Today's observed recession of galaxies pays witness to the sheer force of the Big Bang, and the enormous - almost inceivably large - reservoir of energy that it provided the Universe. During the 1920's, the astronomer Edwin Hubble was the first to realize the largescale expansion of the Univers by examining the "redshifted" spectra of a number of galaxies. (A "redshift" refers to a shift to longer wavelengths in the spectral lines of an object's spectrum, while a "blueshift" represents a shift to shorter wavelengths.)

To relate the recession velocity of a galaxy with its distance, Hubble proposed the relationship: v = H x R. The constant of proportionality, H, is known as "Hubbles' constant" and has units of km/sec/megaparsec. Modern astronomers disagree as to the precise value of H. The most commonly used estimate is around 50 km/sec/Mpc, but astronomical data indicates that the value can range up to 100 km/sec/Mpc.

Of immediate significance is the realization that Hubble's Law, and in particular Hubble's constant, cna be used to set limits on the age of the Universe. Inverting Hubble's costant, and canceling units of distance, actually yields a time estimate for the age of the Universe. For a value of H=50 km/sec/Mpc, we have an age of 10 billion years, while for H=100 km/sec/Mpc, the age is 20 billion years. Hence, we conclude that the age of the Universe, as derived from Hubble's Law, is somewhere between 10 and 20 billion years old.

Two isotopes of uranium, 235 and 238, are especially useful in dating techniques. U-238 decays into lead (Pb-206) with a half-life of about 4.5 billion years. Assuming we know how much of the lead isotope was originally contained in a given sample, we can determine its age by comparing present abundances of U-238 and Pb-206. This dating method has been used to assess the ages of the oldest rocks on Earth, which have been found to be about 4 billion years old. Lunar rocks and the oldest meteorites yield ages of about 4.6 billion years. Similar dating techniques exist for isotopes of strontium, krypton, and rubidium. Taken together, we conclude that from the age measurements of meteorites, lunar and Earth rocks, that the Solar System was formed around 5 billion years ago.

The heavy elements in the Universe were largely produced in supernova explosions. In order to estimate the age of the Universe, we must combine the age of the radioactive elements with the supernova age. The rate of supernova events in our Galaxy is estimated to be about one every 30 years or so. If we add in the time for galaxy formation, around 1 billion years, we find an estimate for the age of the Universe in the range between 11 and 18 billion years.