Frequently Asked Questions

If people frequently asked any questions about the CMB, then these might be among them!

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What, another acronym?

CMB stands for Cosmic Microwave Background. It is also sometimes called the CBR, for Cosmic Background Radiation, although this is really a more general term that includes other cosmological backgrounds, eg infra-red, radio, x-ray, gravity-wave, neutrino. The CMB contains hugely more energy than any other cosmic radiation source, however, so it is the dominant component of the overall CBR spectrum. Other acronyms, such as CMBR, are also sometimes used!

What's "Cosmic" about it?

We refer to it as "cosmic" because the only known source of this radiation is the early universe. It can now be firmly concluded that the CMB is the cooled remnant of the hot Big Bang itself.

Why "Microwave"?

Light comes in a range of wavelengths, from the shortest wavelength gamma-rays to the longest wavelength radio waves, with common-or-garden visible light in the middle. All of these signals are manifestations of the same underlying physical phenomenon, travelling packets of oscillating electric and magnetic fields, called electro-magnetic radiation. All of the forms of electromagnetic radiation travel at the same speed, the speed of light, which is 300,000 km/s. E-m radiation of different wavelengths will interact with matter in different ways. For example, radio waves are picked up by a radio receiver, your eye detects visible light, infra-red radiation warms your skin, x-rays penetrate your body, gamma-rays can give you radiation damage.

Microwaves are the name given to radiation between the infra-red and radio region, with wavelengths typically in the 1mm to 10cm range. Some specific wavelengths of microwaves can be used to excite the molecules in foodstuff, so that you can use them to cook. It turns out that if you had a sensitive microwave telescope in your house you would detect a faint signal leaking out of your microwave oven, and from various other man-made sources, but also a faint signal coming from all directions that you pointed. This is the Cosmic Microwave Background.

Why is it called a "Background"?

We refer to this radiation as a background because we see it no matter where we look. It clearly doesn't come from any nearby objects, such as stars or clouds within our Galaxy, or even from external galaxies. It is clearly a distant, "background" source of radiation. You can think of the whole Universe as being filled with this background of microwave photons.

How do I pronounce "anisotropy"?

If you've never come across this word before, then (obviously) it's new to you, and so even professional cosmologists sometimes pronounce it wrongly. This then is a good question, but hard to answer in plain text! Basically, the stress is on the third syllable, and the common mistake is to stress the fourth. The confusion presumably arises from knowing how to pronounce "anisotropic", and then thinking that you just pronounce it the same way, but without the final consonant.

Why does the CMB support the Big Bang picture?

The basic point is that the spectrum of the CMB is remarkably close to the theoretical spectrum of what is known as a "blackbody", which means an object in "thermal equilibrium". Thermal equilibrium means that the object has had long enough to settle down to its natural state. Your average piece of hot, glowing coal, for example, is not in very good thermal equlibrium, and a "blackbody" spectrum is only a crude approximation for the spectrum of glowing embers. But it turns out that the early Universe was in very good thermal equilibrium (basically because the timescale for settling down was very much shorter than the expansion timescale for the Universe). And hence radiation from those very early times should have a spectrum very close to that of a blackbody.

The observed CMB spectrum is in fact better than the best blackbody spectrum we can make in a laboratory! So it is very hard to imagine that the CMB comes from emission from any normal "stuff" (since if you try to make "stuff" at some temperature, it will tend to either emit or absorb preferentially at particular wavelengths). The only plausible explanation for having this uniform radiation, with such a precise blackbody spectrum, is for it to come from the whole Universe at a time when it was much hotter and denser than it is now. Hence the CMB spectrum is essentially incontrovertible evidence that the Universe experienced a "hot Big Bang" stage (that's not to say that we understand the initial instant, just that we know the Universe used to be very hot and dense and has been expanding ever since).

In full, the three cornerstones of the Big Bang model are: (1) the blackbody nature of the CMB spectrum; (2) redshifting of distant galaxies (indicating approximately uniform expansion); and (3) the observed abundances of light elements (in particular helium and heavy hydrogen), indicating that they were "cooked" throughout the Universe at early times. Because of these three basic facts, all of which have strengthened over the decades since they were discovered, and several supporting pieces of evidence found in the last deacade or two, the Big Bang model has become the standard picture for the evolution of our Universe.

Can I see the CMB for myself?

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.

How come we can tell what motion we have with respect to the CMB?

Doesn't this mean there's an absolute frame of reference?

The theory of special relativity is based on the principle that there are no preferred reference frames. In other words, the whole of Einstein's theory rests on the assumption that physics works the same irrespective of what speed and direction you have. So the fact that there is a frame of reference in which there is no motion through the CMB would appear to violate special relativity!

However, the crucial assumption of Einstein's theory is not that there are no special frames, but that there are no special frames where the laws of physics are different. There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe. But for doing any physics experiment, any other frame is as good as this one. So the only difference is that in the CMB rest frame you measure no velocity with respect to the CMB photons, but that does not imply any fundamental difference in the laws of physics.

What sort of telescope is used to observe the CMB?

Like light at any other wavelength the general system is a dish to collect and focus the radiation, a way of feeding the radiation to the instruments, and then the instruments themselves which are used to detect and record the signals. For microwaves the dish, or set of dishes, is made of a material (metal) which reflects microwaves. The focussed radiation is transported to the receivers by means of "wave-guides", which are pipes specially tuned to transmit microwave signals.

Then the detectors come in two types. "Bolometers" involve technology developed to detect infra-red radiation. They are essentially tiny pieces of special materials which absorb the microwave radiation. This in turn induces a minute change in temperature which is detected by a thermal sensor. These temperature variations are picked up in an electrical circuit and stored on computer. The other technology involves high performance transistors, which work in much the same way as the input circuitry of a radio receiver, only very much more efficient at picking up microwaves. Again the signal is then picked up and stored electronically.

If you are interested in more detail you might want to check out a nice concise text like "Detection of Light from the Ultraviolet to the Submillimeter", by G.H. Rieke, Cambridge Press, 1996.

Where did the photons actually come from?

A very good question. We believe that the very early Universe was very hot and dense. At an early enough time it was so hot, ie there was so much energy around, that pairs of particles and anti-particles were continually being created and annihilated again. This annihilation makes pure energy, which means particles of light - photons. As the Universe expanded and the temperature fell the particles and anti-particles (quarks and the like) annihilated each other for the last time, and the energies were low enough that they couldn't be recreated again. For some reason (that still isn't well understood) the early Universe had about one part in a billion more particles than anti-particles. So when all the anti-particles had annihilated all the particles, that left about a billion photons for every particle of matter. And that's the way the Universe is today!

So the photons that we observe in the cosmic microwave background were created in the first minute or so of the history of the Universe. Subsequently they cooled along with the expansion of the Universe, and eventually they can be observed today with a temperature of about 2.73 Kelvin.

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Douglas Scott
Last revised: 14th November 2001