But the hope is that by studying cosmological information in more detail (and the CMB in particular) we'll learn more about the "why" type questions that relate to the origin of everything, and may find some explanations for the Universe being the way it is.
If the anti-particles and matter particles annihilated each other for
the last time and by some asymmetry some more matter particles were
present that resulted in the formation of the modern day galaxies,where
does the anti-particles come into existence now?as they must have been
annihilated in the early universe,otherwise they would not allow matter to
be formed if they were present to this very day..
Unless some barrier results in their isolation from the matter
particles.Could u please solve this analogy?
Submitted by 06/06
There are no anti-particles today! At least there's a negligible amount left over from the early Universe. And the only existing anti-matter particles are formed from high energy collisions in the interstellar medium. When a very high energy particle collides with a heavy nucleus it breaks off lots of little bits, often including some anti-electrons, anti-protons, etc. We can detect some of these in Cosmic Ray experiments, for example. But only a very very small amount of the matter in the Milky Way is in the form of anti-matter, and the same goes for today's Universe as a whole.
What is the significance of the Cosmic Microwave Background?
Submitted by dbrunn1662"AT"rogers.com 07/06
The existence and spectral shape of the CMB provides very strong evidence for the "hot Big Bang" picture for an evolving Universe which used to be very much hotter and denser.
The "anisotropies" observed in the CMB provide constraints on mechanisms in the even earlier Universe which made the variations in density which grew into all the structure we observe around us today, and at the same time they lead to precise measurements of several quantities which describe the contents and evolution of the Universe.
i have a few questions for you if you could answer then for
me that would be great ok.
the wavelenght
where the radiation come from
how to detect the radiation
how the radiation can be useful
other interesting information
Submitted by justin.19"AT"hotmail.com 08/06
I suggest that for a start you read some of what is on this page! There's also general information at places like Wikipedia.
The short answers to your questions are: the CMB has a range of wavelengths; the radiation came from the hot early Universe; it is detected with a radio, microwave or infra-red detector; it is useful because structures in images of the CMB sky can give us precise information about the Universe on the largest scales.
At some time, won't the distance to the surface of last scattering
increase sufficiently so to be outside our observable Universe, and thus
eventually CMB photons will no longer be visible to us?
Submitted by mbohon"AT"comcast.net 10/06
It's easiest to think of the "Observable Universe" as being the volume from which we can detect photons from earlier times, and hence the "Last Scattering Surface" is essentially identical to the edge of the Observable Universe. The size of the Observable Universe is a little bigger than the distance to the Last Scattering Surface, but only a little (in astronomical terms), and moreover it's hard to see beyond this surface (because of the scattering of course!), and so for all intents and purposes you can consider them to be the same.
We can in principle see neutrinos coming from much earlier times, and hence the "neutrino scattering surface" is slightly bigger, and closer to the true size of the Observable Universe. Gravitons could also come from much earlier, etc.
When these issues are discussed in popular cosmology texts, things are often simplified, to say that we can't see the part of the Universe moving away from us faster than the speed of light, and that the edge of the observable Universe is set by the distance at which the recession speed is the light speed. However, this is a simplification! The whole business of speed in cosmological models is something where one has to take great care, since there can be several different definitions of position and of time (you have to ask things like: "the speed of light measured when and using what distance coordinate?"). The truth is that using the most natural definition of what speed means, you can in fact see the light that left some objects which are now moving away from us faster than the speed of light! I realise this may sound a bit confusing, but I wanted to at least lay this out so as not to be telling untruths! To get a deeper understanding I'm afraid there's no substitute for studying cosmology in its full mathematical glory.
But the bottom line is that you should think of the CMB "LSS" as defining the edge of the Observable Universe.
One wrinkle on this is what happens in the future of a Dark Energy dominated model though! Because as the Universe expands faster and faster the fraction of the Universe that you can see actually shrinks.
And with that I'll stop adding elaborations, since you're probably now even more confused than before!
Hi Douglas, would you mind explaining the evolution of the CMB, ie: it
started at a high temperature, therefore, different wavelength, and
continues to cool, presently at microwave frequency, but not for ever?
Submitted by 909453"AT"@bigpond.net.au 10/06
You have it just about right!
The CMB started in the early Universe when everything was very high energy. You can think of it as a "gas" of photons existing everywhere and having a "thermal" distribution of energies (or wavelengths) characterised by a very high temperature. As the Universe expanded the CMB cooled, a bit like a regular gas does when you let squirt it into a big empty cavity. At the moment the CMB has a temperature of about 2.7 Kelvin, meaning that its spectrum peaks at microwave wavelengths. And in the future it will get increasingly colder, presumably for ever!
I have just finished "The Big Bang" by Simon Singh and it made me think
about the CMB.
As I understand it, the CMB was produced when the universe cooled enough for
plasma to condense into gas and the photons suddenly found themselves in a
transparent universe. My question is what about before that? When the
universe was younger/hotter and the nucleons were bashing about freely the
gluons would be constantly interchanged between them. But when the universe
cooled enough for atomic nuclei to form wouldn't there be the equivalent of
a burst of CMB but with Gluons instead? So shouldn't there be a Cosmic
Gluon Background (CGB or "Cosmic Glue". The detection and variations of
which may yield more information about the early universe.
Submitted by rich_mar"AT"sympatico.ca 10/06
This is a good question!
You're sort of right. But you can't have freely-floating gluons, since they have to be bound up with quarks. What does happen though is that during nucleosynthesis there are photons (gamma-rays in this case) emitted during the nuclear reactions which are going on - basically the binding energy of the nuclei is liberated as photons. These photons add to the CMB and raise the temperature a little. But our Universe has about a billion photons for every nucleon, and so the temperaure of the CMB is only raised by about a billionth!
At slightly earlier times there were lots of interactions involving neutrinos (converting protons and electrons into neutrons for example). When those reactions stopped happening as much, the remaining neutrinos (which unlike gluons, don't interact with much of anything) formed the cosmic neutrino background, which is indeed just like the CMB. And at even earlier times, annihilations between electrons and positrons (anti-electrons) - and between higher mass particles and anti-particles - led to burst of gamma-rays, which boosted the CMB temperature. So in fact the "origin" of CMB photons are particle/anti-particle annihilations in the early Universe.
I am confused about which paper actually
first predicted the CMB. I have looked at the Alpher
Bethe Gamow paper, but cannot distinguish where there
is a direct prediction. Also, the Wikipedia site is
not consistent in attribution, citing both the above
paper and the Alpher-Herman paper. Beyond this,
Alpher and Herman later revised their predicted temp.
to 28K, so it seems they might not have really nailed
down the theory.
Given the above, if I mention the CMB as being
evidence for a Big Bang, do you think that it would be
sufficient to cite textbook souces like Misner Thorne
and Wheeler?
Submitted by bicycle_physics"AT"yahoo.com 11/06
The question of when the CMB might have been predicted is a very murky business! Alpher, Herman and Gamow certainly worked out some details of what we now call the "Big Bang" picture. And they published several versions of a prediction for the temperature of the background radiation - although exactly how seriously these calculations were taken is unclear. Nevertheless, Dicke certainly predicted that there would be a background and set out to detect it, being beaten to the post by Penzias & Wilson, who originally didn't know the significance of what they had found.
And that's the short version!
The evidence for the Big Bang picture (i.e. the Universe once being very much hotter and denser) came later, when it was clear that the CMB has a very nearly thermal ("blackbody") spectrum. So precisely who you should cite for that is also unclear!
But if anyone asks, you can tell them I said it in 2006!
I have been doing some research on the possible effects the CMBR
could have on our natural world. I have taken the frequency down by 29
octaves and found it to be a perfect D. I then made a number of
high-definition audio recordings of various natural sounds, such as a
river and a heavy rain shower, bird calls in a forest, the ocean, etc.,
and put them through a spectrum analyzer. I thought that I might find
dips, or interference, in the spectrum at that frequency, or its
harmonics. What I have found is truly amazing! In every case there seems
to be some influence at that frequency. Do you know of any studies that
have been done, or are being done, about this subject? Or do you know
anybody that might be interested?
Submitted by tedargo"AT"comcast.net 11/06
This sounds intriguing as an artistic project. But I'm afraid that the connection with anything scientific is fairly tenuous. The CMB spectrum is a spectrum of photons, not sound waves. And it's a "continuum", which peaks at a particularly frequency but is fairly broad - hence it could never sound anything like a "perfect D", but would rather sound like noise if intrepretted as sound.
For a different view on "the CMB as sound", used as a pedagogical device for understanding the "acoustic signatures" in the CMB power spectrum you should look at the web-page of Professor Mark Whittle of the University of Virginia, which I've linked here.
The temperature of the background radiation has dropped in due time.
Question: where is the energy, which should be freed by the drop down of
the temperature of the radiation ??
Submitted by Eric.Hoyng"AT"hetnet.nl 11/06
This is a commonly asked question, and so there are other relevant answers on this page.
The short answer is that the energy per unit volume is conserved, and goes into the expansion of the Universe. It's the same as an expanding gas in a box - the pressure of the gas does work in expanding the box, and this "work" is the energy lost in cooling the gas.
I do not understand why the
photons that comprise the CMB have not arrived at thermal equilibrium. I
think you answered the question by saying that these photons are a
different kind of "stuff." Coiuld you be more specific? Are they a
different class of particle?
Submitted by fish9999"AT"netvision.net.il 12/06
Photons are massless particles which carry energy and momentum and interact through the electromagnetic force. So they are quite different from matter particles, like atoms or ions or electrons.
The CMB photons have arrived at thermal equilibrium. That's shown through their "blackbody" spectrum, with a temperature of 2.725 Kelvin.
The photons achieved this equilibrium at early times, when the Universe was very much hotter and denser, and interactions between the photons and matter particles were common. After the Universe stopped being a plasma (at redshifts around 1000, or about 400,000 years after the Big Bang) the photons stopped interacting much with the matter, but cooled with the expansion of the Universe, retaining the blackbody spectrum with an ever decreasing temperature. And that's what we observe today.
I have one more question that I
hope you will answer. What is meant by a "massless" particle. Since a photon
carries energy, and mass and energy possess an equivalence, so doesn't a
photon have to possess mass?
Submitted by fish9999"AT"netvision.net.il 12/06
No, a photon doesn't need to have mass in order to have energy. Much of what we learn about concepts such as mass, energy, momentum and speed are learned for "Newtonian mechanics", i.e. the laws of motion which apply to everyday objects moving at relatively slow speeds. One of the basic motivations of "Special Relativity" is to understand how things behave as you go to much faster speeds.
One outcome of this is that you have to slightly change your notions of those physical quantities. Energy is a thing which can exist in many forms, but is always conserved (e.g. rest mass has an equivalent energy, and you can in principle destroy rest mass to create more kinetic energy). Photons possess "pure energy" with no rest mass. They also have momentum, even although they have no rest mass (and Newton would say that momentum is just mass times velocity). In Special Relativity the quantity called momentum is the thing that stays constant if there's no applied force. You know that photons have to have momentum, since they exert a measurable force ("radiation pressure"), e.g. when you bounce them off something.
You can think of photons as having just pure motion energy, with no rest mass. If you converted the photon's energy into rest mass then you'd have a massive particle, which would have no motion energy - or you could make a lighter massive particle with some motion energy (i.e. travelling at some speed, less than the speed of light). A photon is a very special particle in the sense that it really has zero rest mass and is travelling at exactly the speed of light!
If the theorized Big Bang happened in a specific point beginning the
space/time we experience, why anisotropy and this spectrum of microwave
radiation found in all directions instead of from that point or points in
the singularity event?
Submitted by ajwps"AT"yahoo.com 12/06
Let me say a few things in response to this: (1) the Big Bang is indeed a "theory", but this means that it is a well supported idea, rather than merely an untested hypothesis; (2) when cosmologists talk about the "Big Bang" theory, they mean the model in which the Universe was once hotter and denser and has been expanding, they explicitly do not mean a theory for the first instant that started that expansion (despite what you might think from the name); (3) the expansion happened everywhere at once and not at a point in space; (4) the question that you ask is very similar to others answered on this page, along the lines of "why don't the CMB photons come from one direction?" or "why haven't the CMB photons already passed us?"
... I am also required to understand and explain what
Cosmic microwave rdaiation is. I read your article and am still having a
difficult time understanding exactly what it is. i have searched on the
internet and have yet to come across an explation that i can understand
well enough to explain it in my own words. I was wondering if you could
send me a brief but clear defintion of what CMR actaully is. [abridged]
Submitted by stephanierandall"AT"rogers.com 1/07
Since I don't know precisely what your background is, it's hard to figure out how to explain it to you. Besides which, you should do your own homework!
The CMB is radiation left over from the early Universe, which has cooled as the Universe expanded, and today has a spectrum consistent with a single temperature of slightly less than 3 degrees above absolute zero.
I have a question regarding an answer you gave to sombody else
about why the CMB hasn't 'passed us' already.
I too have wondered the same thing as this guy many times, and often
received a similiar answer. It seems to me that an assumption is being
made (but not stated) in this answer, that being: The inflation of the
(early?) universe must have been much faster than the speed of light..?
The logic being that when you mention photons from this point (here)
having travelled away from here at the speed of light for 14 billion
years - they are still 'getting somewhere' i.e. the universe has gotten
larger than 14 billion light years in diameter, yet has acheived that in
around 14 billion years.
Is that correct or am I also interpreting things wrongly? I see in your
other answers that you say the early universe is also infinite but closer
together, yet I'm still a little puzzled about it.
Submitted by Brett.Melling"AT"uea.ac.uk 1/07
You're pretty close to the right picture I think!
But you don't need inflation in order to have the Universe big enough and full of CMB photons. What you need is some sort of acausal process which starts off everything at once. "All" it takes to make a Big Bang is to have an infinite space which is all very hot and expanding, with the "starting gun" being the same everywhere. (Remember the "Big Bang" isn't the very first instant of time - which no one can picture - but the expanding phase just after that!). Then we see the CMB photons from the parts of the early Universe which are the light-travel-distance away from us. That's really all there is to it. The Universe has always been infinite, or at least much much bigger than 14 billion light years in size.
The thing that inflation gives you is a kind of explanation for that apparently acausal behaviour. A small patch, which was in causal contact at some very early time (i.e. smaller than the light travel time in the age of the Universe) expanded exponentially, at effectively faster than the speed of light, so that it appears to be acausal. But everything in that patch "knew" to start the Big Bang at the same time, and hence the CMB temperature is the same in every direction within the observable Universe.
I read about the neutrino oscillations theory. According to
this theory neutrinos are supposed to have small but non-zero masses.
I am enthusiastic to know about what is the effect of this theory on
CMB spectra.
I also read that there is a problem with the mass hierarchy in
neutrino oscillations and due to their small massed it's difficult to
determine it. However, if one studies the early universe then we have
a huge number of neutrinos and their interactions with rest of the
constituents. So, effectively there is a considerable mass of
neutrinos and effects of different hierarchy should be different. Is
it possible to know about the neutrino mass hierarchy by studying the
implications of neutrino oscillations on cosmology?
Submitted by suchita.kulkarni"AT"gmail.com 1/07
Neutrinos have an effect on the CMB anisotropies because they contribute to the mass-energy density budget of the Universe, and so make it expand faster. The number of neutrinos per unit volume is essentially known (because we know how they interacted in the early Universe, and hence how many of them are left). Hence the CMB (plus other cosmological probes) can be used to constrain the total mass of the neutrino species. At the moment this is just an upper limit, but there may one day be a detection of the neutrino masses in this way. Put together with other experiments, which typically measure neutrino mass differences, one can piece together details of the model parameters describing neutrino oscillations.
The additional effects of neutrino oscillations on cosmology are far more subtle, and I'm not aware of any realistic way of directly constraining the physics of neutrino oscillations using cosmological data.
If an aircraft flies out , and an observer looks up with a cmb
telescope , should he be able to observe the aircraft by its
cmb sillouette ? Is cmb radiation able to pass thro metal
or dense carbon material ?
Submitted by alikaren"AT"blueyonder.co.uk 2/07
There's nothing magical about CMB photons, so you're right that they wouldn't pass through some thick piece of opaque material. In principle you could therefore see the silheuette of an aircraft if you had a high resolution microwave telescope. However, in practice the aircraft will be emitting strongly in the infrared, and so because it's much hotter than the CMB, it will be appear very much brighter!
As well as being an emitter of radiation, the atmosphere itself absorbs some of the CMB photons, this being a strong function of wavelength, and of where you observe from (e.g. the water molecules in clouds will absorb very strongly in a whole bunch of spectral bands). Intergalactic space is pretty empty, as is the space between stars and planets. So most CMB photons travel happily through the Universe from when they were made, shortly after the Big Bang - but a tiny fraction of them happen to be arriving at planet Earth right now, mostly absorbed by the atmosphere or the ground, but a few reaching the detectors of CMB experiments, and allowing us to learn about the Universe!
This started when I read something about the "dark ages" after +380kyears
big bang. Shouldn't that be "dull red" ages? So I did some calculations
and came up with a SWAG of 4 Byears ago to get to about 373K and 2 Byears
ago to get to 273K, which means that that the early univerese was hot, and
no liquid water before 373K and no ice before 273K. So pasturized? Final
conclusion - The Drake equation assumes an equeal distribution through
time for ETI's, if 373K was only 4 Byears ago and life started on earth
3.7 Byears ago, we could be #1 in the galactic intellegence game. Is SETI
a waste of money? Are we alone out here? Is the universe too young to
have generated an equeal distribution of ETI's?
Submitted by markgoll"AT"wt.net 3/07
I think you have the right general idea, but unfortunately you're calculations aren't completely correct.
The relationship between temperature and time in the expanding Universe is a slightly complicated one, because of the different components (radiation, matter and dark energy) which dominate the evolution at different times. The simplest thing to do is to take the age at the "last scattering epoch" for CMB photons and work forwards.
The last scattering epoch was at a redshift of about 1000, corrsponding to
an age of about 300,000 years. Your question asks about conditions in the
Universe when T=273 Kelvin, i.e. when the redshift was about 100 compared
with today (so that the wavelengths of all CMB photons were 100 times
smaller and therefore the temperature was 100 times bigger). It turns out
that during this phase the Universe evolved so that age was proprtional to
redshift to the negative 1.5 power (t
z-1.5). This means that 300,000 years at z=1,000 implies
an age of about 10 million years for z=100.
Hence when the temperature of the CMB was at the boling point of water, the Universe was only about (1/1000)th of its present age. So this was long before there were any alien civilizations in existence. In fact, since structure builds up from very small contrasts in density at early times (the variations in density giving us the CMB anisotropies), there was very little of anything in the Universe at that time. Everything was very smoothly distributed, and stars hadn't yet had a chance to build up the heavier elements. So not only was there no alien life, there weren't any planets or water yet either!
Hi, is it possible to see the radiation with the naked eye (not via the
TV set)? oes it have a frequency on the visible spectrum?
Is there a website with more detailed info about it?
Submitted by joely"AT"framedestination.com 3/07
The short answer is "no". The CMB spectrum eaks in the microwave part of the spectrum, which is several thousand times longer than the wavelengths that your eyes can detect. However, since the CMB cools in the expanding Universe, then it used to be much hotter. So if you had lived in the first few hundred thousand years of the history of the Universe, then the CMB would have peaked in the visible part of the spectrum. Then it wouldn't only have been visible, it would have been really bright!
Could you provide me with a good average number for cosmic
background radiation levels at the Earth's surface near the 1.57542GHz
frequency in dBW? I have heard a lot of numbers thrown out, but because
in circuit thermal noise is usually of more concern than CMB the numbers
are usually left vague. Anyway, I was hoping to get a number to set the
record straight. Also, if you know of a database for current CMB levels
I could reference that would be helpful for the work we are trying to
do.
Submitted by Daniel Healey 3/07
I'm not sure you are really asking about the Cosmic Microwave Background here, or about other potential sources of background noise for terrestrial-based instruments operating at microwave frequencies (or centimetre waves in this particular case).
But if it's really the CMB you're after, then the answer is simple, since it's just a blackbody. So the amount of energy per unit time per unit area per unit frequency is just given by the Planck formula (which you can easily look up if you're not familiar with it) with temperature T=2.725 Kelvin. You can plug in the frequency you're interested in, and use whatever units you prefer. If you want a total power unit, then you'll need to multiply by your bandwidth and the area of the thing you're interested in. E.g. at 1.57GHz, with a 1MHz bandwidth then a square 1mm across sees about -200dBW (in these units), which I think is pretty small by any standards!
I'm wonder, two things, actually. First, can
you tell me the flux of the cmb (in watts per metre square)? And
do you have any idea why it has been so impossible for me to find
that value anywhere on-line?
Submitted by misterpink"AT"mail.com 3/07
The reason this value isn't written down explicitly (very often) may be because it isn't usually the most relevant quantity, but is easily calculable anyway.
