Some of the earliest models of the Universe to incorporate Einstein's general relativity were proposed by Aleksandr Friedmann (1888-1925) and Abbe Georges Lemaitre (1894-1966). They derived solutions to Einstein's field equations (Friedmann in 1922 and Lemaitre in 1927) which predicted that galaxies should be receding from each other due to cosmic expansion.

They found three distinct classes of models:**the open Universe**: a low matter-density Universe which expands forever**the closed Universe**: a high matter-density Universe in which gravity eventually stops the expansion and the Universe re-collapses**the critical density (flat Universe) Universe**: in which the density of matter is just enough to theoretically stop the expansion, but only after an infinite length of time.

Somewhat surprisingly, current observational data favours the delicately balanced flat over both the open and closed models.