A universe which is constantly growing in size and in which the constituent parts (galaxies, clusters, etc) are flying ever further away from each other. Although contrary to the static universewhich had been assumed throughout most of history, an expanding universe was confirmed by Edwin Hubble’s 1929 observations of the redshifts of distant Cepheid variable stars, and is consistent with most solutions to Albert Einstein’s general relativity field equations. It also suggests that, in the distant past, the universe was much smaller and ultimately had its beginning in a Big Bang type event.