A more relevant quantity might be the flux density, i.e. the energy per unit time per unit area per unit frequency, or perhaps that quantity multiplied by some bandwidth. That's because the CMB spectrum is quite wide, and most kinds of detectors have a much narrower range of frequencies to which they are sensitive, and hence wouldn't see all of the CMB flux. Another issue for real detectors is that if they have some directionality (i.e. only receive radiation from a limited solid angle) then they'll only see part of the CMB.
If it's really the total flux that you're interested in, then that is just obtained from the 4th power of the CMB temperature times the Stefan-Boltzmann constant. This comes out to be about 3.13 × 10-6 W m-2.
...then I'm wondering, is there any difference in flux, or flux density, of the
cmb at different latitudes (like there is with the Sun). My (very limited)
understanding of the cmb was as a constant source everywhere and in all
directions.
My interests are actually _climatological_. Though the cmb is quite small,
as you say, I'm curious if it produces any surface or atmospheric effects,
if it gets 'knocked around' by greenhouse gases, if there's a 'cmb
constant' (like the Solar constant) contributing energy to Earth systems
- things like that.Submitted by misterpink"AT"mail.com 4/07
The CMB doesn't depend on latitude on the Earth, and as you suggest, it comes equally from all directions.
I very much doubt that there are any significant effects on the Earth's climate, weather or anything related that might affect human beings. This is not only because the magnitude of the CMB is small by terrestrial standards (it's only important because it's absolutely everywhere in the Universe!), but also because the driving effects for weather and related phenomena are differential, i.e. it's changing factors which have big effects, while the CMB is about the most constant thing you can think of!
It is said that the expansion of the universe "cooled" the radiation.
What is the mechanism by which the wavelength of the photons is changed??
Submitted by suegoss"AT"ecentral.com 4/07
There are several different (and equivalent) ways to think of this. Probably the simplest is just to realise that any expanding gas gets colder. "Radiation", i.e. a "gas" of photons, behaves pretty much the same as any other gas in this regard, although actually it cools at a different rate than a regular gas of particles, since the pressure for radiation isn't negligible like it is for "matter". So the explanation is just that this is part of the usual laws of thermodynamics.
Is this microwave background radiation the empty space between objects?
Is this mbr what is behind the universe in expansion? Imean if the
universe is expanding there must be something behind waiting to be
fullfill? If the universe is 74% dark energy,22% dark matter and only 4%
atomos that fill the visible universe? where this mbr come from?
Submitted by jacomiz"AT"hotmail.com 4/07
The contribution of the CMB to the energy density budget of the Universe is quite small, about 0.01%. So in today's Universe it has a nearly negligible effect on how the expansion evolves. The dominant effect today comes from the Dark Energy, although in fact the CMB dominated in the earlier history of the Universe.
I've been trying to find a figure for the Cosmic Background power
density (Wm^-2) as received here on Earth. I could probably arrive at a
figure based on a black-body cavity model, but I don't have the
necessary figures. I've seen graphs for power per unit wavelength, and
integrating the graph should again produce a figure.
However, I suspect that someone, somewhere, has it written down.
Submitted by sidhedark"AT"yahoo.co.uk 4/07
You can work it out directly from the CMB temperature, under the assumption that it's a "blackbody". In that case the total flux is just the 4th power of the temperature multiplied by the Stefan-Boltzmann constant (about 5.67 × 10-8 in SI units). This gives approximately 3.14 × 10-6 W/m2.
So, the radiation was emitted by gas that was around half the temperature
of the sun. Why does it now take the form of a 2.73 K blackbody?
Submitted by bigpapaskills"AT"gmail.com 4/07
The Universe is expanding and so the radiation cools, while retaining its blackbody form. One way to think of this is that the wavelengths of all the photons get stretched as they travel through the exanding space. The CMB corresponded to a temperature of around 3000 Kelvin at the time of "last scattering", i.e. the "cosmic photosphere", which corresponds to a redshift factor of about 1100. This means that we observe the CMB today as a blackbody which is about 1100 times cooler, meaning about 2.73K.
suppose that the universe will expand forever because of hubbles law.
what will eventually become of the microwave background radiation???
Submitted by NONIKALS42"AT"aol.com 5/07
It will simply continue to cool, i.e. the wavelengths will continue to stretch, so that the peak of the spectrum moves into the radio regime, and then further and further into the long wavelength radio waveband. This will make it harder and harder to detect (assuming that we're talking about billions of years into the future!). So in some sense we live at a special time when it's relatively easy to use the CMB to find out about the Universe!
Why does the CMB has blackbody spectrum?
Submitted by eozmelek@su.sabanciuniv.edu 5/07
That's a simple question to answer!
The CMB photons come from a time when the Universe was very much hotter and denser than it is now - so that the photons were created in "thermal equilibrium". This means they could be described as a distribution of particles with one temperature, i.e. they had the blackbody spectrum when they were born. As the Universe expands the blackbody spectrum keeps its shape, with all the photons losing energy together, so that the temperature of the blackbody decreases with time. The blackbody shape is preserved, even although the photons stopped strongly interacting long ago.
how do we know that
cmb occurred around 380000 years after the Big Bang, and why has its
state not perceptibly changed?
Submitted by randshindery"AT"yahoo.com 6/07
What we know is that the CMB sky that we observe is made of photons which last interacted with matter at about that epoch - we call this the "last scattering surface". We know that the temperature of the CMB photons is decreasing as the Universe expands, and we know the evolution of that expansion pretty well, since we now have reasonably good estimates for the present expension rate, plus the densities of ordinary matter, dark matter and dark energy. The "last scattering epoch" is determined by the time when the matter (mostly hydrogen) went from being ionized (because it was so hot) to being neutral (once it was cool enough). When we put together the dynamics of the expanding Universe, plus the estimates of the cosmological parameters, plus the physics of hydrogen atoms, we come out with a pretty robust determination of this epoch, which is around 380,000 years.
I'm not sure what you mean by asking why the CMB has "not perceptibly changed". The photons interact very little with neutral matter, and so we're simply seeing back to the time when the photons were last scattering strongly. This is just the same reason that we see the surface ("photosphere") of the Sun, rather than some closer or deeper surface.
if the universe continues to expand forever, what will eventually become of
the cosmic background radiation?
Submitted by joohyun.lee@yale.edu 7/07
That's a great question! Strangely enough I've just written a paper on this topic, along with my colleagues Jim Zibin and Adam Moss. So you can read the technical answer to your question here.
The short answer is that the temperature continues to fall, while the features on the CMB sky change in various ways, depending of course on whether the future continues to be dominated by the Dark Energy.
I understand that you wrote the review of CMB for PDG's Review of
Particle Physics chapter 23. You cited the value of CMB temperature as
2.725+/- 0.001 K from J. C. Mather, D. J. Fixsen, R. A. Shafer, C.
Mosier, and D. T. Wilkinson, Astrophys. J. 512, 511 (1999). But I surfed
this paper and found that they use 2.725+/- 0.002 K (95% confidence)
instead of +/-0.001K. I also noticed that you cited 0.001K as 1 sigma, so
my question is
Where did you find the data and can you give me a reference? Thanks
Richard Loogn
Submitted by richard.loogn"AT"gmail.com 7/07
The Mather, et al. 1999 paper is curently the best study on the CMB temperature obtainable from FIRAS. But unfortunately it's not 100% clear about what temperature to use! I think they never really thought that people would be looking for a place to cite for a better temperature measurement than the older FIRAS one. So the paper doesn't make the size of the uncertainty very clear. The reason of course is that it's dominated by systematic error, so it's nothing like Gaussian. What they quote is something like a 95% confidence limit. For Gaussian errors this is +/-2sigma. So 1sigma is half of 0.002, which is 0.001. I explicitly asked Dale Fixsen if that was what I should use, and he said if I insisted on a 1sigma error bar, then that's the best he could suggest!
What experimental evidence do we have that the CMB fills the entire
universe as required by theory?
Submitted by vorleons"AT"hotmail.com 7/07
I don't think this is "required by theory". The ubiquity of the CMB is what drives the theoretical picture (not the other way around).
The CMB is observed to be very close to isotropic, i.e. the same in every direction. Since CMB photons travel at the speed of light, then it would take an extremely controived universe to only have CMB photons arriving at us here and now from every direction, without having them everywhere else too! In fact the tiny amplitude CMB anisotropies have a pattern which constrains details of the cosmological model - and part of that is the need for radiation which fills the Universe and which dominated the total energy density at early times. I can't think of any way of avoiding having CMB photons everywhere.
There's a question that arises if you consider the CMB in the context
of thermodynamics:
The energy of a "photon gas" - Bose gas with zero chemical potential -
is a constant times T^4 times the volume. If T scales like 1/R, and V
scales like R, the total energy decreases like 1/R. If the CMB is a
photon gas, the expansion is actually isentropic (V*T^3 = const), or
reversibly adiabatic.
The question is where does this energy go? It has to be work done by
the gas expanding - but against what? It isn't interacting with anything
since the time of decoupling.
I should add that the "classical" assumptions - photons in a container
with volume V and temp T - don't necessarily apply to the CMB. There is
no container and, since the CMB doesn't interact with anything since
recombination, the numer of photons is constant. But if it's a dilute
Bose gas with a nonzero chemical potential, the spectrum isn't black
body.
Submitted jfjanak"AT"verizon.net by 8/07
This is a good question, which shows that you've been thinking hard!
There are several answers to similar questions already on this page. The reassurance that everything must be OK, comes from the fact that General Relativity (the theoretical cornerstone for our models of the large-scale behaviour of the Universe) manifestly conserves energy, at least locally. So if I consider any small volume of the Universe, then the energy "lost" from the CMB goes into "work done" in the expansion (since there's a pressure and a changing volume, then there must be "PdV work").
But you also raise the issue of how to regard the Universe, since it isn't really a closed system, except that in a sense it's the most closed system there is (since it is in fact everything!). So one can get into knots trying to think about the "total energy" in the Universe. But the picture we have of simple homogeneous spaces means that every bit of the Universe is (on average) the same as every other bit - so that tells you that the best way to think about the Universe is just to consider any little bit of it, and the same will apply to all the other bits.
Since we have moved from the Big Bang much more slowly than any radiation
given off at that time, why is there still CMB present in the universe?
(It ought to have outpaced the growth of the universe immediately and
disappeared into the void that the universe is expanding into.)
Submitted by bobjt"AT"talktalk.net 8/07
This is the most asked question about the CMB!
I encourage you to seek out many other answers on this page. But the short answer is that we haven't "moved from the Big Bang" at all, and that you have a mental picture which has to change before you'll be able to come to grips with this question.
The Big Bang happened everywhere. There are distant parts of the Universe which are right now detecting the CMB photons that our region of space gave off at very early times. If you can understand that, then you're a long way towards getting the right image for the expanding Universe that we live in.
Is it fair to say that today's microwave background is the redshifted
version of what were originally gamma rays?
That is, that the Big Bang produced gamma rays which filled the small
early universe, and that expansion caused these original gamma rays to be
converted to microwaves?
Submitted by mdrnhart"AT"yahoo.com 8/07
That's right! Today's CMB photons were once much higher energy gamma-rays. They've been redshifted down to lower energy as they travelled through the expanding Universe.
The second part of your statement is perhaps not as clearly correct, however. The early Universe wasn't "small". You should think of it as being very very big (even at early times) - but always expanding. So those early gamma-rays were everywhere (and moving at the speed of light in random directions from each point) - with the resulting CMB photons being everywhere today.
Several hundred years after the Big Bang the photons were decoupled
from the matter. Let us consider N photons that decoupled at that time and
follow them during billions of years until they arrive to our detectors.
The temperature was much higher than it is today, thus each of these
photons had higher energy. They have traveled in the Universe without
interaction for several billion years (thus they did not scatter, they
were not absorbed etc.). Finally they arrive to our detectors. However,
their energy is much lower now than it was earlier. WHERE DID THIS ENERGY
DISAPPEAR, if they had no interaction with anything? Does this mean that
in an expanding Universe the energy conservation does not hold anymore?
Generalising: does this mean that the total energy of the radiation in the
Universe is much lower now than it was when the radiation has decoupled?
(Not considering obviously the radiation coming from the stars.)
Submitted by sukosd"AT"reak.bme.hu 11/07
This question is answered several times on this page already! The short answer is that the energy per unit volume is conserved, since there is a "work done" contribution due to the expansion. In other words one could alternatively ask: "Since there's a pressure coming from the photons, and also the volume is changing in an expanding universe, then where do I get the energy from to cause the expansion?" The answer to that would be: "from the fact that the photons are losing energy!"
In certain books the "cooling down" of the CMB is explained as a
consequence of a "scaling" in the expanding Universe. They say, that
because of the expansion of space every distance is scaled by an universal
factor. Since the wavelength is also a distance-like quantity, it is also
scaled, causing the whole CMB to shift toward larger wavelengths. My
question is: if we take serious that EVERY distance-like quantity is
scaled, then for example the radius of the H-atom (and many other physical
quantities) should scale as well, since it is also distance-like! This is
probably nonsense, since the radius of the H-atom is determined by
fundamental contants (e, h, c, electron mass, etc.). If it scales, then
either these fundamental contants should scale somehow as well, or the
physical laws should change with time... I think that none of these could
be accepted.
Submitted by sukosd"AT"reak.bme.hu 11/07
This is an excellent question! It shows the sort of thing that can happen when one gives a relatively simplified answer to a basic question, but the questioner then goes on to think about a deeper question, which shows the limitations of the pat answer!
It's really space that's expanding, not the things within that space. So atoms etc. are not getting stretched. However, photons are kind of special, because they are purely energy, being defined only by their wavelength. The rest mass of a particle (for example) isn't changed at all by the expansion of the Universe, but photons have no rest mass. When they travel through space their wavelengths really do get stretched, while the properties of more ordinary particles do not.
If that still sounds a bit suspicious, then the real answer is that there's a rigorous mathematical picture underlying all of this, which is General Relativity. And when you follow photon trajectories in an expanding medium within General Relativity, you do indeed find that they lose energy. This is of course confirmed empirically by Hubble's law, which is just that the redshift of a distant object comes from the fact that this "universal factor" was smaller when the photons left the source.
If the wavelength of the photons of the CMB radiation were
"redshifted" during the years they travel, it is plausible that the same
redshift occurs to the photons emitted by distant stars. This redshift
does NOT depend on the actual velocity of the star, since it depends only
on the time the photon travels until it hits our detector. Question: is
THIS universal redshift taken into account (deduced) when determining the
velocities of distant stars? With other words: when we determine Hubble's
constant, is this universal redshift of photons taken into consideration?
Submitted by sukosd"AT"reak.bme.hu 11/07
The redshift measured for a specific source of radiation has several distinct contributions. Physically you can think of the main "Hubble flow" redshift, which comes from the ratio of the universal "scale factor" between the time of emission and observation of the photons. But secondly there's a contribution coming from the "peculiar velocity" of the source relative to a smoothly expanding Universe. And thirdly there's the peculiar motion of the observer, which includes the rotation of the Earth, orbit of the Earth around the Sun, etc.
See in this way, I think it's obvious that there has to be these additional "Doppler" redshift contributions. That's because the answer you get must depend on whether you make a correction to a frame in which the Earth isn't rotating (a small, but measurable effect), then there's the 30 km/s motion of the Earth around the Sun (so that normally redshifts are expressed in a "heliocentric frame"), and for extragalactic objects one must decide whether to remove the Sun's orbit around the Galaxy, the Galaxy's motion relative to nearby galaxies etc.
Assuming that the Universe will expand forever, what will eventually
become of the CMB?
Submitted by Lcep72"AT"aol.com 12/07
It will continue to cool as the wavelengths stretch, and so will become the CRB (Cosmic Radio Background).
My question is that whether Big
bang theory has any thing against the assumption that there 'universe of
anti-matter'? and if at all an anti-universe exists, is there possibility
of CBR? if so, what can be its nature; red shift or blue shift?
Submitted by saumya_mohapatra"AT"yahoo.co.in 12/07
I'm not sure I fully understand the question - but let me try to answer it anyway!
Ever since anti-matter was discovered there have been suggestions that maybe there are parts of the Universe filled with anti-matter, just as our part is filled with matter.
There are strong constraints on such ideas, particularly when you realise that there's nowhere in the Universe which is truly empty (e.g. the vast intercluster medium pervades all the space between galaxies, with a density of hydrogen and helium which is only a thousand or so times less dense than in clusters of galaxies) - so there are no "voids" to separate the speculated regions of anti-matter from the regions of matter. That means there should be gamma-rays being formed where the regions meet - and we don't see anything like this.
Inventing deep voids just to separate these regions of matter and anti-matter sounds pretty contrived, and not much like the dynamical Universe we live in. The voids would have to be big enough not to have changed much in the age of the Universe. And if they were that big they'd strongly affect the CMB anisotropies on large angular scales - which again we don't see.
So the idea that there are anti-worlds etc. out there, although it has a certainly science-fiction-type appeal, is not something which appears to happen in the real Universe!
In fact there's a big puzzle from the "hot Big Bang" picture, which is why the Universe chose to be mainly matter, rather than a balance of matter and anti-matter - but that's another question that I'll leave for another time!
I was wondering if you know the mass density of radiation from the CMB with
temperature 2.73K.
Any information on how to calculate this would be much appreciated.
Submitted by thomas.carpy2"AT"mail.dcu.ie 01/08
This sounds like someone asking for help on a homework question! I'm afraid I have a very strict rule about not doing other people's homework without getting paid!
There's an old joke about how you use a barometer to tell the height of a building. Along those lines, the answer to this question is: "Find the owner of the Universe and tell him that you'll give him a very nice Cosmic Microwave Background of temperature 2.73K if he'll tell you its mass density".
...However, I personally am unconvinced that CMB came from the Big Bang itself
(please understand that I am not necessarily questioning the big bang
itself...I'm just questioning CMB as EVIDENCE of the Big Bang). To me, CMB
looks like just another manifestation of Dark Energy! In fact, I think that if
Dark Energy had been discovered before CMB, then CMB never would have been
attributed to the Big Bang. You have to admit it, the two (CMB and Dark Energy)
seem quite similar, at least in there pervasiveness throughout the universe and
the timescale of their existence (I'll talk more about the latter shortly).
[abridged]
Submitted by chadswhite"AT"juno.com 01/08
Actually the CMB and the Dark Energy are physically completely different. The CMB is composed of particles moving at the speed of light, which have a significant positive pressure. This turns out to mean that in a Universe with an expanding "scale factor" (i.e. the Universe in which we appear to live) the energy density of the CMB decreases as the 4th power of the scale factor. The Dark Energy, on the other hand, appears to be similar to a "cosmological constant" or pure "vacuum energy". This has a negative pressure, which means that in an expanding Universe the energy density doesn't change at all. This Dark Energy behaves as a very bizarre sort of stuff, entirely different from what we normally think of as "matter" or "radiation". Hence this new name was coined, to emphasize the distinction.
Another major difference is that although you can "squeeze" the CMB (so that bits of the Universe are a little bit hotter or colder than other bits), you can't squeeze the Dark Energy (or at least not very much) - it's pretty much a uniform fluid that fills the Universe.
Another issue that makes me doubt the link between CMB and the Big Bang is the
following: According to current theory, the microwaves of CMB would have been
generated ONLY during a certain early phase of the Big Bang. Why then are we
STILL getting hit by the microwaves? Is it just a big coincidence that we happen
to be alive during the (cosmologically) brief period in which the radiation
generated from this early event hit the earth? If we had lived a billion years
ago, would this burst of radiation not yet have arrived? If the human species
still exists in a billion years, will the burst be over?
Submitted by chadswhite"AT"juno.com 01/08
You've just posed the single most asked question about the CMB! It occurs in several different forms on this page, and so I encourage you to look for the answers.
The fundamental resolution for what you are thinking is that your mental image is wrong. The "Big Bang" is not a specific place, but is everywhere!
I would
just like to ask a few questions that have been puzzling me, and i need
to answer for an assignment could you help me please?
. At the point of recombination how much had the CMBR deteriorated from
its original gamma rays?
. Is it possible to calculate the rate of degridation from the CMBR now
and hence determine when it might end?
[abridged]
Submitted by onephatwookie"AT"hotmail.com 01/08
I try not to do people's homework for them!
There should be plenty of information on this page (and others on the internet) to help answers those sorts of questions.
I just selected out a couple that were posed here, since I don't really understand them - but on the other hand they might still be interesting to answer!
It's true that CMB photons were once gamma-rays, of arbitarily high energy. They "formed" (in the sense of when they stopped being significantly created and destroyed in particle interactions) when the Universe was about 1 year old. So I guess you could consider this to be when the photons that we detect today came into existence. Then you could figure out the temperature at that time, and hence the ratio of typical photon energies today compared with back then.
