The Universe today is old, cold, low-density, and expanding. If we run

the expansion backwards, we will eventually find a Universe where all

the matter was in one place where the density and temperature are nearly

infinite. We call this hot, dense initial state of the Universe the Big

Bang. This lecture introduces the Big Bang model of the expanding

universe, and how the history of the Universe depends on two numbers:

the curretn expansion rate (H0), and the relative density of matter and

energy (Omega0). Combined with observations, these give us an estimate

of the age of the Universe of 14.0 +/- 1.4 Gyr. Recorded 2006 February 27 in

1008 Evans Laboratory on the Columbus campus of The Ohio State

University.