The big bang model of the universe makes a number of profound and testable predictions. In a uniformly expanding universe, galaxies would have been closer together in the past. Early in the universe, the density (and temperature) of matter would therefore have been very high. The discovery of the expansion of the universe by E. Hubble in 1929, the discovery of a (now) cool remnant cosmic background radiation by A. Penzias and R. W. Wilson in 1965, and the observed relative abundances of the lightest elements (notably, hydrogen and helium) provide compelling evidence in support of the big bang model.