But the photons continued to scatter off the matter (although not changing energy very much) until the "last-scattering epoch" which was at an age of about 400,000 years. The temperature at that time was about 3000 Kelvin and the photons at that point were optical (or mildly infrared, with typical wavelengths of about 1 micron). You could also calculate the factor by which the photons have lost energy since that time, and the answer is around 1000 (which is the redshift of the last-scattering epoch).
As for what will happen in the future, well the CMB photons will continue to lose energy forever. Or perhaps, in a vacuum-dominated model, they'll eventually have wavelengths around the size of the observable Universe, and hence the concept of a CMB temperature will eventually lose meaning. But that's quite a long way off, when they've reduced in energy by another factor of around 1028 - and who knows what else might have happened by then!
They say that the total mass-energy density in the universe is about 9.9 x 10-30
g/cm3. According to the following link, it breaks down thus: 4% Atoms, 23% Cold
Dark Matter, 73% Dark Energy.
http://map.gsfc.nasa.gov/m_uni/uni_101matter.html
My question is: Is the CMB even considered in this break down? If so, in which
category does it belong? If not, why is the CMB not considered in the total
energy density of the universe?
Submitted by chadswhite"AT"juno.com 02/08
When you work out the numbers, a 2.725 Kelvin thermal spectrum of photons corresponds to an energy density which is quite negligible compared with the energy density in matter and in dark energy.
The CMB contribution to that budget is about one hundredth of one percent, i.e. a fraction of around 10-4.
However, in earlier stages of the history of the Universe things were quite different. That's because in an expanding Universe the ratio of energy densities of radiation and matter decreases with time. So at earlier times the photons were more important. In fact if you go back early enough the photons made a bigger contribution than everything else, and we call this the "radiation-dominated" phase. This occurs when the Universe is younger than about 100,000 years.
when we look at CBR, are we seeing microwaves that have been
travelling TOWARDS us since the moment the universe stopped being opaque,
or are we picking up microwaves that are bouncing around (and have been
bouncing around since the universe stopped being opaque) the same "chunk"
of space that we now inhabit, i.e. they really are BACKGROUND and have
simply expanded along with us as our part of the universe expanded?
Submitted by andy_kovacs"AT"yahoo.co.uk 02/08
It's the former. We are seeing microwaves which have travelled straight towards us since they last scattered at this opaque era (the "last scatttering surface"). We're seeing the early Universe in all directions, because space has always been really, really big!
Hi...I was just wondering if you knew what the relationship is between the CMB
and why a machine can never run at 100% efficiency.
Submitted by CYVERIA1"AT"MOUNTAINCABLE.NET 03/08
I'm not sure there's any direct connection. They are both related to the laws of thermodynamics - but that's about it.
Unless someone else ses that I'm missing something here?
[follow-up question]
......the answer was in the laws of thermodynamics.
At one point the article spoke of heat loss, etc. It went on to say 100%
machine efficiency would have to be at absolute zero. My understanding of 0
degrees Kelvin is that it can never be achieved because of the 2.7 degrees
Kelvin from the CMB.
Submitted by CYBERIA1"AT"MOUNTAINCABLE.NET 04/08
It is possible to reach temperatures below that of the CMB. It's done all the time in low temperature laboratories, and in fact the detectors used to study the CMB are usually cooled to well below 2.7 Kelvin!
Of course it isn't easy to reach such low temperatures, requiring lots of energy (and increased entropy, and all those other good thermodynamics ideas!). It would be genuinely hard to do it for very long, since the CMB is everywhere, and so if you wait long enough (in laboratory units, but certainly short on a cosmological scale!) your experimental system will tend to reach equilibrium with the CMB.
i have to write a small esay about Cosmic Microwave Background Radiation
and the question that i have the answer is what is it. so can u tell me
in general terms what is it?
Submitted by antoine_washington1@yahoo.com 04/08
It's a background of radiation that comes from the Cosmos!
But seriously, you can find the answer by reading the basic page which is above this one, i.e. here, and by reading answers on this very page.
Hi, I read your FAQ on the CMB online and i had a couple questions. so
lately i've been trying to understand this theory. I understand that we
can sense this uniform background radiation in uniform all across the
universe. And this radiation is evidence of the early universe, but how
exactly is it evidence of the early universe? we know the universe is
expanding from the red shift in galaxies. But i guess i'm just confused
on how exactly the CMB confirms the big bang. in simple terms if you
could just fill that info in it would be appreciated. and the radiation
is at 2.73 kelvin today (does the 2.73 represent what it is today or many
eons ago?), how does that tell us of the early universe? that the
universe is cooling because it is expanding?
Submitted by tbmcquade@gmail.com@yahoo.com 04/08
The basic fact is that the CMB has a spectrum (i.e. brightness as a function of wavelength) which is extremely accurately described by a "blackbody" shape (i.e. the shape that a body has which has a single temperature). The only explanation that we have for this is that the radiaton came from a time when the whole Universe was in good "thermal equilibrium" - a situation which you get very naturally if the Universe used to be very much hotter and denser. The alternative, "obvious" source of the CMB would be some relatively local cool material which is distributed more or less uniformly - however, there's no way to do this without getting emission or absorption features, so all such ideas were ruled at least 30 years ago.
On top of that, we now have precisely measured CMB anisotropies, covering a wide range of angular scales. These are very well fit with a model of evolving density perturbations in a universe which starts off hot and expanding. So it is very hard to avoid the conclusion that the Universe was once very hot and that our basic understanding of its evolution is in good shape (although of course, there are still many unaswered questions).
In an expanding Universe which contains radiation, it is expected that the radiation is cooling with time, and hence was hotter at earlier times. So the CMB fits in very well with the expanding Universe picture, or what is often referred to as the "Hot Big Bang picture".
can't seem to find if CMB is flat , ie freq v. amplitude, or decreases with freq
or if it has a peak
at some freq. What I'd love to see is noise temp v freq, so that if I had a
perfect mw antenna,
perfect lna with 0db NF and both flat from say 50MHZ to 10 GHZ, what would I see
looking
at it wqith a perfect spec an with differebt IF bandwidths.
Submitted by patr@tac.com.au 05/08
The CMB spectrum is a pretty much perfect "blackbody spectrum", with a low frequency slope, a peak, and then a high frequency exponential fall-off. You can look up the function in any basic physics textbook (or on the internet).
The precise form will depend on the units you are using. If you are using "noise temperature" in the radio (long wavelength) part of the CMB spectrum, then noise temperature is constant with frequency.
If I look at a plot of the Hubble data, and want to include the microwave
background as data, I have to extend the x-axis out to 14 billion
parsecs. If I then extrapolate the linear Hubble trend to 14 billion
parsecs and read the recession velocity from the y axis, I read 1,040,000
km/sec. This is about 1/3 the velocity of light. Is this correct? Before
I conducted the exercise, I was expecting that the microwave recession
velocity would be very close to the velocity of light (not equal to c
because then the redshift would be infinite). If this estimate is
correct, is there some way to understand why this number and not some
other. Does it make sense to think of the source of the CMB to be
receding same as any other distant source?
Submitted by Dave.Kennedy"AT"Inteq.com 06/08
If you try to interpret the very distant Universe in terms of a recession velocity, you quickly get in trouble. This is because we're dealing here with General (not Special) Relativity, and you also have to take into account that as you look at distant objects you're looking back in time (so it becomes unclear when you are measuring the speed!). If you do the full calculation within the expanding Universe model, then you find that the CMB photons were indeed emitted from a surface which is expanding away from us at close to the speed of light.
However, things are actually much simpler if looked at in terms of expansion rather than velocity. The Universe is expanding uniformly, which means at earlier times everything was closer together than they are now. The CMB photons were last scattered when this "scale factor" was about 0.001, meaning that the photons have been redshifted by a factor of about 1000. That's why we observe the "hot" early Universe in cool microwaves.
Does the cosmic background radiation permeate,travel through the
planets or just surround it ?
Submitted by info"AT"drnabilhassan.org.uk 06/08
The planets (and stars for that matter) are opaque to most radiation. So just as you can't see stars through the Moon, you can't see the CMB photons either. The planets (satellites, asteroids, etc.) also emit microwave radiation of their own, however. This is because they are glowing at some fairly cool temperature, so that they give off radiation which typically peaks in the far-infrared part of the spectrum. These bodies are actually much brighter than the CMB, and that can make them useful as standard sources of radiation which can be used to calibrate CMB experiments.
I am also a bit puzzled about the fact that the CMB is centered on the 3K
wavelength. This is said to translate into a radiation that originates from the
universe when it had a temperature of 3000K. As the universe cooled is seems to
me that there ought to be radiation originating at every degree lower than
3000K, e.g. at 2900, 2800, 2700, etc. degrees. With the redshift caused by the
expansion this should translate into something like 2.9, 2.8, 2.7 etc K. That
is, there ought to be a continuous CMB at wavelengths larger than the 3K
wavelength as I see it. Why isn't that the case?
Submitted by carl"AT"ullerup.com 07/08
This is because the CMB photons are last scattered at redshifts around 1000, when the temeprature was about 3000K. After that the photons travel freely towards us, stretching in the expanding Universe. So we are seeing back to this early time of intense scattering (between radiation and matter). We can't see radiation coming from, say, redshift z=100 when the temperature was about 300K, because there was no interaction with matter back then. Although of course we can if we like consider the CMB photons as being 300K radiation from z=100 or 30K radiation from z=10, or whatever, since it's all equivalent to seeing 3K radiation today. The point is that the radiation only last scattered at z=1000, and so that's the surface around us where we see the matter distribution being imprinted on the CMB anisotropies.
I have no idea ?how this may sound to you ?or how unusual this may be,
but I believe I can see the cosmic microwave background. ?For ?as long
as I can remember I've seen static everywhere, all of the time. ?A few
years ago I discovered that other people didn't see this static, so I
went to eye doctors to out if there may be a problem ?with my vision. ?I
am near-sighted, but I see the static better with my contacts in and
there was ?no sign of any other problem. ?I a feeling that this may be
of interest to some one. ?I don't really know who to contact. ?I would
like to find out more about this. ?Is it even possible that I could be
seeing CMB?
Submitted by brooke_simpson27"AT"yahoo.com 07/08
I'm sure you are experiencing some sort of sensation in your vision, but it isn't possible that this is the CMB. The energies involved are dramatically lower (more than a factor of 1000) than what is required to excite the optical receptors in our eyes.
Human vision is a complicated thing, and not something I'm an expert on! However, I would suggest that you try to find more information on the internet to learn about how vision works. Hopefully you'll find something that explains what you may be experiencing.
why is it no one has thought of cosmic B.R. as an alternative sourse of
energy
Submitted by info"AT"drnabilhassan.org.uk 09/08
Although the CMB is the dominant photon background in the Universe, it is completely negligible by terrestrial standards. We live in a very special part of the Universe! We are in a very overdense galaxy, very close to a bright star, on a rocky planet, surrounded by gases, etc. The CMB is the same here as it is everywhere, but there are spectacularly higher local sources of energy (specifically coming from the Sun or from radioactivity from previous generations of stars).
Was CMBR temperature 2.725 K calculated for the local potential where CMB was
investigated (taking into account CMBR blueshift due to potential wells of the
Earth, solar system and the Galaxy as a whole) or evaluated in homogeneous
space? In other words, what would be CMBR temperature measured in a place where
it is possible to neglect a curvature of space? As within the Galaxy the
average density is around a million times the average density of the Universe,
the original small fluctuations of density-temperature became huge (while
matter have clumped) and a difference between these temperatures expect to be
essential.
Submitted by astronet"AT"yandex.ru 09/08
Although the local density is very much higher than for average parts of the Universe, the gravitational potential (or equivalently the curvature of space) is not very different. You need to be a black hole to have a genuinely large amount of curvature (and hence a large "gravitational redshift"). So the CMB temperature measured at the Earth only differs from the "empty Universe" value in about the 8th decimal place.
You say that the density of the cosmic microwave background radiation is being
diluted as the universe expands. How do you know this? Is there some
experimental data that shows that this radiation is becoming less intense over
time?
Submitted by chadswhite"AT"juno.com 10/08
It's really required in an expanding Universe. And several things would go completely wrong (e.g. fitting the CMB anisotropies, measuring the Sunyaev-Zel'dovich effect) if it were not true.
But there's also some direct proof, which comes from making estimates of the CMB temperature in very distant gas clouds (i.e. at high redshift, z, where the CMB temperature should be (1+z) times higher). These estimates can be made from looking at the ratios of the strengths of particularly molecular lines in the gas clouds. This gives the local "excitation temperature", which can't really be lower than the CMB temperature - hence the lower envelope of these measurements should track 2.725 × (1+z)Kelvin, which is indeed found to be the case.
I don't mean to beat this topic to death, but it is important for me to
understand this better.
First, let me see if I understand the direct evidence. You're saying that they
have measured the lowest temperature of distant galaxies, and it came out to be
about 2.725 K? Is this what you're saying?
Also, I don't understand how the interaction of high energy electrons with the
CMB (Sunyaev-Zel'dovich effect) shows that the intensity of the CMB is getting
weaker with time...
Lastly, how does anisotropies of the CMB show that the CMB is being diluted?
Submitted by chadswhite"AT"juno.com 10/08
Temperatures measured in very distant galaxies are consistent with 2.725 × (1+z)Kelvin, not with 2.725 Kelvin. So for example in a galaxy at z=2 you see temperatures around 10 Kelvin.
The "Sunyaev-Zel'dovich effect" is like a "hole" in the CMB caused by scattering through the hot gas in clusters of galaxies. The spectral shape of this signal is what you expect if the gas is scattering CMB photons which have a temperature of 2.725 × (1+z)Kelvin for a cluster at redshift z.
The detailed "power spectrum" of CMB anisotropies is very well explained by a model having about 6 parameters within the "hot Big Bang" framework, meaning that the Universe used to be hotter and the CMB has been cooling. This is a crucial part of the whole picture, and there's no way to make sense of the CMB anisotropies without the "last scattering surface" at redshift around 1000.
I have a question concerning our conception of space in the big
bang theory. Are we entitled to ask this question: where did the big bang
happen? If it happened at a certain point in the universe far from us, how
come the CMB is coming from all directions? and how come the earth reached its
current position before the CMB photons reached it?
Submitted by T.elsayed"AT"ThPhys.Uni-Heidelberg.de 12/08
Yes you are entitled to ask that question!
The answer is that the "Big Bang" happened everywhere at once. If you can get your head round that, then you've come a long way towards understanding how the Universe works! And you can find more detailed answers on this page.
Another question regarding the CMB photons, if their frequency has
decreased from the instant of their creation till now due to the expansion
of the universe, where did their energy difference go?
Submitted by T.elsayed"AT"ThPhys.Uni-Heidelberg.de 12/08
It goes into exapnding the Universe, which takes energy!
What causes us to be able to see the CMB. What material is it reflecting
off of ?
Submitted by Kenneth.C.Herr"AT"aero.org 12/08
The CMB photons are scattering off charged particles which existed in the hot early phases of the Universe. We are seeing them when they last scattered off this plasma about 13 billion years ago!
I am aware of the cosmological principle,but if all aroud us at approx 14
billion light years is the cmb ;doesnt that infer we are in the middle.Forgive a
layman Im sure theres a reason this is so,but I had to ask .Its something
Im not getting
Submitted by Toonheid"AT"aol.com 12/08
We're in the middle of our own "Observable Universe"!
We see the CMB photons which are reaching us right now from all directions, having travelled for (most of) the age of the Universe. Other observers will be seeing other parts of the early Universe, and will be at the centres of their own patches.
Is the strength of the microwaves now constant or still diminishing??
Submitted by jawepret"AT"blueyonder.co.uk 01/09
The CMB is still diluting as the Universe expands. It will continue to cool forever! Although the timescale for this variation is very, very long, so the change in the CMB temperature over your lifetime would only show up in about the 8th decimal place.
Where can I find the audio or sound of the Cosmic Microwave Background
Radiation?
Submitted by wynnwolfe"AT"pacbell.net 04/09
Of course the CMB isn't sound, it's a form of low energy electromagnetic radiation. However, it can be represented as sound, and this has been done in a few different ways, which are pointed out elsewhere on this page.
My question is: with the CMB sitting out there at 13.7 LY, all we can see
or experience is its translucent image. If this can be spoken of as
the outer limits of our ability to see "the beginning", can a case be made that
it too is now "clear" in real time? If we could see through this CMB what
would it show us?
Submitted by stephenbierce"AT"yahoo.com 04/09
That is of course a very hypothetical question!
The answer depends on which bits of physics you're willing to push aside, and which bits you're happy to keep. So I don't have a specific answer.
If the question is something like "given that we see the CMB sky pretty much as a snapshot at 380,000 years after the Big Bang, what would it have looked like if scattering was a bit less and we saw it instead at some earlier epoch, like 100,000 years after the Big Bang? or 1,000 years?"
The answer to this more specific questions is that if the CMB "last scattering surface" was (for some reason) a bit earlier, then obviously the CMB sky wouldn't look drastically different. The pattern of anisotropies would have a different dependence on angular scale, that's all.
If you're imagining seeing back to much earlier times, then the question is much harder to answer! That's because the physics you'd have to change to affect the transparency of the CMB would also affect some of the other things going on, like the oscillating sound waves, or potentially things like the coupling of neutrinos, the balance between matter and antimatter, the details of nucleosynthesis, etc.
The CMB photons that we see were created (although there's some ambiguity here over precisely what is meant by "created") something like a year after the Big Bang. So in a sense we're seeing the Universe as it was at about that time, except for photon trajectories being scattered.
The fluctuations that we see on the CMB sky were probably generated much earlier than this. So there's also a sense in which we're seeing conditions that existed when the Universe was only a fraction of a second old. If we ever manage to probe primordial gravity waves, then we can claim to be seeing back to an epoch when the Universe was "opaque" to gravitational radiation, which is this same very early time (perhaps when the Universe was inflating).
I have a couple of questions about the Cosmic Microwave Background Radiation
Experiment.
1.) What is the Hypothesis of the Experiment? ... [abridged]
Submitted by j.torrez44"AT"yahoo.com 06/09
Although CMB instruments are "experiments", they are a little different from the traditional "scientific method" type of experiment. The usual model we have for how science progresses is that one designs an experiment to test a hypothesis, refines the hypothesis, designs new experiments, etc. But astronomy (i.e. the study of the heavens) is a bit different from this (in fact all of science is a bit different from the simplistic "scientific method" - and a good thing too, otherwise it would be very boring!). In astronomy you can't go and visit the objects you are studying, and so you can't change them to see what will happen. You just observe what you can and then draw inferences.
CMB experiments are motivated by a desire to learn about the Universe on the largest accessible scales. There is a fairly simple model which describes the main features of the Universe, and that model is constantly being tested by such experiments. More importantly though (given that the basic model seems to work quite well) the parameters of that model are being fitted using the CMB data. If there is no good fit, then that will tell us that we need to adapt the model - which is how progress is made in understanding how the Universe works.
Is there a way to generate a plot of background temperature vs time from last
scattering to now? Or is there such a plot already made that I could look at?
Submitted by edweinb"AT"sbcglobal.net 06/09
The CMB temperature is just inversely proportional to the "scale factor". In terms of redshift the relationship is simply T(z)=T0 (1+z), where T0 is the temperature today. Epochs in the early Universe can be described by the redshift that they would be observed at by us - so we live at redshift zero and the very earliest moments have very high redshifts.
The complication with plotting the CMB temperature versus time is that the relationship between time and redshift depends in detail on the cosmological model. If you specify exactly how much dark matter, dark energy etc. you have, and the precise value for the expansion rate today, then the relationship between redshift and time is calculable. But if you decided you preferred a different amount of dark matter (for example), then the relationship would be a bit different. For a Universe with multiple components making up its density, like the Universe we appear to live in, the z to t conversion comes from doing a numerical integral. So there isn't a simple formula that can be written down here.
However, here's a plot for something close to the currennt best-buy cosmological model. You should be able to combine this with the value of the CMB temperature today to figure out what the CMB temperature was at different times in the past.
I have question about one of the Horizon problem, which some boks state it as
most crucial problems of the this theory.
As I have understood this problem, it can be stated in two sentences:
1- There are some points in the Universe which are not causally co-related
2- What we see in the CMB is that these points have the same temperature, and in
the explanation of this "same temperature" , we say that these points had been
in thermal equilibrium at the time of the CMB photons formation, and so these
points should have been causally related.
So, we see a kind of contradiction in the above sentences, and this is the
horizon problem.
What I can not figure out is that why do we need to say that these point had
been in thermal equilibrium? These points had started from the same initial
conditions and so they should have the same temperature without being in thermal
equilibrium themselves.
