The Big Bang Theory was formulated by Alexander Friedmann in 1922 by applying Eintein's General Theory of Relativity to the structure and evolution of the universe as a whole. Friedmann began with Einstein's equations and found a solution to those equations in which the universe began in a state of extremely high density and temperature (the so-called Big Bang) and then expanded in time, thinning out and cooling as it did so. One of the most stunning successes of the Big Bang theory is the prediction that the universe is approximately ten to fifteen billion years old, a result obtained from the rate at which distant galaxies are flying away from each other. This prediction accords with the age of the universe as obtained from very local methods, such as the dating of radioactive rocks on the earth.
If this is so, the universe will collapse into a singularity reminiscent of the one from which it was born. The Big Crunch would look much like the Big Bang in reverse except it would be much less uniform. The tiny uniformities in the universe right after the Big Bang are what grew to form the galaxies and clusters. Whether out universe will die in a Big Crunch or Big Chill can be determined by measuring the density of matter versus the rate of expansion. Much of modern cosmology, including the construction of giant new telescopes such as the new Keck telescope in Hawaii, has been an attempt to measure these two numbers with better and better accuracy. With the present accureacy of measurement, the numbers suggest that our universe will keep expanding forever, growing colder and colder, thinner and thinner.