Submitted by anariman"AT"interchange.ubc.ca 06/09
Remember that the Universe didn't start out as a "point", but has always been large! This matters for precisely the issue that you are trying to understand here. When you work through the business of causality (i.e. objects being within the light-crossing distance of each other in the age of the Universe), you find that the causally-connected distance always grows with time in the usual expanding Universe.
This observation is important - because it means that if two points are only just becoming causally connected today, then they can never have been causally connected in the past. Expansion of the Universe just isn't fast enough to beat the growing size of the light-travel distance.
Hence the opposite sides of the CMB sky, which even today aren't quite in causal contact with each other, were horribly out of contact the earlier and earlier you consider things.
There are 2 possible ways out of this dilemma. The first is to say "well, maybe the initial conditions were just set up in some acausal way, so that everywhere had the same temperature at some early time". But that kind of seems like giving up! Unless of course you have a good explanation for the magical process which achieves these early conditions.
But the second possibility is more promising. "All" you have to do is arrange for objects to fly apart faster than the speed of light, so that they start off within causal contact, but then get very far apart. This idea is called inflation, and it turns out to be a very promising idea for understanding what might have happened in the first tiny fraction of a second. As well as solving this "horizon problem", it also gives a fairly natural explanation for why there is any structure in the Universe at all, by stretching quantum fluctuations to become macroscopic perturbations in density!
I read of a study that shows the CMBR temperature 11
billion years ago was 9K. Assuming that the only mechanism for cooling of the
final scattering radiation is wavelength stretching due to expansion, I would
have expected the CMBR to be much higher. After only 1.7 billion years I would
have expected that the background would have still been visible glowing. What
am I missing here?
Submitted by edweinb"AT"sbcglobal.net 06/09
No, you have the timescales quite wrong. The CMB came out of the visible part of the spectrum at around the time of the "last scattering surface". This corresponds to a redshift of about 1000 and a time of about 400,000 years after the Big Bang.
Would the redshifting of the peak of the cosmic background do anything
interesting at various ages of the universe if it
matched an absorption line in some material like hydrogen or helium (or would
the peak be so weak that it would not
make so large a difference?) I mean that running recent history backwards the
peak would reach into the infrared at
sometime and cause things to be baked, no? [abridged]
Submitted by smkolins"AT"mac.com 07/09
The short answer is that any such effects are small enough to be almost negligible.
The longer answer is that if you care about the details, then there is some quite interesting physics associated with the interaction between the CMB and hydrogen and helium atoms as the Universe cools. The "recombination" of hydrogen and helium leads to weak (and broad) emission lines in the spectrum of the CMB. And the interaction between this spectrum and the energy levels in hydrogen and helium can even lead to some absorption features. I've written papers on this topic, as have several other people! A readable summary of recent work on this topic can be found here.
what are the best theories to date with what lies beyond the cmb?
Submitted by Electrodynamic"AT"hotmail.co.uk 07/09
We see the CMB photons coming from the "surface of last scattering", defined as the time when the Universe became transparent to these photons. At earlier times the atoms in the Universe were ionized, and ionized material interacts very strongly with photons. It's like trying to look into the Sun - you can only really see the surface layers, because it is too "optically thick" to see further.
The cosmic "last scattering surface" is at about an age of 380,000 years in the history of the Universe. Before that we have a very good picture of how the Universe had been expanding, getting cooler and less dense. This is usually called the "hot Big Bang model". It is very successful at explaining a great many observational facts about the Universe. However, it gets less an less clear as we try to ask about earlier and earlier times. One hope from the CMB is that we can detect the effect of gravitons as well as photons. The gravitons (or gravitational waves) only scatter at fantastically early times. And so if we can detect them (which is difficult although possible) we'll be "seeing" conditions in the Universe when it was perhaps less than a trillionth of a trillionth of a second old.
What is exactly is cosmic microwave background radiation is simple terms?
Submitted by bbsnsxo2"AT"hotmail.com 10/09
Read this page!
So what I do know is, that the existence of the CMB frame is not a problem
for general relativity, as the only thing that is special to it is, that the
CMB is uniformly distributed from this frame's point of fiew. Physics is
still the same fiewed from both, the CMB frame as well as from any other
frame one might choose.
The part I don't really understand yet is, that it is often said, that the
CMB frame defines a rest frame of the universe.
What exactly is meaned with "universe" in this statement?
(1) Is it the spacetime manifold itself? So would one really be able to tell
if he is moving with respect to space?? -- (what i doubt)
(2) Or is it the frame in witch all particles (including massles ones, like
the CMB-photons) are distributed in the most uniform way? So one would just
be able to say, if one is moving with respect to this frame, and not with
respect to space itself.
Submitted 11/09
The phrase "the CMB frame defines a rest frame for the Universe" is not really a meaningful statement! It is meant to simplify the situation for people who are first thinking about this topic. More sophisticated people will realise that it's not at all clear what it means (if anything!).
The correct picture is more like what you describe as number (2) in your question.
Can the CMB frame really be seen as a rest frame of all particles in the
universe - so for the CMB + all the matter and stuff that is flying arround.
My guess is: No, it can't. - I mean: The CMB is just one component of all the
stuff that was created. One cannot argue that the rest frame of this
component has to be the rest frame of the sum of all componetns as well.
So my guess is, that the CMB frame is really just the frame in that just the
CMB, and nothing else, is distributed uniformly. Not more and not less. So
in this case the grade "rest frame of the universe" would not be a really
good name to give to it at all.
Submitted 11/09
This seems quite perceptive to me, well done!
If you really want to read more on this kind of topic (at the kind of technical level) you could do worse than see a paper I wrote with Jim Zibin, which you can find here.
How do I get an intuitive
understanding of the origin of the acoustic peaks?
through the
phase when the
fluctuation bounces back to minimum temperature, the temperature looks like
a rarefaction? Shouldn't it just bounce back to its original size and
(over) density?Submitted anonymously by a senior colleague!
(These peaks in the "power spectrum" of CMB anisotropies are caused by the effect of large-scale sound waves around the time the photons were last scattered.)
I think that's a good question! The answer lies in the fact that the whole heuristic explanation is a bit confused about whether it's in real space or Fourier space. Plus it avoids any mention of gauge choice (ie exactly what you do above the horizon size) etc. So to some extent the basic picture doesn't stand up too well to such questioning! All that aside, I think it is possible to answer your question simply. You should think of this overdense blob as getting a kick from gravity when it comes inside the horizon (ie becomes causal) - thus it is a driven acoustic oscillation. So you would expect the "starting point" to be the equilibrium then the radiation falls into the potential well, ie it goes to maximum compression, then bounces back (due to the baryon pressure) to overshoot the equilibrium and become a rarefaction. I hope this helps!
I have been reading tons of
technical papers and the idea of an "angular power spectrum" keeps popping
up. It's hard to bridge the gap in this field between the extremely
low-level information and the extremely technical. What is this entity?
Submitted by jlh22"AT"dana.ucc.nau.edu
Different experiments are sensitive to different angular scales. Think of a CMB experiment as measuring a set of temperature of the sky in "pixels" of some angular size (set by the smallest scale which can be resolved by the particular telescope) over some region of the sky. The size of the pixel and the size of the region of sky are the smallest and largest angular scale that are probed. The information obtained from a CMB experiment is then the variation in the temperatures at some particular angular scale, or perhaps a range of different angular scales.
For the COBE satellite, for example, these variations in temperature were measured over a range of angular scales from about 7o up to the full sky. Other experiments tend to have higher angular resolution and also cover just a small fraction of the sky. When we put together a whole bunch of measurements at different angular scales we end up with an estimate of how the temperature variations change as we change the angular scale. A plot of this, ie "temperature variation" versus angular scale, is referred to as an "angular power spectrum" -- it's the amount of "power" in the temperature fluctuations plotted as a function of angle.
In fact you want to be more mathematically precise than this, and instead of "angular scale", you really want to deal with the amplitudes of a set of functions which are independent of each other and which probe different angular scales. Such functions for the sphere are called "spherical harmonics", and have well-understood properties. The spherical harmonic which describes variation over the whole sky is called the "monopole", while variations of 180o are described by the "dipole", variations on scales of 90o are described by the "quadrupole" and so on. The index of the harmonic is the "multipole" number, so that a small multipole number corresponds to large angle, and a large multipole number corresponds to a small angle. What is actually plotted then is the square of the amplitudes of these spherical harmonics versus the multipole number. This is the "angular power spectrum", and it can be interpretted as meaning the variation in temperatures as a function of angular scale (size of "pixel"), plotted so that large angles are on the left and small angles on the right.
I am having difficulty understanding how the odd acoustic peaks in the
power spectrum can correspond to fluctuation compressions, while the even
peaks correspond to rarefactions. It seems to me that
we should see equal power in compression and rarefaction.
Submitted by desai"AT"orca.astro.washington.edu 2/99
Ah, a technical question! The details of how the microwave background fluctuations vary with angular scale can be somewhat esoteric - however, the basics are pretty straightforward: the density perturbations were oscillating, and we catch them "frozen" at the last scattering epoch. The big bump in the spectrum corresponds to the scale that was first feeling the causal effects of gravity at that time, while smaller scales had already been oscillating for a while. Both compressions and rarefactions lead to enhancements in the temperature fluctuations at those particular angular scales, and so the "power spectrum of anisotropies" ends up as a series of bumps and wiggles.
The specific question (posed by desai) concerns the peaks that come from the compressions versus those that come from the rarefactions. The reason they're not exactly equal is that there's an effect from the regular matter (baryons) which essentially shifts the zero point. This is fairly well explained in the excellent summary called "The Physics of Microwave Background Anisotropies", by Hu, Sugiyama & Silk, which appeared in Nature in 1997, and can be found as a postscript file here. If this seems too complex, you might like to start with the article by Scott & White, called "Echoes of Gravity", which is here.
Why is the background microwave radiation found to be so smooth
since surely a lumpy distribution is necessary to explain the
very uneven distribution of matter in the universe?
Submitted by degs"AT"iafrica.com 10/99
This is a good question, and one which many people were asking prior to the detection of the CMB anisotropies in 1992. Up until then there had only been upper limits to the lumpiness of the CMB sky. The point was that it was difficult to construct models in which the COBE satellite would see complete smoothness, and that would be consistent with the idea that the structure we observe in today's Universe grew through the steady action of gravity. Many people suggested that if COBE gave further upper limits, then we would need to consider much more complicated ideas for how structure formed.
In fact the amount of variation observed in the microwave sky is almost exactly what was predicted based on the most popular models (where the Universe is dominated by some form of so-called Cold Dark Matter). So the CMB sky is just as lumpy as it should be (about 1 part in 100,000) to lead to all the lumpiness we see today, with the simple attractive force of gravity being all that is required to increase that lumpiness over the last 10 or so billion years.
Do the tiny CMB anisotropies constrain the mean mass density of the
Universe in any way, and if it does, why?
Submitted by ib6146"AT"bristol.ac.uk 11/99
The anisotropies in the CMB can be measured as a function of angular scale. Schematically, what you do is make a map of the CMB sky with a certain resolution of pixels, and estimate the variance of the temperature in that map - then smooth the map to a bigger pixel size and repeat. You end up with a bunch of numbers that tell you how "lumpy" the microwave sky is at various angular sizes (technically what is measured is called the "power spectrum", which is really just a better defined version of this same procedure).
It turns out that theories for the formation and evolution of structure in the Universe give quite detailed predictions for this "power spectrum". In particular, the simplest models (compatible with a theory called "inflation", which generated the fluctuations in the very early Universe) predict stronger anisotropies around a scale of about 1 degree on the sky. This seems to be pretty much what the most recent expermental results have been showing.
In a little more detail, the robust prediction of the models is for a characteristic length scale on the sky (corresponding roughly to the distance light can have travelled in the age of the Universe at the time when the CMB last scattered off matter). This length scale will look like a different angular scale on the sky depending on the "curvature" of the Universe. Since the curvature is governed by the mass-energy density content of the Universe, then the precise angular scale of maximum CMB variations on the sky depends on the density of the Universe.
The best available data suggest that these simple theories seem to be a good fit, and that the Universe must be rather close to being "flat" (ie not very curved at all). Since the matter that we can account for comprises only about one third of what is required to make the Universe flat, then there seems to be evidence from the CMB for some additional form of energy that helps keep the Universe flat. This is variously called the "cosmological constant", "qunitessence" or "dark energy". But just because it has several names, it doesn't mean we really understand what it is!
Could you give me the angle formed between the rotation axis of Earth
and the axis of the anisotropy dipole of CMB.
Submitted by mcrotti"AT"inlab.com.ar
As you no doubt know (if you've read any of the rest of this page at least!), there is a so-called "dipole" pattern on the CMB sky. In other words one half of the sky is a little hotter than average and one side a little colder. This is a Doppler effect, casused by our motion through the Universe.
Precisely what direction you give for this dipole depends on what other velocities you have subtracted off first. For example, we know that the Sun is moving around our Galaxy, so do you want the dipole in the rest frame of the Sun or the rest frame of the Galaxy? For the Solar System, the answer is that the motion is towards celestial coordinates RA (longitude) = 11.20 hours (or 168o), Dec (latitude) = -7.2o. Since this coordinate system has the "celestial equator" defined as the projection of the Earth's equator on the sky, then our motion is -7.2o from the equator. So the answer to your question is that the Solar System is moving through the Cosmos at an angle of approximately 83o from the Earth's South Pole.
In fact the annual variation in the Dipole, casued by the Earth's motion around the Sun, can also be seen by an all-sky experiment that lasts more than a year. This is a direct measurement of the Earth's motion, and in a sense proves that the Earth is going round the Sun rather than the Sun going round the Earth (not that this has been doubted for a few hundred years!). The COBE satellite detected this annual variation so well that it could be used to calibrate the instrument - since we know precisely how fast the Earth is going round the Sun (30 km/s), then we know precisely how big a variation should be measured.
What are the standard model CMBR anisotropies and how do they come about?
Submitted by mbliss"AT"oberon.ark.com 2/00
The CMB anisotropies come about from a combination of effects due to interaction of photons with perturbations in the density of the Universe on a range of scales. Various aspects of this question are discussed in answer to other questions on this page.
There is really no "standard model" as such. The general paradigm that seems to fit best is that the initial perturbations were of the sort produced during an inflationary period in the very early Universe (which isn't necessairly to say that inflation happened!). These evolved from being initially of roughly the same amplitude at each scale, to the situation we observe today where they vary with scale in a complex way. This variation encodes the values of a number of fundamental parameters which describe our Universe: the average density in each of the important components, including each form of dark matter; the expansion rate; the amplitude and scale-dependence of the initial density perturbations; the contribution from gravity waves; and some recent processing of the anisotropies by scattering in the reionized Universe, gravitational lensing and other effects.
The promise of the new CMB experiments is that we will ultimately know the values of these parameters. Assuming that this "standard paradigm" (inflationsry-insipred cold dark matter dominated Universes) continues to look good, then the experiments will nail down the parameters and provide us with a detailed "standard model". The alternative is that the general picture fails to fit everything, implying either that there is some crucial missing ingredient, or some whole new paradigm is required. The new CMB experiments should tell us that also.
After comparing several sources, I have noticed that there are different
numbers where isotropy is concerned...is it 1/1000, 1/10,000, or 1/100,000?
Submitted by JULIE108"AT"aol.com 4/00
It depends exactly what you mean by isotropy.
The CMB sky is totally smooth down to one part in 1000. At that level you notice that one side of the sky is hotter than the other side of the sky. This is the "dipole" caused by our motion through the Universe. So you can say that the sky is isotropic to 1 part in 1000.
When you subtract off this motion effect you find the remaining temperature map of the sky is incredibly smooth. You don't see any features in this map until you get down to the 1 part in 100,000 level. The sky has an overall temperature near 3 Kelvin, while these anisotropies are differences in temperature which are typically tens of micro-Kelvins. So apart from the motion effect, the CMB sky is isotropic at the level of about 1 part in 100,000.
Assumption #1: There are no preferred inertial frames, leads to general
relativity... but the CMB provides a preferred inertial frame. If the CMB
obeys the laws of physics, and it shows the way to a preferred inertial frame,
then physics as a whole is not the same in all frames, in contradiction to
Assumption #1.
Submitted by smithe"AT"mrcwdc.com 4/00
Although I've answered similar questions before, I know that this concept is still troubling to some people. So let me make a couple of additional remarks here. Firstly, the description of an expanding Universe involves general relativity, and not special relativity. So it's not necessarily the case that you should expect the ideas of special relativity (which are, let's face it, less general!) to apply. Special relativity doesn't really deal with non-inertial frames. So, for example, you can in principle (described consistently within general relativity) discover observationally that the Universe is rotating, even although (within the context of special relativity) your inclination might be to ask "rotating with respect to what?".
But more importantly you have to examine the assumptions more closely. Let us go back to Einstein. A translation of his original 1905 statement is: The same laws of electrodynamics and optics are valid for all frames of reference for which the equations of mechanics hold good. We will raise this conjecture (the purport of which will hereafter be called the `Principle of Relativity') to the status of a postulate...
There is indeed a frame of reference (actually it's expanding, but never mind that!) in which the CMB dipole would be measured to be zero. But there is nothing special about the laws of physics in that frame. Photons behave as you expect, the laws of electromagnetism are no different, and F=ma is still true.
I dont understand why the variance of energy distribution isnt
calculated which depends on the square of planck's spectral fn and Bose
statistics. Is this due to plasma formation prior to photon gas
formation?
I'm afraid I don't understand this question.
The CMB is composed of photons, which follow the so-called Planck function (the blackbofy shape to the spectrum), and are governed by the so-called Bose statistics (since photons are bosons). However, the fluctuations in photon number caused by those Bose statistics have no cosmologically observable effects that I can think of.
How are the parameters calculated and verified from the observations?
Submitted by ian.watson0"AT"tinyonline.co.uk 6/00
This is another great question. However, I'm afraid that a full answer would be very long, and also rather technical! This in fact is what I spend a bunch of my time doing research on, so I could very easily bore you here!
The short answer is that the fundamental observable is the CMB anisotropy "power spectrum", which tells you how the variations in temperature depend on angular scale. This power spectrum can be calculated for simple models of the origin and evolution of the fluctuations - which grow into our present-day structure and leave their imprint on the CMB sky. These predictions give a power spectrum which contains a series of what are often referred to as "bumps and wiggles"! The details of these features depend on the precise values of the cosmological parameters which describe our Universe (densities in each of the species of matter, expansion rate, a couple of parameters to describe the initial state of the fluctuations, some recent astrophysical processing effects, etc.).
When you have a decent CMB anisotropy data-set you can try to fit the power spectrum estimate, and then constrain the various parameters to the ranges where the models fit the data. At the moment the flatness of the Universe (essentially the sum of all the densities in components of matter and energy) is well constrained, and there are some limits on other parameters too. But we're still in the situation where the answer depends to a large extent on how wide a range of models you're prepared to consider. Things will continue to get better as the bumps and wiggles are pinned down more accurately. It is realistic to expect that in the near future the CMB (in combination with other cosmological measurements) will allow quite precise determination of most of the important parameters simultaneously.
There have been many relevant articles at a range of levels, in various magazines and journals. In the UK, you could check out back issues of New Scientist for example. If you can view postscript files, then the most introductory article I wrote is called Echoes of Gravity, and although a few years old it may still be useful. And for a more recent discussion of measurements of the curvature of space you could look at another article I wrote, Still Flat After All These Years!.
I would like to ask an
explanation about CMBR peak. The first Doppler peak as approximately 200
divided by the 
.
Actually where it comes from?
Is there any effects of
on this peak?
Submitted by hsrashid"AT"pcu.helsinki.fi 5/00
This is a question about the angular position of the peak in the CMB anisotropy power spectrum. In other words, what is the characteristic angle on the CMB sky?
The answer is that there is a characteristic length built into the
known physics of the sound waves which are important for the evolution of
small-scale CMB anisotropies. This length scale (essentially the distance
a sound wave can travel in the age of the Universe at the last scattering
epoch) doesn't depend very much on the model. But the angular scale that
it appears on the sky depends on the curvature of space. The main dependence
is on whether space is flat or not, which is determined by the total
mass-energy density parameter tot.
In a Universe with flat geometry the characteristic angular scale is just
below 1 degree on the sky (or a peak at about multipole 200 in the power
spectrum). In a Universe with closed geometry the angle is
larger, and in a Universe with open geometry the angle is smaller. The
dependence is approximately proportional to
. The fact that the peak in
the power spectrum appears to be near multipole 200 (corresponding to an
angular scale just below a degree) is taken as very strong evidence that
the Universe has close to flat geometry. Since the total density parameter
in all forms of matter that we know of amount to perhaps
mat=0.3 to 0.4, then this tells us that
some other form of energy (which does not cluster like matter) makes up the
deficit. And so there is good reason to believe that
=0.6 to 0.7.
This is either a "cosmological constant" (energy density of the vacuum)
or something even stranger!
In detail the position of the peak also depends on other parameters, such
as , but these
vairiations are quite weak.
For a more detailed discussion, which is still (hopefully!) quite readable,
let me refer you (again) to the article
Still Flat After All These Years!, which I
wrote with two of my colleagues.
ok where can i find pictures of backrounds???
Submitted by Lildevil200114"AT"aol.com 5/00
The best images of the CMB sky are probably still those obtained with the COBE satellite. COBE mapped the whole sky, but with an angular resolution of about 7 degrees. So the COBE maps show a projection of the entire sky (typically into a particular oval shape that would have the plane of the Milky Way as a horizontal line through the middle) at any of 3 frequencies (or combinations of all 3), with a fairly accurate representation of the largest-scale features. They can be found at the COBE DMR images page.
The COBE maps only contain information on the biggest angular scales, since the large beam of the COBE telescope effectively smoothed the maps. But more recent experiments have mapped parts of the sky at much better angular resolution. A good example is the map from the antarctic flight of the BOOMERANG experiment, which can be found in various forms on the BOOMERANG Press Page.
I have one really big question....
I'm a student of engineering in first year so i'm not an expert in
physics or something...
Ok the question is... How CMB proves uinverse is flat?
Submitted by soponcho"AT"geocities.com 5/00
I've answered similar questions above. So let me be brief here. For more details you could read, for example, the article Still Flat After All These Years!.
The basic point is that there is a characteristic scale set by the physics
of the evolving density variations in the early Universe. This characteristic
length scale will appear to be a different angular scale on the sky
depending on the overall curvature of space (which you can think of as making
parallel lines diverge or converge in open and closed spaces, respectively).
Recent measurements of the CMB sky indicate that this scale is just about
where you'd expect it to be in the most popular models, provided
that the geometry of space is approximately flat. It could, of course,
still be a bit open or a bit closed, but it has to be quite close to flat.
More explicitly, the total value of the density parameter
= 1.0 ± 0.1 approximately.
Is "anisotropy" pronounced annieSOTTrippy or anneyeSOTTrippy ?
Submitted by george.barnes"AT"tafe.nsw.edu.au 6/00
It's the latter.
We invented this word deliberately as a sort of secret hand-shake -- if you can pronounce it properly, then you're allowed into meetings of the Clandestine Members Bureau.
(Follow-up comment):
why not include a small embedded media file of the correct pronunciation of
anisotropy?
why not tell people to look
it up on Merriam-Webster's site, which includes nifty
little sound
samples to help everyone get their tongues around it. What I can't
get the hang of is not the emphasis placement, but NOT saying a long
"o" for both "o's".
Submitted by frank.j.glazer"AT"verizon.com 3/03
Good idea! Although I suspect that not everyone will be able to play whatever format the file is in. Any volunteers to send me a short audio file that might be widely playable?
Now that you can pronounce it you will be allowed into the Clique for Mellifluent Badinage!
(Another Follow-up comment):
I just went to your FAQ page, looking to find out how to pronounce Anisotropy.
While I appreciate your efforts, the answer you posted was no help at all!! I
was hoping for a phonetic spelling, with syllable breaks and an accent mark!
Oh well, I'll keep looking...
P.S. Note that I do not know how to pronounce anisotropic either, so your
explanation that knowing how to pronounce this adds confusion to pronouncing
anisotropy just makes it more confusing! With explanations like this, I hope
you don't make a living teaching! Thanks for taking the time to post the FAQ
page, and please take these comments as the good hearted ribbing that it was
intended to be.
Submitted by jigger"AT"Mafi-Trench.com 10/04
Ah, thanks for those words of praise! As you say, it is indeed lucky that I don't try to make a living from teaching!
As to providing a phonetic pronunciation guide, complete with upside-down letters and obscure alien accents, I'm afraid I don't know how to produce them in a way which is readable on different sorts of computers and browsers and which is comprehensible to the average cosmologist who didn't take a linguistics course!
I have a book beside my desk called a "dictionary", which specialises in this sort of thing, and indeed has a pronunciation entry for both "anisotropy" and "anisotropic". If you really want a phonetic transcription, you might try there.
But I made an attempt at copying this - here's the jpeg, and pdf versions.
Your explanation of the correspondence between CMB hotspots and last
scattering surface overdensities was intriguing. Is the overdensity the
cause of positive delta T but gravitational redshift ? Can these two cancel
out?
Submitted by george.barnes"AT"tafe.nsw.edu.au 6/00
This may get a little technical, so let me take a deep breath and give this a go!
The gravitational redshift results in a decrease in the temperature for a postive enhancement in the density. The effect of the density, on the other hand, depends on what type of fluctuation we have. In the most popular sort (called "adiabatic", where the source that made these perturbations didn't change the entropy, and is what you most easily get out of inflationary models, as well as appearing to fit the current data pretty well), the density effect gives more temperature where you have more matter. So the two effects partly cancel. On the other hand there's a different kind of perturbation you can set up in the early Universe (called "isocurvature", where the overall energy density is unperturbed), in which the radiation is overdense when the density enhancement is negative. In this case (which is much less popular, since it doesn't fit the data at all well) the two effects have the same (negative) sign.
Either way, you tend to have a negative temperature fluctuation when the overdensity is positive. In principle the two effects could exactly cancel out, but that would require that the kinds of matter and energy which dominate the Universe would have to be very different from how we think the Universe behaved at the time the CMB photons last scattered and got these temperature variations imprinted.
Why is it exactly that we study the polarization of the Anisotropy? What
is it that we intend to learn from this?
Submitted by noam"AT"asiaa.sinica.edu.tw 7/00
This is a very good question.
There are several parts to the answer. Firstly, the CMB is predicted to be slightly polarized through the scattering process by which it last interacted with matter. No one doubts that the polarization is there, and so it's detection will be a confirmation of the basic paradigm for the physics of the generation of CMB anisotropies. And if there is no polarization, that would be much more exciting of course!
Once the polarized CMB sky begins to be mapped in detail, then there is information which can be extracted from these signals which complements what can be determined from the temperature anisotropies. In particular the polarization is a much cleaner "snapshot" of the "last scattering surface", and so encodes more direct information about conditions back a few hundred thousand years after the Big Bang. And also the existence of very large scale gravity waves (which might be left over from certain kins of processes which happened in the very early Universe) leaves a distinct imprint on the CMB polarization. If this imprint could be detected then we would learn important information about the origin of the seed perturbations which gave rise to all the structure in the Universe.
The CMB sky is predicted to be polarized at about the level of one part in a million, so it will be challenging to map the polarization in detail. But the pay-off is sufficiently exciting that several experiments are underway to do just that.
I'm interested in knowing what the actual measurement differences were in
obtainin the background radiation values.
Submitted by KBeran2140"AT"aol.com 9/00
I assume what is meant here is the size of the temperature differences measured in CMB maps. All CMB anisotropy measurements are essentially differential, i.e. experiments measure the difference between two temperatures. Since differences are measured then what is being studied are the variations in temperature relative to the average value of about 2.725 Kelvin. These variations need to be measured with an accuracy of about 10µK (or at least the average of a whole bunch of measurements of the same thing has to be this small) before you have much of a hope of detecting the anisotropies. The hottest feature seen in a CMB map has a temperature of about 100µK or 0.0001 Kelvin. And the coldest feature has a temperature of about -100µK (relative to the background temperature).
Where can I find out more about cosmic variance, sample variance
and window functions? Can you point me to any helpful papers?
Submitted by TelfordRE"AT"Cardiff.ac.uk 10/00
A technical question!
For readers of this page who are wondering what the question means, let me first explain the terminology. "Cosmic variance" refers to the idea that when we measure properties of the CMB sky we might be able to do so with arbitrary precision, but the information we can extract about the properties of the cosmological model cannot be obtained with arbitrary accuracy. The reason is that our particular sky is one realisation of a statistical process described by the model (the model predicts the variance of the temperatures as a function of angular scale, not tha actual pattern of temperatures).
The fact that individual experiments only have a finite amount of information about the sky is usually referred to as "sample variance". This is just the same thing as asking any statistical question with limited information. For example asking what fraction of the time a coin comes up heads when you have only tossed it a finite number of times - this will always result in a particular level of uncertainty which depends on the number of coin tosses. For the CMB, the "sample variance" is smallest when an experiment has mapped the whole sky, and then the "sample variance" is the "cosmic variance". And you can't do better than this, because there's only one sky to observe!
A "window function" is the name given to the function which tells you the range of angular scales that a particular experiment is sensitive to. This depends on the amount of sky covered, the beam-size (angular resolution) of the telescope, and the particular scanning strategy adopted.
I'm not aware of anything terribly informative on these topics which is not written at a technical level. For details on "sample variance", you could start with a paper by myself and a couple of my collaborators (Scott, Srednicki & White, 1994, Astrophysical Journal, volume 421, pages L5-L8), which you can get here. A good paper on window functions is this one by White & Srednicki (1995, Astrophysical Journal, volume 443, pages 6-10), which you can get here, and another, by Knox (1999, Physical Review, D60, 103516), is here.
I would like to know what the fluctuations in the CMB tell us.
Submitted by hurstds"AT"muohio.edu 11/00
Many, many things!
At a basic level, the fluctuations tell us that the density variations on large scales in the early Universe were about 1 part in 100,000. This turns out to be about the right amplitude for gravitational instability (i.e. the fact that overdense regions become more and more overdense with time) to have formed all the structure that we observe today over the history of the Universe. So the simple observation of these fluctuations confirms that gravity was the force that grew all of the structures (galaxies, clusters of galaxies, etc.).
By measuring how the fluctuations vary with angle, we hope to be able to determine a large number of the fundamental parameters which define precisely what sort of universe we live in. Already it appears that the CMB fluctuations tell us that the curvature of the Universe is very close to flat. And additionally that the sorts of very early density variations which the Universe possessed appear to be like those obtained in so-called inflationary models of the early Universe. We fully expect to learn vastly more in the coming years!
Could you point me to some resources that may contain
information on the calculus behind CMB.
Submitted by pufffy13"AT"yahoo.com 12/00
The mathematics behind the general concepts involving the CMB is in fact quite straightforward. It's when you come to try to understand the anisotropies in the CMB that things get more challenging! I suggest that you start by going to your local bookstore or library and looking for any books they have on cosmology which are at some intermediate level between the popular text and the full-blown research textbook. Only you will be able to tell if it's the right next level for you!
Eventually you might be able to work up to the full mathematical treatment of the anisotropies. One of the most elegant and complete discourses is a paper entitled "A Complete Treatment of CMB Anisotropies in a FRW Universe" by Hu, Seljak, White and Zaldarriaga, which you can get as a postscript file here.
On the right side scale of to-day's picture there are negative
temperatures in microkelvin. I thought that, by definition, the minimum
temperature in kelvins was 0. What is your definition of the kelvin
temperature scale?
Submitted by lermanma"AT"csolve.net 12/00
The picture you are referring to is a map of part of the CMB sky made by the MAXIMA experiment, which was featured on the Astronomy Picture of the Day site for 30th December 2000.
The temperature scale for that map is for temperature anisotropies rather than absolute temperatures. In other words what is measured is the temperature difference relative to the average temperature of the CMB sky. The CMB temperature is a little less than 3 Kelvin, while the temperature differences shown on the map range between about -0.0003 Kelvin and +0.0003 Kelvin (or 300µK, i.e. 10,000 times smaller). In fact the experiment only measures temperature differences, and isn't sensitive to the overall average value (which requires a different sort of experiment to measure). It would be possible to add back in the roughly 3 Kelvin "DC level" for the map, but then the anisotropies would be such low contrast that you wouldn't be able to see them.
The 2.7K CMB seems to consist of standing waves to me.
I would greatly appreciate your comment on this aspect of the
2.7K CMB. [abridged]
Submitted by henry"AT"govital.net 1/01
Let me first define a "standing wave", for others who may read this answer. Think of a standing wave as a pattern (on a string, say) with a fixed wavelength but with amplitude getting bigger and then smaller with time. There are points on the pattern (the nodes) which don't move at all), while between the nodes the displacement is biggest. There it goes through maximum, back to zero, then negative, back to zero etc. There's a nice animation here. This is what waves on the strings of a musical instrument behave like. The wave pattern itself doesn't travel along the string, and this distinguishes standing waves from "travelling waves". In travelling waves the pattern of the wave moves along at some particular speed (like waves on the surface of water for example).
The CMB does not consist of standing waves, but of travelling electromagentic waves, i.e. photons. These waves move at the speed of light, and were emitted in all directions by every part of the very early Universe.
However, standing waves do play a role in the physics of the CMB. The bumps and wiggles in the power spectrum of CMB anisotropies can be thought of as standing sound waves. By precisely measuring the anisotropies over a wide range of scales we can determine the amplitudes of all these sound wave harmonics, which tells us about the fluctuations in the early Universe, and how they evolved. This in turn tells us about fundamental properties of the Universe that we live in.
In the maps of CMB can you see two different size blobs corresponding to
the first and seconds peaks of the power spectra?
How do they make the power spectra from the map?
birdjo"AT"Spole.gov 3/01
If you stare hard at a good CMB map (or a simulated one!), you can tell that there's a characteristic angular scale. This corresponds to the scale represented by the main peak in the power spectrum. Probably it's hard to tell by eye that there's a second peak, since there's power over a wide range of angular scales. But for a sufficiently low noise map you should be able to discern that there's a characteristic size for the blobs and that there's structure on smaller scales too. For an experiment with a small enough beam-size, you would notice a lack of structure in the maps on the smallest scales, but still large enough that they're not just smoothed out by the beam - this would be the fact that the power spectrum itself has no power at the smallest scales.
Now to answer your second, more technical question, which is a good one. The crudest way to make power spectra is just to Fourier transform the map. You can bet that that's about the first thing that's done once the maps have been produced! But you need to be much more careful when producing a final estimate of the power spectrum. For a start you need to use spherical harmonics rather than Fourier modes, since the sky is curved, not flat. Then the finite size of the map introduces correlations between the modes. So you need to make binned estimates over various ranges of k, and estimate each of these in a way which is independent of the other values. A fair amount of theoretical work has been done figuring out how to do this efficiently while not losing any information content. The final estimates of power spectra from experiments like BOOMERANG and Maxima take days to run on a supercomputer!
I need to find some good colour images of the CMB for us to use as the
basis for an animation. There are 3 main images I'm after - to show the
differences between open, flat & closed Universe (ie. small, medium &
large 'blotches' of colour).
www.pioneertv.com 6/01
As a start you could look at a picture I made (with Martin White) for the US Decadal Survey in Astronomy, which you can find as a postscript file here. Basically you want a picture of coloured blobs, with stronger contrasts at smaller scales for the open model and the opposite for closed models. So in fact if you're just trying to show artistically what happens, you want to do something which looks like blowing up the small-scale features while keeping the large-scale features the same.
There's also a picture on the Boomerang web-site which is very nice for showing the connection between the CMB and the geometry of the Universe. Unfortunately it doesn't keep the same "phases" for the 3 maps, i.e. each one is a separate random simulation. So it doesn't show the effect as well as it could. But I think you get the idea that open Universes would have much more small-scale features than closed models.
what are some irregularities and what techniques are being used to
detect irregularities in CBR?
Submitted by mo_poon"AT"yahoo.co.uk 10/01
The "irregularities" in the CMB are technically referred to as "anisotropies". They are deviations from smoothness discernible when a map of the microwave sky is made. These deviations occur over a wide range of angular scales. So whether you make a map of a large fraction of the sky with big pixels, or a map of a small chunk of sky with smaller pixels, you will still see variations in brightness. In fact the way that these anisotropies vary with angular scale carries the precious information about our Universe which researchers are now trying to unravel.
To make such measurements you need to be sensitive to brightness differences corresponding to only a few microKelvin, or about 100,000th of the brightness of the CMB as a whole (which corresponds to about 2.7 Kelvin). This can only be done with specially designed experiments to make ultra-sensitive maps of parts of the sky at microwave frequencies. Right now there are a range of such experiments, operating from the roofs of Physics departments, from high mountain observatories, from balloon-borne platforms or from space. Together they promise to build up the full picture of the cosmological information content buried in the CMB sky.
What are the implications for various cosmological models of the
universe?
Submitted by mo_poon"AT"yahoo.co.uk 10/01
Things are really the other way round: we learn about the cosmological model by measuring the anisotropies. The precise variation of the anisotropies with angular scale (how "noisy" the maps look as a function of the size of the pixels) encodes information about the Universe in which we live. We can learn about the initial seeds from which all the structure formed, as well as the amount and composition of dark matter, the rate of expansion, the flatness of space, the overall shape of the Universe etc. That we have the ability to learn all these things by measuring the microwave sky is an incredible opportunity that the Universe has given us.
Since the CMB was emitted prior to star and galactic
formation, why do we not see images in "negative" of
these material structures? I.e., surely the moon
blocks the CMB from a telescope. Surely the sun does
as well. Why not Alpha Centauri? Andromeda?
Submitted by bopbone"AT"yahoo.com 11/01
Absolutely!
The Sun is a local strong emitter of radiation at just about every wavelength. So to make a map of the CMB sky, you have to make sure the Sun isn't in your picture! It's even worse than that though, since you're looking for variations of one part in 100,000. So you have to make sure you don't even see a glancing reflection of the Sun. CMB experiments go to great trouble to reduce any effect of the Sun by observing well away from the Sun's position, as well as having all sorts of shields etc.
The Moon reflects all sorts of light from the Sun, and also glows in the mcrowaves (because the Sun heats it up). So you have to avoid the Moon too.
Even the most nearby stars are still so far away that their effects are negligible. However, that's not true for the whole agglomeration of stars in our Galaxy (the Milky Way). So you have to make CMB maps well away from the Galactc Plane.
Other galaxies will have a smaller effect. This certainly has to be thought about. They block the CMB, it's true, but they also emit radiation on their own. So they will typically be bright spots. There's another effect though that can casuse a "hole" in the CMB at certain wavelengths. That's called the "Sunyaev-Zel'dovich" effect (after the two Soviet scientists who first suggested it might be observable). The idea is that hot gas in the outskirts of galaxies, or in clusters of galaxies can scatter some of the CMB photons. Some of the low energy photons gain energy, and so with a radio telescope you see a hole in the CMB in the direction of a galaxy cluster (and at shorter wavelengths you'd see a hot spot in the same place). This effect has in fact been observed for many of the biggest clusters of galaxies. And presumably as observatioins continue to get better, we'll eventually be able to see this effect for individual galaxies too. It actually turns out to be a useful thing to observe, since it tells you information about the obstructing object that would be hard to tell any other way.
In the CMB rest frame you measure no velocity with respect to the CMB
photons?
I thought that the speed of photons was "c" when measured from any and
all frames of reference. What am I missing?
Submitted by 12/01
It may be that some people (even me, gasp!) have been sloppy with language when explaining the CMB "dipole" effect.
Of course the photons are always moving at the speed of light. So what you detect is not that there's a different speed for the photons in one direction. You simply see more photons (or equivalently a brighter intensity or higher temperature) in the direction you're moving towards, and fewer in the opposite direction. Think of it as a bunch of photons moving about in all directions, and then imagine how many photons will cross a unit area of detector per unit time. If you are in the rest frame with respect to the CMB photons, then you measure the same flux of photons no matter where you point your detector. But if you now move your detector very fast in one direction then you'll get more photons per unit time. It's just like how you get more raindrops in your face when you run in a rainstorm!
So the effect is that if we're moving in some direction relative to the "CMB rest frame", then there will be more photons (per unit area per unit time) apparently coming from that direction, and fewer coming from the opposite side of the sky. That the sense in which we can detect whether we're moving with respect to the CMB photons.
I was recently reading how the COBE has observed very slight variations in
intensity of microwave radiation as a function of direction, but I do not know
what causes this. Could you provide any insight as to the source of these
variations?
Submitted by gte878n"AT"prism.gatech.edu 3/02
Since you can find more detailed answers here on this very web-page, let me just provide you with a brief answer here. They are variations in density observed at the epoch when the photons last interacted with the matter in the Universe, about half a million years after the Big Bang, when the Universe was very much hotter and denser. Sveral other experiments have detected such variations, at a range of angular resolutions, and we're now building up a detailed picture of these minute variations in temperature, which hold valuable information about the structure of the Universe on large scales.
I'd like to know if it would be possible to orient oneself in space by
comparing extremely accurate CMB maps. For example, if one was to make a
hi-res CMB survey map and if one could (by magic!) travel to another part
of the universe hundre ds of millions light years away *instantly*, would
it be possible to determine o ne's location (relative to the origin point)
by taking new CMB measurements and comparing them to the original
measurements? If not, how might such a feat be accomplished?
Submitted by damiensk"AT"pacific.net.au 8/02
This is a very interesting question!
The CMB anisotropies come from the "last scattering surface", which is an imaginary sphere around us, where we can see back to a time when the radiation last interacted with matter. The hot spots and cold spots on the sky come from the overdensities and underdensities on this sphere. Each observer has a different surface, determined simply by the distance that light can have travelled in each direction in about the last 15 billion years. So if we moved, say 1000 light years in some direction, then the CMB map would look pretty much identical. But if we moved some significant fraction of 15 billion light years, then we'd be seeing different density blobs, which would cause different sets of hot spots and cold spots. In fact the "thickness" of the last scattering surface is something like a few hundred thousand light years, and so if you moved further than that, then the features you'd see on the CMB sky would no longer be correlated.
So I suppose if you found yourself instantly in a part of the Universe where the CMB sky looked similar, but a little different from our own, then you could infer that you'd moved by some amount in some direction. It should be possible to quantify that amount, and know where you were. However, if you moved to some part of the Universe millions of light years away, then the maps would have nothing in common (other than the overall level of fluctuations etc.), and so you'd have no idea where you were, other than further than something like a million light years.
How do the COBE results conform with a model with a positive cosmological
constant? Is this model scale invariant too? If not, why do the
physicists bother themselves about it? [abridged]
Submitted by 11/02
The results of the COBE satellite indicated that the initial conditions for the density perturbations (which were laid down in the early Universe and which gave rise to all of today's structure) were "scale invariant". This means equal amounts of fluctuations at all scales. This says nothing about the geometry of the Universe, or about whether or not there is a cosmological constant.
The COBE data are equally well fit with the sort of model which is currently popular, i.e. about 2/3 of the total energy density in a cosmological constant (or "Dark Energy") and the other 1/3 in Dark Matter. However, it's also true that very large amounts of Dark Energy would have left a noticeable effect on the COBE data, which were not seen (so we know the Universe isn't very closed, with more than enough cosmological constant, for example).
In fact the most recent CMB anisotropy data, from several experiments which collected data at smaller angular scales than COBE, also point towards models which contain about 2/3 Dark Energy.
My question concerns
the image generated in the early 90's from the COBE satellite. What was the
significance of this image, apart from confirming the existance of the CMB?
Did the image change our ideas of the background radiation in any way?
Submitted by Andres.Donaldson"AT"stpauls.richmond.sch.uk 11/02
The data from the COBE DMR instrument were the first to demonstrate that the CMB sky contained anisotropies. So the fluctuating pattern in the images showed regions of the early Universe which were a little hotter or cooler than average. The CMB itself had been detected in 1965, but all images made of the CMB had shown it to be extraordinarily smooth (apart from the "dipole' pattern showing our motion through space).
The COBE data measured the amplitude of these temperature variations. And they were pretty much exactly what was needed for the popular "cold dark matter" models to have left those impressions in the early Universe and then grown all of today's structures through gravitational instability by the present day.
So the anisotropies in the COBE data were very important in confirming that the basic cosmological picture was on the right track. That picture is composed of: a hot early Universe, expanding and cooling, containing low contrast density perturbations at early times, which grew over billions of years to produce all of the rich structure of today's Universe. If COBE had detected no anisotropies, or ones with the wrong amplitude or variation with angular scale, then our basic paradigm would have to have been dramatically changed (and there were many cosmologists at the time who were anticipating such a change!).
Newer CMB anisotropy results have further bolstered this picture, as well as filling in some of the details, like the form of the initial density perturbations, the amounts of dark matter and dark energy, etc.
I've read some introductory notes on CMB anisotropy. I saw that most
calculations assume the matter content of the universe as it is, without
considering the fact that the matter content must somehow evolve via
CP-violation processes to produce a net amount of matter.
I wonder if there has been any attempts before to include this
CP-violation processes in CMB anisotropy calculation and if there will be
any observables due to this.
Submitted by haryo"AT"hep.fsu.edu 01/03
It's clear that the vast majority of the Universe is made of matter rather than "anti-matter", and that there has to be a good particle physics explanation for this eventually. At the moment there is no definitive answer to the process which favoured matter over anti-matter, although there are a number of ideas.
The effects which generate this asymmetry occur at very high energies, long before the CMB anisotropies that we see were generated. I'm not aware of any detectable effects of such processes on the CMB anisotropies. But I'd be happy to hear of the details of any idea that you work out!
The uniformity of CBR suggests that the Universe was
very smooth during the first 300000 years. On the
other hand dark matter should have been in clumps that
attracted ordinary matter after the decoupling of
radiation and matter, to form galaxies. But these
clumps of dark matter would affect the geomerty of the
Universe. So the Freedman's smooth viewpoint of the
primeval Universe is wrong and should be replaced by
inhomogeneous models.In this case the CBR ought to
have irregularities. What's wrong with this picture?
Submitted by tomp044"AT"yahoo.co.uk 02/03
This is a very good question!
The answer is something that cosmologists refer to as "gravitational instability". This is the fact that slightly overdense regions increase their contrast relative to the average density (and obviously undersense regions get more underdense) as the Universe gets older. This happens in a regular static medium, and turns out also to happen in an expanding medium (i.e. the Universe as we know it), although at a slower rate. You can think of an overdense region expanding a little slower than a typical part of the Universe and hence increasing its density contrast with time.
The Universe at early times had very low amplitude inhomogeneities. We see these at a time of about 300,000 years through the CMB anisotropies, which have amplitudes of about 1 part in 100,000. These amplitudes have grown by the present time (say 14 billion years later) to form galaxies and the rest of the structure in today's Universe.
What is anisotropy?
Submitted by mriccobo"AT"ucsd.edu 03/03
The dictionary definition is something like "the state of having different properties in different directions". In CMB studies (as in some other fields, like geophysics, crystallography, etc.) this word is used to refer to the quantification of the degree to which something is not isotropic. Specifically, CMB anisotropies are the description of the temperature variations on the microwave sky, i.e. the pattern of hot and cold spots which we can detect using sensitive CMB experiments.
I'm doing my ninth grade science fair project on the theory of a finite, dod
ecahedral universe. ... Most of the words I don't know the meaning of I can do
without, but the main one that I can't seem to get around is "spectrum".
Do you think you
could explain to me the meaning of this word, and what exactly its relationship
to my topic would be?
Submitted by jewelz1088"AT"comcast.net 01/04
Here "spectrum" has a slightly more general meaning than you might have come across. A spectrum is usually the light spread out into all its wavelengths (colours). This might be more properly called the frequency spectrum.
But we can also imagine taking an image and splitting it up into the amplitude of waves in space - we can plot amplitude versus (spatial) wavelength, for example, as a way of describing the information in the image. More usually physicists like to plot the amplitude squared against the inverse of the wavelength, and that's called the "power spectrum". A plot of this power spectrum is a good way of statistically describing the content of an image. In particular it gives a way of seeing how much "power" the image has at different scales (i.e. is it very blotchy on small scales, or large scales, or what?). This is the standard technique for describing the statistical content of CMB maps. For large enough maps (where the sky isn't flat, but curved) you need to use something slightly different, which works on the surface of a sphere rather than on a flat plane, but it's still called the power spectrum.
The point is that theoretical models of the Universe can predict the shape of this power spectrum. And so by careful statistical comparison of the observed power spectrum with a set of theoretical ones, we can infer things about the model which describes our Universe.
Would you be kind enough to tell me who was the first cosmologist to propose
acoustic oscillations (as opposed to density fluctuations)as the origin of the
CMB anisotropies. The more I think about it, and the more I'm convinced it is
one of the most brilliant ideas in the whole field of modern cosmology!
Submitted by georges_melki"AT"hotmail.com 02/04
This is an excellent question!
The main idea is that density perturbations oscillate as sound waves, driven by gravity and with the restoring force provided by pressure between the baryons (regular matter) and photons. The CMB sky can be thought of as a snapshot of these oscillating modes - kind of like a set of standing waves with random phases, spread over the sky. The evolution of the sound waves makes particular angular scales special, resulting in the peaks and troughs seen in the power spectrum of CMB anisotropies.
The best answer is that the idea was already fairly clearly formulated by Jim Peebles and Yacob Zel'dovich (and their collaborators) around 1970. There are papers by these US and Soviet groups about that time which show power spectra with such oscillations present. At that time there was most focus on the signature of the oscillations on the matter disctribution (non-baryonic dark matter wasn't really talked about then!), but over the next few years there were published predictions of CMB anisotropy power spectra too. Particularly important papers with such predictions were those by Doroshkevich, Zeldovich & Sunyaev in 1978 and Silk & Wilson in 1981. Precisely which paper contained the definitively earliest prediction of the CMB acoustic peaks is probably a matter which will be debated by future historians of science!
The idea for acoustic oscillations at all in the Universe goes back to Andrei Sakharov (also famous as a dissident) in the mid-1960s. However, Sakharov's picture was of a cold universe, since this was before the CMB had been discovered. Hence there were no anistropy oscillations in his picture, since there was no CMB at all!
Some people like to give some of the credit to Sakharov. But my own view is that although he played an important role, the real progress was made by Peebles, Zel'dovich and co., when they formulated the modern ideas around 1970.
Could you pls elaborate on the Fourier development of the CMB power spectrum in
your FAQ section.To my knowledge,Fourier analysis applies to periodic functions,
and I can't see anything periodic in the CMB!
Submitted by georges_melki"AT"hotmail.com 02/04
You can certainly carry out Fourier analysis for functions which are not periodic, provided that they're bounded. For example, if you have an image of something in a rectangular array of pixels, then you can Fourier transform the image to understand the amplitudes of all the waves (which fit to have nodes at the boundaries of the image) that you have to add together to contruct the image. In 2-dimensions you can separate the problem into waves in the x-direction and waves in the y-direction. If there's no special direction in the image, then the information in the 2 directions will be statistically the same, and you'll care about the amplitudes (and possibly phases) of the modes as a function of the modulus of the vectors in Fourier space.
The analysis of CMB images is exactly the same, at least for small maps. There are no special directions in the CMB sky, and the phases of the CMB anisotropies are random (this is both justified from theoretical prejudice and from empirical testing of real CMB data). So you can statistically describe the structures in the map by looking at the power spectrum of Fourier modes, i.e. estimates of the squared amplitudes of the Fourier modes as a function of scale (or technically the wavenumber, the reciprocal of scale).
For CMB maps which are larger, you can't ignore the curvature of the sky. So you can't use Fourier analysis, which only work for a flat image. Instead you use a different set of functions to decompose the CMB sky - functions which are appropriate to use on a sphere. These are called spherical harmonics. The CMB power spectrum is plotted as a function of "multipole", which is an inverse angle. This is simply the curved sky version of the Fourier power spectrum of a flat 2-D image.
was universe really re-ionized ? how to detect it's
signatures on the CMBR. What is means by optical
depth and how to measure it for reionization ?
Submitted by jprasadb"AT"yahoo.co.in 04/04
The Universe became neutral at about 300,000 years after the Big Bang, which is the time the CMB photons last scattered. Except that the Universe became ionized again fairly recently (by cosmic standards) at maybe a few hundred million years after the Big Bang - and this has a small effect on the CMB photons. "Optical depth" is the term used to quantify the amount of scattering. The bigger the optical depth between us and the epoch of reionization, the bigger the effect on the CMB. These effects are measurable on the CMB anisotropies, and give a particularl signal in the large-angular scale CMB polarization data. Such a signal was seen by the WMAP satellite, and through measuring it in more detail we should be able to learn about the processes which reionized the Universe.
so, if the big bang was lumpy, we could see changes in the CMB flux
as time goes on, or if we could go to another point in the universe?
Submitted by rich"AT"concordma.com 07/04
This applies to the lumpiness of the Universe at the "last scattering surface", when the CMB photons last interacted with matter. We see a certain pattern of anisotropies on the CMB sky because of these irregularities. If we could instantaneously move to a different part of the Universe, then we'd be seeing a spatially different surface in the past, and hence see a different pattern of anisotropies. However, you'd have to travel a cosmologically significant distance in order to see much difference! And the amplitudes of the temperature variations as a function of angular scale on the sky (a.k.a. the anisotropy "power spectrum") would be statistically the same as we see on our sky.
It's also true that if we wait long enough we'll be able to see the CMB sky changing, as the last scattering surface moves back through space. But again, you'd have to wait a cosmologically significant amount of time before you'd notice the difference!
I have read some papers,books,reviews.But I can't understand why in angular
power spectrum of CMB the main contribution in low multipoles(l) is mainly due
to SW-ISW effect?I mean t o say that we can explain low "l" contribution to
angular power spectrum conside ring SW-ISW effect,why?
Submitted by kanan"AT"cts.iitkgp.ernet.in 04/05
This is alittle hard for me to answer, since I don't know how much you already understand. But let me have a stab anyway.
"SW" is the Sachs-Wolfe effect, first pointed out in a paper by Sachs and Art Wolfe in 1967. The physical effect is that fluctuations in gravitational potential cause fluctuations in CMB temperature. Basically we can see correlated variations in gravitational potential on the last-scattering surface on a wide range of angular scales. These scales include those which are large enough that causal physical processes cannot have affected them between the time the fluctuations were laid down (the inflationary epoch, say) and the last-scattering time. This corresponds to an angular scale of about a couple of degrees on our sky. So on angular scales measured in degrees we're seeing the "initial conditions" in the variations in gravitational potential, unaffected by causal processes - in other words we see the Sachs-Wolfe effect at low multipoles.
I hope this helps!
At http://snews.bnl.gov/popsci/spectroscope.html it is said that "a
Doppler-shifted blackbody spectrum from a body at one temperature doesn't
look exactly like a blackbody spectrum produced at any other temperature".
Is that statement really correct and does it mean that the Doppler shifted
CMB dipole is not a blackbody?
Submitted by vorleons"AT"hotmail.com 08/05
The CMB dipole does not have a blackbody spectrum, but a spectrum which is the frequency derivative of a blackbody. You can see this by realising that the dipole is the CMB pattern minus the "monopole", i.e. you need to take a difference. For any anisotropy measurement, the function you use to convert from intensity fluctuation to temperature difference is the derivative of the Planck (blackbody) function.
And actually it's a little more complicated that that, since really all we've done by using the derivative of the Planck function is use the first term in the Taylor expansion. In principle higher order corrections are also there, and potentially measurable as small (calculable) deviations from the monopole (times the Planck function) plus the dipole (times the derivative of the Planck function).
If one could actually measure the absolute spectrum at every point (rather than making relative measurements), then of course it would be an exact blackbody everywhere, with the temperature varying with position.
If you'd like more technical details, there's a paper in 2003 by Kamionkowski and Knox, called "Aspects of the Cosmic Microwave Background Dipole", which you can find here
Could you also comment on the accuracy of the following comment in a
Wikipedia article about the CMB (http://en.wikipedia.org/wiki/CMB): "There
are however very small yet significant variations (anisotropies) from the
black body spectrum. The most pronounced is the dipole anisotropy (180
degree scales) which is at a level of about 10^−3 of the monopole." I
thought that no spectral deviation from a blackbody has so far been detected
in the CMB and that those anisotropies are just temperature variations from
point to point.
Submitted by vorleons"AT"hotmail.com 08/05
I wonder who wrote that?!
I think it should say something like "There are however very small yet significant variations (anisotropies) from the uniform temperature observed on the sky".
Maybe you'd like to edit the wikipedia entry? You're obviously smarter (or at least more careful) than whoever is responsible for that statement!
[Abridged] ... whereas they go into elaborate mathematical developments, there
is always something arbitrary on the physics side(or so it seems to
me at least).Take for example Zaldarriaga's "An Introduction to CMB
anisotropies", which I find excellent regardless of the numerous
typos.Here is a typical statement from this paper:
"We start by considering perturbations produced by density
modes.When working with linear theory in a flat universe, it is
convenient to use Fourier modes because they evolve
independently.These modes are the eigenfunctions of the Laplacian
operator.."
Well, apart from the last statement, which is trivial[exp(ik.x) is
definitely an eigenfunction of the Laplacian...], all the rest
seems to me to be imposed on the reader! To start with, I don't
agree with the "flat universe" bit,it is misleading:the universe
may be spatially flat...
Then,why is it convenient to use Fourier modes to describe density
perturbations?Because they evolve independently? And how do we know
they are the only modes?Is this a postulate?
A few pages further, we have another "deus ex machina"
statement:"The anisotropy field is characterized by a 2x2
tensor".Again, what is the reason for this?
Submitted by brans_dicke"AT"yahoo.com 09/05
All of those statements seem pretty obvious to me, but then I was trained as a physicist and not a mathematician!
When we say the Universe is "flat", we mean that its spatial sections are Euclidean. That's a very good approximation in the early Universe, since it's pretty much true today and all the models evolve away from flatness.
The Fourier modes are a complete set of modes. In other words I can write down any scalar function of position (describing the density field) and construct it as a superposition of 3D Fourier modes.
It's convenient to use Fourier modes, because they evolve indpendently in "linear perturbation theory". Linear theory is a very good approximation at early times since the dimensionless amplitude of the perturbations is about 10-5.
And the business of the 2x2 tensor, that's for describing the full polarized CMB anisotropy field. The reason for this choice is just geometry - that's the mathematical object you need to describe the algebra of linear polarization. You can look this up in standard textbooks on polarization - there's nothing about this which is specific to the CMB.
I hope this helps to dispell at least some of the apparent acts of faith!
In a nut-shell, CMB anisotropies can be thought of as the line-of-sight
projection of various plane-wave temperature and polarization
fluctuations.
Amazingly, the acoustic peaks are due to contributions in a direction of
observation which is orthogonal to the wave-vector.
Isn't there a paradox here? And how do you explain this?
Submitted by georges_melki"AT"yahoo.com 10/05
You're right. Except that the "spatial" part of the phases of the plane-waves is completely random (to a high degree of accuracy at least). So you can think of a sea of fluctuating lumps, whose specific positions aren't organised in any way (or alternatively think of a set of plane waves with all sorts of directions and random positional phases). It's the "temporal" phases of all the fluctuations which are coherent, i.e. all the lumps of a particular scale are oscillating in and out together, like balloons being inflated and deflated (or like a set of standing waves if you're thinking in the wave picture).
The CMB "last scattering surface" is just like a slice through the 3-dimenstional distribution of lumps at a particular time. There are places (in space, or projected on the sky) where the amplitude is high (both positive and negative) and places where it's low. But on a particular scale they're all evolving together in time. So the variance of the amplitudes of the fluctuations is just a function of time, for a given scale. Now if we take a snap-shot of the pattern, we see this variance being a function of scale, which we measure as the angular spectrum of the CMB anisotropies. And it doesn't really matter whether you think of this scale as being in the line-of-sight direction or the transverse direction, because there are no special directions in the Universe.
could you please explain why the fluctuations in CMB map produced by the
WMAP satellite allow cosmologists to observe structure in the universe further
back in time than do observations of galaxies at high red shifts.
Submitted by Lilac4moi"AT"aol.com 10/05
When we see the CMB, we are seeing the structures in the Universe at the time the photons were last scattered. This turns out to be about 300,000 years after the Big Bang. The CMB photons have been stretched by a factor of about 1000 since that time, i.e. we're observing back to redshifts of about a thousand. So by measuring the CMB anisotropies we can learn about the structure in the Universe at very early times, when the density contrasts were still low (i.e. before any actual objects had formed).
When we observe galaxies, we're seeing the photons that were made in stars in those galaxies. The most distant galaxies are seen at redshifts of about 6 - and obviously most galaxies we see are at much lower redshifts than that. So we know a lot about structure in the Universe are relatively low redshift. Obviously you can't see galaxies unless there's starlight, and we think the first stars were formed at redshifts below 20 (although this value is still very poorly known). Se we're never going to see individual galaxies back to redshifts anywhere like as high as where we're seeing the CMB anisotropies.
The neat thing is that by comparing the structure at low redshift with the structure at earlier times seen on the CMB sky, we can learn a huge amount about how structure has formed in the Universe.
In an article by Hu and White entitled "A CMB polarization primer", one
reads the following statement:"The degree of linear polarization is
directly proportional to the quadrupole anisotropy in the photons when
they last scatter".
So, is this the same quadrupole that gives the differential background
temperature between two points on the sky separated by 90 degrees?Or is
it some property of the radiation at every
point in the sky?
Submitted by georges_melk"AT"yahoo.com 10/05
It's both of what you suggested.
Every observer in the Universe sees their own last-scattering surface. You can also think of the scatterers on our surface seeing their own "sky". On that sky there will be a quadrupole, and this is what acts as the "source" for the polarization. The polarization we observe is the sum over all the patterns made by all these scatterers.
I have - on a number of recent occasions - received conflicting information
regarding red spots versus blue spots on the CMB. For instance, in a webcast
lecture at the Space and Telescope Science Institute, Rachel Somerville
describes the red spots as being hot along with having high density
characteristics. In contrast, she describes the blue spots as being cold along
with having low density characteristics. Therefore, Dr. Somerville argues that
the high density, red spots are a reflection of photons falling (collapsing)
into gravitational wells, in effect, contributing to this red "high density"
signature. On the other hand - in a webcast lecture at Perimeter Institute -
Rachel Bean describes reds spots as being hot yet having properties of low
density. In contrast, she describes the blue spots as being cold yet having
properties of high density. Therefore, Dr. Bean argues that low density, red
spots are a reflection of photons escaping (expanding away) from gravitational
wells, in effect, leaving behind a "low density" signature. In other words,
Someville's red spots are correlated with high density and the "contracting
dynamics" of the photons falling into gravitational wells. In contrast, Bean's
red spots are correlated with low density and the "stretching dynamics" of
photons escaping from gravitational wells. Is this a simple case of one
scientist being right while the other being wrong? Or is this a more complex
case of both scientist being right? If this is a typical example of
duality/equivalence in physics where Nature can be expressed separately yet
equally on two sides of the 'same coin of reality," please enlightened me on
how both scientist can derive these contradictory yet correct conclusions.
Despite my obvious naiveness regarding the broad yet convoluted subject of CMB,
I am - needless to say - receptive to a response
Submitted by cynholt"AT"bellsouth.net 12/05
So this is a battle of the Rachels!
I believe that Rachel B. is (largely) correct here. Although Rachel S.'s confusion is entirely understandable.
For a start, the colours are completely arbitrary! It just depends which colour-table you choose. Physically one might have imagined that red would be cool (because of redshift) and blue would be hot (because of blueshift). But that disagrees with the "bathroom tap" convention, i.e. members of the general public (who aren't usually physicists) "know" that red things are hotter than blue things. So the COBE DMR team chose to use the "bathroom tap" convention, where red is hot and blue is cold. And many other CMB map-makers have adopted the same convention.
The next complication is that there are several different physical effects happening. If we restrict ourselves to the largest angular scales, then things are probably the simplest. Here we have a combination of "intrinsic" temperature fluctuations and gravitational redshifts/blueshifts. A region which is overdense in gravity will be overdense in both matter and radiation (this is a consequence of the perturbations being of the "adiabatic" sort typically produced in early universe inflationary models). This means that an overdense region is also hotter (redder). However an overdense region is also hard for photons to climb out of, i.e. there's a gravitational redshift, and so the radiation appears cooler (bluer) to a distant obsever. It turns out that the gravitational redshift has a bigger effect that the intrinsic temperature variation (see the question: "Where does the 1/3 come from?" on my "Advanced FAQs" page), and hence overdense regions appear cooler than the average (and hence blue). The same is true for underdense regions, which appear hotter than average (and hence red).
On smaller angular scales things are less clear though, because the effects of oscillations compressing and rarefying the material can be larger than the gravitational redshifting effects, but these depend on scale. And there are additional effects due to velocities (i.e. Doppler shifts), as well as lots of other effects as one goes to even smaller scales.
So on large scales hot spots are underdesnities and cold spots are overdensities. But on smaller scales the best answer is "it depends"!
After checking
your faq page, I still have doubts about the precise
relation between CMB anisotropies and the Cosmological
Constant (and dark energy). I would be very glad if
you could also provide me some graduate/post-graduate
level bibliography on the subject.
Submitted by alice.holden"AT"yahoo.com 06/06
Dark Energy mainly affects the evolution of the Universe at low redshift (i.e. relatively recent times). So there is little direct imprint on the CMB sky, since the anisotropies are mainly formed around a redshift of 1000 in the fairly early Universe. But what Dark Energy does affect is the distance to the last-scattering surface, which changes the angular structure of the CMB anisotropies. Precise measurement of this angular structure gives an accurate value for the last-scattering surface distance. This in turn gives a constraint on how the Universe has been evolving. Combining this with any of a number of other cosmological constraints (e.g. the measured amount of Dark Matter) then gives a good estimate to the amount of Dark Energy. The fact that this compares very well to what is inferred from observations of distant supernovae is the main reason why Dark Energy is taken so seriously.
You can find much more technical discussion of this and related issues in my article with George Smoot, a pdf version of which is here
Some very basic questions, perhaps an omission in my physical knowledge.
What is meant exactly with one part in 100.000?
What is angular scaling (degrees) in relation to the band filtering of a
piece of the CMB map, I don't understand the proces of transgression of
only 2 coulors (low L or many degrees) tot a whole colour mosaic (several
100 L's or 1 degree)?
What is the practice of measuring the temperature differences in every
scale, I mean do they use a standard distance between points or is it a
mean diifference between hottest and coldest points in evere scale or is
it totally different?
Submitted by rulf0000"AT"planet.nl 07/06
"One part in 100,000" (or equivalently 10-5) is a way of saying that variations in the CMB temperature (in this case) are at the level of 1/100,000. In other words, since the CMB temperature is 2.725K, then the temperature variations are at the level of about 30 micro-Kelvin.
I'm afraid I don't really understand your second question. But it suggests that you've been looking at CMB anisotropy maps (from the WMAP satellite for example) and trying to relate those to the "power spectrum" of Cl. To fully appreciate that, you need to understand what a "Fourier spectrum" is, and then how to extend that idea to a curved sky with "spherical harmonics". But the simpler version is that CMB experiments measure the "variance" (i.e. statistical scatter) in a CMB map, and they do that as a function of different smoothing scales. You can think of the Cl plots as depicting this variance as a function of angular scale of the smoothing, with large smoothing corresponding to small multipole "l".
Your third question is related to the second. The quantity measured is essentially the variance, i.e. the average of the squares of the differences from the mean temperature in the map.
I measure the temperature at one point on the sky [ mean temperature on a
small surface- pixel] , than on another point with angle alfa between
these points. I do it many times on the whole sky and make average of
this difference of temperature. Than I repeat the procedure with
different angle and so on. I have function- difference of temperature viz
angles. Now mathematica machinery Legendre transformation and spherical
harmonics and POWER SPECTUM ! Am I right ? [ Of course it is only great
lines of reasoning.]
Submitted by skarbzbig"AT"neostrada.pl 08/06
I think you've got it!
Although to correct you a little, it's the variance among the set of pixels that you measure. First remove the overall CMB temperature from the whole set of pixels, then calculate the average of the product T(i)*T(j) for positions "i" and "j" separated by some fixed angle. Then do this for a whole bunch of different angles. This gives you the "correlation function" of the CMB sky as a function of angular separation. Finally, the "power spectrum" is the "Legendre transform" of that!
Just reading through the CMB FAQ and came across the question on
Anisotropy and its pronuniciation. I have found that a phonetic
pronunciation helps me to pronounce certain words. I looked up
Anisotropy on www.dictionary.com and found this pronunciation:
an-ahy-so-truh-pee
Submitted by jg318206"AT"att.net 10/06
Seems pretty good to me! Thanks.
My question is this:If the basis for the measurements is looking at "dark
spots" in the Universe. How can we be sure that the areas we are looking at
is dark.
When the Hubble telescope was aimed at a dark spot in the universe it found
1000 galaxies.
So are the dark spots used for the microwave background radiation readings
.... dark ?
Submitted by ericfarmer"AT"adelphia.net 6/07
I'm afraid that this is one of those times when an attempt to simplify an explanation only results in more confusion!
The CMB sky is described by a set of measurements of temperature variation at different sky positions, or equivalently a set of brightness measurements for those positions at microwave wavelengths. There are variations all over the place, however the map of the CMB sky is quite unlike most other astronomical images: the contrast level between "hot spots" and "cold spots" is really very low, and there are no well-defined "edges" to any features. So it's not very helpful to speak about specific shapes on the sky, "dark spots" for instance. Instead one needs to statistically describe the average contrast level for different sized features. This is techincal called the "power spectrum of anisotropies". And it is this power spectrum that encodes the information which tells us what sort of universe we live in.
It appears that whatever you were reading was trying to describe this statistical pattern by saying that CMB experiments detect "dark spots" and "bright spots". But these have nothing to do with visible light, they are also quite ill-defined (since it depends what angular scale you are concentating on), and in any case they are only variations ati the tens of micro-Kelvin level compared with the roughly 3 Kelvin temperature of the CMB sky as a whole.
Since the dipole moment is observer-denpendent and could be removed if
we choose an appropriate coordiante. But this dipole velocity still plays
a cruitial role in the theoretical studis of the acoustic peaks. For
example, in W. Hu and S. Dodelson 2002 , the continuity equation includes
the photon velocity which can be treated as the dipole velocity. Why does
this removable dipole still underlies the anisotropic spectrum? Is this a
coordinate-choice problem?
Likewise, the monopole is the mean temperature of the CMB. They say
"The temporal and spatial variations of the monopole and dipole determine
the anisotropic pattern". How to understand this?
Submitted by richard.loogn"AT"gmail.com 7/07
This is a genuinely hard technical question!
There are some subtleties here, but the monopole and dipole are certainly observable - the complication comes when you think about how this relates to causality, choice of coordinates etc. In fact I'm working on a paper right now with some colleagues on this very topic. So look out for that in the next few weeks. [This is a long-winded way of saying that I don't really know the answer!].
As to the damping tail on the anisotropic spectrum, yous review 2006
says that this damping tail can be explained by either the finite
thickness of last scattering surface, or the imperfect coupling of the
photon-baryon fluid. Does this mean that we really don't know which is
the true reason, but just model-fittings?
Submitted by richard.loogn"AT"gmail.com 7/07
Another technical question! We know that when we put the right physics into a computer, then the damping tail comes out, i.e. we see less and less anisotropies at the smallest angular scales. So there's no real mystery about what causes this. The issue is how does one describe the basic physical reason behind the effect, and the answer is that there's more than one thing going on, and more than one way of looking at some of the effects too. If you want the gory details, then read the 1997 paper by Wayne Hu and Martin White, which you can find here.
I am just finishing a set of small sky charts ...
These charts show the positions
of important objects on the sky, for students who want to find these
objects. He has requested two charts showing the positions to which, and
from which, we appear to be moving within the CMB from the small
temperature asymmetry for the Cosmology chapter. These positions are
labelled Leo and Aquarius respectively in textbooks. However, unless we
are missing something vital, neither the WMAP or COBE pole positions are
within these constellations, nor is your quoted (Scott and Smoot) position
after correction for solar system motion. Is there some "historic" reason
why these names should be applied to these pole positions?
[abridged]
Submitted by taclark"AT"ucalgary.ca 9/07
The "dipole" of the CMB has a very well defined direction - but the issue is what reference frame do you want to specify for that motion?
The direction of the dipole that we measure gives the speed of the Earth (or a satellite like WMAP, which is pretty much the same thing) relative to the CMB. Typically you would remove the variation caused by the Earth's motion around the Sun (which has a ~30km/s amplitude and a 1 year period). Then you're left with the motion of the Sun (or the "Solar System barycentre" if you like) relative to the CMB. That could certainly be said to be "the dipole" and hence you could indicate that direction if you want. But the Sun's motion around the Milky Way Galaxy is a big part of that, is quite well known and isn't really very "cosmic" - and so if you want to know the Milky Way's motion relative to the CMB, then you'd subtract off the Sun's vector (which is about the same magnitude as the overall dipole and in roughly the opposite direction). Then you get the ~600km/s value that's usually quoted as "our motion through the sea of CMB photons".
I think the Solar-CMB motion is perhaps in the direction of the constellation Leo (but just at the southern edge), while the better "Milky Way" or "Local Group" dipole is further south, maybe in the direction of Hydra. The direction that we're moving away from in the "cosmic dipole" is whatever is on the opposite side of the sky from that.
I was reading your CMB FAQ page
(http://www.astro.ubc.ca/people/scott/faq_basic.html) recently. I do not
understand your answer to "How come we can tell what motion we have with
respect to the CMB?" It seems to me that the CMB photons are traveling
at speed c relative to us and relative to any other frame in the
universe. There is no frame in which light is at rest. If you could
explain to me what I am missing here, I would appreciate it.
Submitted by ceciliavogel"AT"augustana.edu 12/07
You're right that the CMB photons are travelling at the speed of light no matter what frame you're in! However, there are measurable effects of your motion.
The correct way to work this out is to calculate the effects of a "boost" within the theory of Special Relativity. But I'll dispense with the full mathematics here, since you can understand the basic physical effects quite simply.
Just imagine that you're surrounded by a sphere of people pointing flashlights at you - this is a crude model of the Universe! Now, if you move reasonably quickly in some direction then there will be several noticeable effects. Firstly all the photons in the direction you're moving towards will be blue-shifted (higher energy), while those in the opposite direction will be red-shifted (lower energy). But also the flashlights in the direction you're moving will appear to be denser together on the sky than those in the opposite direction (that can be understood because you're travelling to "meet" the light rays at a slightly different angle).
The net effect of all this is that you will see a "dipole" pattern on the sky, i.e. more brightness for one "pole", smoothly varying towards less brightness at the other "pole". All of the photons are moving towards you at the speed of light, but you can still tell your motion relative to the "rest frame". In CMB language that means we can measure our motion through the Universe relative to the "CMB rest frame". Here the phrase "CMB rest frame" just means "the frame in which there's no CMB dipole".
I hope this has helped demystify this issue for you!
My question is: why are there two dots, presumably at the apex and antiapex of
the sun's motion, in the COBE map in today's APOD?
(http://antwrp.gsfc.nasa.gov/apod/ap080309.html)
They make it look like a stretched yin-yang symbol. If I'm interpreting the
colors right it look like at those points the temperature is a little cooler
than in the adjacent regions. My guess it is some artifact of the measuring
process and not a cosmological feature, but I couldn't find out any mention of
the features much less an explanation.
Submitted by jjb"AT"vt.edu 03/08
This is an image of the microwave sky made by the COBE satellite, with temperature (or brightness) colour coded, with warmer parts being red and cooler parts being blue. It is a map of the whole sky projected onto an oval shape, with the Galactic Plane running horizontally across the middle, the Galactic North Pole at the top and the Galactic South Pole at the bottom. The image has had the average temperature removed, so that this is a map of temperature differences relative to the average. Apart from that though, it has had nothing removed.
The "yin and yang" shape is the "cosmic dipole", which is caused by our motion through the Universe. We see the CMB being a bit hotter in the direction we're moving, and a bit colder in the opposite direction, with a gradual variation in between.
The other stuff that you see in the image is from the Milky Way Galaxy that we live in. Parts of the Milky Way are brighter than others, and at these wavelengths the brightest parts are about as bright as the dipole.
Those spots that you see are parts of the Galaxy, along the middle of the image. When one wants to study the "anisotropies" from the distant Universe, the Galaxy gets in the way. Despite the best efforts (by observing at a series of different wavelengths, for example) it's not possible to completely remove the effects of the Galaxy, and so for studying the distant Universe a horizontal strip is cut out of the image, and the dipole is also removed. Then the much fainter primordial anisotropies are revealed (as seen in other maps from COBE or WMAP, for example) - which tell us so much about the Universe.
I understand there was a mathematician collaborating with Einstein at
Princeton who speculated, based on his work, that the Universe was/is
essentially rotating and not expanding, it's just that the scales are so
large that our thinking is being influenced along the lines of a more
simplistic and linear perspective for purposes of social interaction.
Possible?
Submitted by jmar55"AT"yahoo.com 03/08
The Universe could certainly be rotating in principle. There are a set of cosmological solutions to Einstein's field equations which have rotation, specific examples of what are called the "Bianchi solutions". However, these models predict specific patterns of anisotropies in the CMB, which are not seen. Hence one can place very tight limits on the amount of rotation.
I was
wondering what the new total mass-energy density parameter [omega.uc.gif]
tot was from the most recent 5-year WMAP data.
Submitted by jeffakkerman"AT"yahoo.com 07/08
If you are asking about the mass contribution in matter, the value decreased slightly compared with the previous data. The new value is 0.24 +/- 0.03. But if you are asking about the "total" (including the Dark Energy), then the answer is still consistent with unity, with an error bar which is only 1 or 2 per cent. This means that the Universe appears to be quite close to "flat", i.e. like good old-fashioned Euclidean geometry.
on your page http://www.astro.ubc.ca/people/scott/faq_basic.html, you have a
question under the header "How come we can tell what motion we have with
respect to the CMB?"
I should say I'm no physicist and it was the first time I read anything
substancial about CMB. But the above question intrigued me. However, reading
your answer I didn't get any wiser. In fact, I have a feeling that something
is missing or assumed in your answer.
You write that "in the CMB rest frame you measure no velocity with respect to
the CMB photons". There is something weird in this statement for a physics
noob like me - from the little I have read about physics, that sounds alot
like the idea of an ether to me. You say in many places that the photons move
at the speed of light. How then would it be possible to see them at rest? You
would have to be moving along with them at the speed of light? Secondly, you
say that the CMB photons are moving in all directions. How then, can there be
a certain frame relative to which all the photons are at rest? Third, you only
mention the theoretical aspect, "that it is possible", but don't say anything
about how you actually measure it. Measuring the speed of the photons? But
they should all be moving at the speed of light?
Please enlighten me! :)
Submitted by mikael_eriksson76"AT"yahoo.com 09/08
I'm afraid I was being a bit sloppy with language here - sorry that this has confused you.
The CMB photons are always moving at the speed of light. There is a frame in which they are isotropic, in other words you see as many of them coming at you from one direction as any other direction. If you move relative to that frame, then you see more of them coming at you from the direction towards which you are moving and fewer in the direction you are moving away from (it's a bit like running in the rain).
So, to be a little more explicit, the number of photons per square degree (or other unit of solid angle) changes over the sky. This means that the brightness of the CMB also changes over the sky, and we observe the familiar dipole pattern. The direction and amplitude of this pattern tells us how fast we are moving compared with the "CMB rest frame". Here the phrase "CMB rest frame" is short-hand for "frame in which the CMB has no observable dipole, so that the sky is isotropic on large angular scales".
reguarding the cmb,
How does studying the radiation lead to our abiblity to determine the
composition of the universe, ie baryonic vs non-baryonic materials??
Submitted by gx_cully"AT"laurentian.ca 11/08
The idea is that there are variations in the temperature (or brightness) of the CMB sky. The strength of these variations is a function of angular scale. Experiments like the WMAP satellite measure the CMB sky accurately, so that this function can be calculated. The shape of this function (technically called the "power spectrum of anisotropies") encodes information about the initial fluctuations in the Universe and how they have evolved with time, which depends mainly on the composition of the Universe. So by accurately mapping the anisotropies it is possible to derive the values of several cosmological quantities, like the fraction of matter that is in the form of atoms as opposed to dark matter, etc.
Is the dipole anisotropy due to the Sunās orbital motion about the galactic
center, some motion of the galaxy in the local group, or is it something
bigger?
Submitted by soule1061"AT"verizon.net 10/09
The dipole is caused by the vector sum of all the relevant motions. This includes the "something bigger" which you allude to, namely that the Local Group of galaxies is moving at several hundred km/s towards a particular direction (sometimes called the "Great Attractor"). But the motion also includes the motion of our Galaxy relative to the centre of mass of the Local Group, the Sun relative to the local centre of mass, the motion of the Sun's neighbourhood around the Galaxy, the Earth's motion around the Sun, and even the rotation of the Earth (for a terrestrial experiment).
Kate Land and Joao Magueijo of Imperial College London labeled the dipole
anisotropy the "Axis of Evil". Can you elaborate?
Submitted by soule1061"AT"verizon.net 10/09
The so-called "axis of evil" is not the dipole. Instead it is a particular direction relative to which the largest-scale anisotropies (excluding the dipole) appear to be mildly correlated. I think it's fair to say that most cosmologists regard this is just a statistical fluke (perhaps affected by some systematic mapping effects), rather than an indication of some new physics. The statistical significance of the correlation is not very strong, and has not become stronger as the quality of the data has improved. It will be interesting to see if something similar is seen by the Planck satellite. But if the effect doesn't become more striking then this will get shelved under "interesting quirks" rather than leading to any Nobel prizes!
Can you describe the significance of the Canadian BAM experiment?
Submitted by barrys"AT"creation.portal.ca
BAM is the Balloon-borne Anisotropy Measurement, an ongoing effort to measure the CMB anisotropies, headed by Mark Halpern at the University of British Columbia. The first thing to say is that Mark's experiment is an incredible tour-de-force, given the resources he's had! Compared with US colleagues, the funding for BAM is very modest, and it's enormously to his credit that he has ever flown a balloon. The 1995 flight was largely a success, although there were some minor problems. The result is a significant detection of a signal which is most easily explained as being primordial. BAM's relatively wide frequency coverage means it can do better than most experiments at separating the primordial signal from the local (ie Galactic) signals which get in the way. So the BAM results are certainly among the most reliable indications that there really are primordial fluctuations at angular scales of a few degrees.
Detecting temperature differences of 1 part in 100,000 is pretty difficult! But to make real progress in understanding the Universe as a whole, it is important to measure those fluctuations over as large an amount of sky as possible. BAM is essentially ready to fly again, and is awaiting the return of stable winds to the balloon-launching facility in Palestine, Texas - this won't happen until about May, so that is when BAM is due to fly. This next flight will have increased sensitivity, and several other improvements, making it possible to scan a much larger amount of sky. The results from that flight ought to be really worth waiting for.
Could you let us know about the new satelites after COBE which will
study the CMB. What are they going to measure precisely?
Submitted by biswa"AT"media.physics.indiana.edu 11/98
There are two planned satellites. The Microwave Anisotropy Probe (MAP) is a US mission, which is due to launch in November 2000. A European-based satellite, the Planck Surveyor, is scheduled to be launched around 2006.
The satellites have similar science goals: to map the CMB anisotropies over the whole sky as well as possible. This means covering a wide range of frequencies, and having as high an angular resolution as possible. MAP will provide maps which are a huge improvement over the crude resolution and relatively low sensitivity of COBE. Planck aims to be the definitive CMB mission, mapping the sky at all relevant frequencies, and with an angular resolution matched to expectations for the anisotropies. You can think of them as the second generation and third generation CMB satellites, after COBE.
Rather than me saying any more, let me point you to the horses' mouths. Many more details can be found at the web sites for MAP and Planck.
What are the advantages of using interferometry, such as CAT and DASI,
in obtaining data about the CMBR?
Submitted by micky"AT"acon.com.au
Interferometers use correlations between multiple dishes to make maps of the sky. These maps contain information with an angular resolution (visual acuity) which is in principle the same as a telescope with diameter equal to the spacing between dishes. In practice a set of observations with different "baselines" (distances between the dishes) has to be combined in order to build up the full picture, with both large and small scale information fully represented in all directions.
Because of this correlation nature of interferometers, they are naturally taking differences between sets of signals on the sky. Hence they are very good at subtracting out the effects of the atmosphere, as well as effects due to lack of complete stability in the detectors. These kinds of "systematic" effects can be reduced by being very careful in taking and analysing the data from single dish experiments, but certainly this should all be easier with an interferometer.
It is also generally easier to build an interferometer which probes the smallest angular scales. This is because the angles that you probe in the sky are inversely proportional to the length scale of your system. And it's obviously easier to build an interferometer with small dishes 100 metres apart than to build a 100 metre dish.
Over a range of angular scales in which there is lots of current interest (namely from several degrees down to several minutes of arc) the single dish and interferometry approaches are both competitive. Hence there is one prototype interferometric experiment in existence (the Cosmic Anisotropy Telescope) and at least 3 more ambitious experiments being built: the Cosmic Background Interferometer (CBI); the Degree Angular Scale Interferometer (DASI); and the Very Small Array (VSA).
I am interesting in the CMB since my thesis is dealing in a noise processing
method. I believe that the CMB looks like a "noisy" signal in the time domain.
I am interesting in exploring its characteristics.
Submitted by sarafian"AT"inter.net.il 1/99
Interesting question. I'm not entirely sure what it is that would be most useful to you however. The COBE time-series signal was a fairly large set of noisy temperature differences on the sky. This had to be reduced to a set of averaged data, with various systematic effects removed from the data. Then the temperature differences between a huge set of pairs of positions had to be converted into a map of temperatures on the sky. More recent balloon experiments are similarly a set of temperature differences, and information about where the instrument was pointing at each time. Most raw CMB data files will be large - probably larger than you might want to deal with!
In general the "noise" in the data will have non-trivial properties, such as correlations in space and in time. These have to be removed in order to reveal the cosmological signal. Obviously this whole business can get quite tricky, particularly for large data sets. An additional issue for CMB data sets is that generally the time-stream of data has a very low "signal-to-noise ratio", unlike many other kinds of data which you might encounter. It's only when the data are averaged together that the signal can be seen. And even then, the nature of the signal is that sky has "excess variance" compared with the expected noise. This means that what you end up detecting is a set of splotchy patterns. It's reproducible (because it's real), but it isn't a "neat" signal, like a high contrast picture with nice sharp edges. There's a growing literature on ways of analysing signals from CMB experiments. However, much of the stuff used for more conventional signal processing is unfortunately not useful.
Probably you want to choose some simpler kind of data set to look at. But if you really want time-ordered data from a CMB experiment, then I have no doubt that many of the experimental groups would be happy to give you their GB worth of data to play with!
Why doesn't other sources of microwaves interfere with the smoothness of
the CBR? Wouldn't other sources of micowaves and other sources of
radation at other wavelengths that may get shifted into the same
wavelength as CBR make it hard to measure the CBR?
Submitted by deanf"AT"ecluboz.com 6/00
This is a most excellent question!
The answer is, of course, yes. It is crucial in measuring these tiny signals to investigate whether sources other than primordial temperature variations can be contributing. This is done through various means. The best way is to measure the anisotropies at a number of different wavelengths, and then test for whether the wavelength variation is consistent with other known sources of radiation in the local Universe. Gas and dust in our own Galaxy are particularly problematic. This means that it is hard to get decent CMB data near the Galactic plane, but away from the plane it turns out that these local sources of confusion are quite small. And if you make maps at several microwave wavelengths then it is relatively easy to tell them apart.
You're absolutely right that this is a source of concern for CMB experimenters, and they work very hard to check whether they are seeing some faint dust in our Galaxy, for example. But, for the best CMB data-sets (e.g. from COBE or the BOOMERANG maps) there can be no doubt that most of the signal seen is due to genuinely cosmic structure.
I'm currently an undergraduate doing research work in astrophysics and
cosmology. I'm currently trying to convince my
over-seers to allow me to do research with CMB whether on a balloon or from
ground.
I was wondering if you would either be able to lead me in the direction of a
information source that would suit my needs for general economic, logistic,
and instrumentation information?
Submitted by pelcher"AT"hotmail.com 7/00
This sounds very ambitious!
The answer depends on what exactly you had in mind. I can imagine a very instructive and useful undergraduate research project to determine (say) the temperature of the CMB at some particular wavelength. This is far from impossible, since in fact the CMB is a very bright source. I'm not saying it would be easy, since probably it would involve a lot of hard work. But it's probably well within the scope of an undergraduate project, and could well teach you an amazing amount.
However, attempting to put together an experiment to measure the anisotropy in the CMB would be an extremely ambitious project. The temperature differences are very small, and it is very easy to measure things other than variations in the CMB sky.
I suggest that the most useful thing might be for you to pay a visit to the CMB experimental group which is nearest to your home, and ask them for specific ideas. If you tell me which college you are attending, then I can try to put you in touch with someone. Maybe you could even do something to help out part of one of the current CMB experiments?
Good luck!
I was a co-op student
(Electrical Engineering) at BTL in the 1960s, about when the CMB was first
discovered by Penzias et al. There were a lot of jealous comments from the
Murray Hill solid stated physicists to the effect that "... all those guys
did was clean the pigeon s*** off their antenna, and for that they're going
to get a Nobel Prize ..."
Submitted by a.ross"AT"ieee.org 10/00
Thanks for sharing that!
The whole history of the discovery of the CMB is quite fascinating, with several twists and turns. The role of pigeon's in this story is nicely recollected in a section of Marcus Chown's popular book "Afterglow of Creation", called they died for Science!
I am currently researching the CMB in general and have a wide range
of information on the CMB theory and implications. But I am having a
really hard time trying to find information on the experimental
methods used... is there any chance you could direct me to a sutible
web page or journal artile etc..?
Submitted by R.C.Simpson"AT"newcastle.ac.uk 12/00
Every experiment is of course different in several ways, and so you should look at information about a few separate projects. Most of the experimental groups have their own web-page. I have a fairly up-to-date list here. Many of these web-pages contain technical details explaining how the experiments are carried out. I suspect there is more than enough here to keep you going!
Do you know of any books or scientific journals written on the
COBE and MAP missions that I might have access to through my local
library?
Submitted by bjserink"AT"home.com 3/01
There are at least 2 books written by members of the COBE team, with two different points of view. One is by George Smoot and Keay Davidson, called "Wrinkles in Time" published by Avon Books. The other is by John Mather and John Boslough, called "The Very First Light : The True Inside Story of the Scientific Journey Back to the Dawn of the Universe", published by Basic Books. MAP hasn't flown quite yet - but you can bet there will be books eventually!
I notice that every experiment I've read about, from the very first
Penzias/Wilson observation, used a cryocooled detector.
Is the background bright enough that some backyard experimenter could detect
it with today's low-noise electronics at room temperature?
Submitted by fwamsley"AT"wirelesscorp.com 9/01
This is an excellent question!
So excellent in fact that I asked the advice of my colleague Mark Halpern, who is an expert in CMB experiments, having actually built them, rather than just thought about them like me. Here's roughly what he suggested.
You can in fact probably detect the CMB with modern electronic detectors at room temperature. For example you could use a digital satellite receiver. But you probably have to do a couple of crucial things in order to be successful. First of all you want the power from your receiver over a much wider bandwidth than the electronics deals with. So probably you need to get the output as near to the dish as possible, before it has been processed into narrow bands etc. There ought to be low enough noise in such an off-the-shelf system to allow you to detect the CMB provided that you use a broad enough band (GHz if possible).
The second thing you have to do is figure out how to believe you've detected a non-zero constant signal. The easiest way to do this would be to make measurements at a number of different angles from the zenith. Then fit for the expected atmospheric variation (1 over the cosine of the angle) and see if there's a constant offset remaining. That offset is the CMB sky. You'd also have to try to avoid looking at the Galactic plane while you do this, because that will also give a detectable signal. But it should be possible to do this.
I haven't heard of anyone trying this recently with "Radio Shack" type equipment. So I'd be interested to know if you succeed. And if anyone else reading this knows of a useful article I'd also like to know about it.
do they(CMB radiations) get
scattered or reflected continuously from the different living and non
living object of this world? if the answer is yes then my next question
is- is it possible to detect or receive(experimentally) these CMB
radiation scattered or reflected from a particular object?
Submitted by swati_v"AT"isical.ac.in 12/01
The first thing to understand is that the CMB radiation, although it fills the Universe, is a very low energy density. So it is totally swamped by emission from warm material in our Galaxy, from stars, from the Earth's atmosphere, from the ground and from warm bodies (like you or me!).
Although it is certainly possible to detect it, you first have to subtract out the very much stronger local sources of radiation.
The answer to your question about absorption or scattering is that you can easily reflect CMB photons off the right sort of material (what you make a mirror out of for a CMB experiment, for example), but that many materials will just absorb it.
So if you put a CMB detector out in space, and tried to look through the Earth, you might expect to see a big hole there. Except that it's typically hard to make things (even space) colder than the CMB. So in fact the relatively warm Earth would of course appear like an incredibly bright hot-spot in your image. But it would certainly be impossible to observe the actual primordial CMB photons (tracing out the anisotropies, say) through an object like the Earth.
How did Penzias and Wilson know they had 3 degree radiation?
Didn't they just have the one reading at 7.35 cm? That could be in
any black body spectrum I want couldn't it?
Submitted by wrx"AT"mail.cruzio.com 12/01
That's correct. Penzias and Wilson had no way of knowing how close the CMB was to a blackbody. However, their measurements were on the long wavelength side, and there the brightness is easily related to a temperature, which they estimated to be about 3 Kelvin. It was only when measurements of the brightness had been made over a wide range of wavelengths (including both sides of the peak) that it was clear that the spectrum was close to blackbody. And we now know that it's very close of course.
According to te latest MAP data the product of the Hubble parameter and the
age of the universe is approximately 1 instead of 2/3. Please explain.
Submitted by LABELE"AT"aol.com 03/03
If the Universe had always been expanding at the same rate, then the time in the past when everything used to be in the same place is just 1/H0. However, this can only be the case if the Universe is completely empty. Models which contain matter will be slowing down due to the positive attraction of mass, and hence the Hubble parameter will have been larger in the past. Hence the age of the Universe will be a bit smaller than 1/H0. For a Universe with critical density in matter, it turns out that H0t0=2/3. However, the existence of Dark Energy makes things more complicated, because the Universe is now accelerating. It turns out that you get close to H0t0=1, even although the Universe used to be decelerating and now is accelerating. This depends on the precise values of the parameters which describe the model. But for the currently favoured values it turns out to be true that H0t0 is quite close to 1.
Why is microwave radiation used for measurement instead of other
wavelengths like Infrared or Gamma ray?
Can the same measurements be done with a radio receiver?
Submitted by 0nthony"AT"unihedron.com 5/03
The CMB radiation peaks at microwave wavelengths. But it is still measurable at longer (radio) wavelengths and shorter (infrared) wavelengths. However, it's entirely negligible at very short wavelengths like optical or gamma-ray.
Because the best range for observing (around a wavelength of 3mm) lies between the radio and infra-red bands, the measurements are done with a combination of radio techniques and infra-red techniques.
In fact the term "microwave" conventionally refers to the frequency range 300MHz (corresponding to a wavelength of 1m) up to 300GHz (corresponding to 1mm). So the CMB lies at the upper frequency (lower wavelength) end of the microwave region of the electromagnetic spectrum.
How weak is this radiation at
ground level? Is it significantly attenuated by atmosphere? i.e. would it be
much stronger on the orbit?
Submitted by 0nthony"AT"unihedron.com 1/04
You are absolutely right that the atmosphere attenuates microwave radiation. Unfortunately the details depend on the precise frequency that you're interested in, as well as other factors such as the weather conditions, altitude, angle to the zenith etc. A lot of the effect comes from water (and other) spectral lines, and so at some particular frequencies the effects are especially bad. But in some frequency bands (particularly lower frequencies) the effects are very modest. CMB experiments designed to measure the overall brightness (or spectrum) will obviously need to correct for this attentuation. For anisotropy experiments the correction becomes essentially part of the calibration procedure - one needs to know how many Volts in the detector corresponds to a given source brightness, with bright objects of known flux (e.g. planets) used to calibrate.
You can certainly find more information on the web. Perhaps the sort of data you are looking for would be given by people who study remote sensing of the earth from satellites, e.g. here
What is the difference between lenses used in microwave devices e.g.
horn lens antenna and binoculars or telescope lenses?
Submitted by TSM"AT"brightok.net 1/04
This is a topic on which I am far from an expert!
There are lenses used for focussing microwaves. They are formed using materials with appropriate refractive indices, or using a mesh of waveguides for focussing the wavefront. However, I think what you're talking about is an antenna rather than a lens.
Optical telescopes come in 2 basic varieties: refracting or reflecting. Typical small telescopes that you buy in the store will use lenses, and these are called "refracting telescopes". Larger amateur instruments and almost all professional telescopes are of the "reflecting" type, i.e. they use mirrors to focus the light. A microwave telescope is basically of this sort too, with a collecting dish which focusses the microwave light onto the detectors, or perhaps onto a secondary mirror which then directs the light to the detectors. Some microwave telscopes (the original of Penzias and Wilson for example) use a "horn antenna" to direct the microwaves into a waveguide through which they pass to the detector. It's more efficient to collect the radiation in a dish and then pass that to a "horn antenna" or "feed-horn" - so that's what many modern CMB telescopes have, rather than the horn pointing directly at the sky.
Is there a way to hear the CMB, that is to say, is there a specific way to
tune in to it so you know your reading it?
Submitted by blindmellojelly"AT"charter.net 12/04
There are answers to similar questions above on this page.
I assume by "hear" you mean detect the radiation and see evidence that you were detecting it in real time? In principle it isn't too difficult to detect the CMB for yourself, since it's about the brightest thing in the sky at microwave wavelengths. However, I'm not aware of any artcile or web-site describing how to go about building a home-made CMB detector. If anyone has seen such a thing, they should let me know.
Does the CMB show ANY deviation from a true black body, no matter how
small, from the results of recent measurments? I've looked at the available
energy spectra I can lay my hands on, but thought it would be useful to
actually ask someone more intimately involved with CMB than I am.
Submitted by ma"AT"star.ucl.ac.uk 02/05
Right now I'm not aware of any evidence for any deviation from a pure blackbody spectrum. The upper limits are pretty tight, depending on the form of the deviation you have in mind of course. The measurements come from a range of different experiements, dominated by results from the FIRAS instrument on the COBE satellite at the higher frequencies, and with many experiments contributing measurements at lower frequencies (including the ARCADE experiment most recently).
[abridged] one
thing that has been puzzling me for a while is whether the CMB should appear
as uniform as it does (in both spatial distribution and polarisation
distribution) when one takes into account quantum uncertainty that surely
must be encountered at the 'last scattering surface'? The release of
photons from the photon-baryon fluid seems to be too uniform to my way of
thinking if the only process was cooling through expansion?
Submitted by ma"AT"star.ucl.ac.uk 03/05
There are no quantum effects that I'm aware of, happening at the last scattering time, that are even close to being significant for the CMB anisotropies. It may be, of course, that you have something in mind that no one else has thought of - that would be very interesting!
It's also true that in the now conventional inflationary model for the early Universe, all the structure in the Universe comes originally from quantum fluctuations, i.e. the CMB anisotropies, galaxies, and everything else. So at very early times, quantum effects certainly are important.
how do you measure cmbr?
Submitted by bgy3reb"AT"leeds.ac.uk 03/05
If this question means how do I measure the CMB, then the answer is that I don't! I'm a theorists/data analyst, and leave measuring the CMB to people who actually know how to do it!
On a question: Can I see the CMB for myself? - you answer:
- In fact you can! If you tune your TV set between channels, a few percent of
the "snow" that you see on your screen is noise caused by the background of
microwaves.
What proofs? Exact and sensitive devices may show Cosmic Microwave Background.
The TV set cannot be named such device, - assert a repairman (TV sets)
Submitted by nonstop"AT"fort-dks.dp.ua 08/06
This isn't supposed to be "proof" for the existence of the CMB, just a cute idea. In fact modern TV's filter out noise and automatically tune into broadcasting channels, so you can't easily do the experiment!
You do need a very senstive device to measure the CMB anisotropies, since the variations are so minute. But you don't actually need a very sensitive detector to measure the existence of the CMB itself, since it's actually pretty bright at microwave frequencies. The trick is to build a device which is capable of measuring the absolute level of the microwave sky, which isn't as easy as it sounds.
how did Penzias/Wilson remove from their measurements
the radiation from the horn itself or radiation
from the atmosphere ?? Since these bodies are
above absolute zero, they have an emission spectra albeit very low,
from photos I have seen neither the horns nor the
chopper at the mouth of the horn are cooled to liquid helium as was the
detector.
Submitted by jnz9876"AT"yahoo.co.uk 10/06
I really wouldn't claim to be an expert on making absolute measurements of the CMB! And certainly I don't know exactly what Penzias and Wilson did. But I do know that removing all of the other sources of noise is a difficult procedure, and something that Penzias and Wilson took great care over. This is the main reason that they took so long to convince themselves that they were really seeing a cosmic signal. So you are right to wonder about how this is done in practice!
All such experiments are intrinsically "differential", i.e. they measure the difference between two things. This may be the "sky" versus a known "source", or it may be the difference between positions on the sky. By making many such measurements and calibrating things very carefully, it's possible to separate the emission coming from the telescope, from the ground, from the atmosphere, from the plane of the Galaxy and from the distant Universe.
I'm a high school researcher at
Harvard-Westlake School in Los Angeles. I'm intrigued by the continuing
analysis of cmb, and though I realize that an advanced background in
astrophysics is necessary to appreciate many of the recent developments
in the field, I would like to increase my understanding and develop
research in whichever ways possible.
As you study cmb, perhaps you have some suggestions for starting very
basic research into cosmology.
Some ideas I think might be within the realm of possibility for high
schoolers are
-building a homemade radiometer to "see" the background radiation for
myself (or obtaining discarded university equipment to do so)
-measuring the peculiar velocity of our local cluster through the cmb via
analysis of the dipole
-matching up anisotropies in the cmb with observed locations of galaxies
Submitted by gold.adam.07"AT"hwstudents.com 12/06
These are very good ideas, and I wish you luck on your quest to perform research on the Cosmic Microwave Background!
Building a detector of the CMB is in fact fairly simple (although I confess that it might be beyond my own experimental abilities!). The trick is to be able to make an "absolute" measurement. I'm not aware of anything written to describe how one might do this in practice. But if anyone knows of a relevant article or web-site, I'd be very glad to hear about it.
Measuring the dipole starts to become something difficult, since it's about one thousandth the amplitude of the CMB itself. So I suspect that this would be too ambitious a project for the High School level. But don't let me stop you if you're keen!
Something involving analysing existing CMB data is certainly feasible, and probably the easiest of the 3 suggestions. Data from the WMAP satellite are publicly available and easy to download from the internet. The trick is figuring out how to manipulate those data, which can be done using a variety of computer applications. To do this probably requires a fair level of competence on a computer, e.g. the ability to program in some language (or at least write scripts for some image processing application). There are many things which could be done with the data-sets once you know you can download and manipulate t
