The expanding universe is described by the Friedman-Lemaître cosmological model. This elegant and simple theory, derived independently by A. Friedman in 1924 and G. E. Lemaître in 1927, predicted the expansion of the universe that was observed by E. P. Hubble in 1929. According to this model, usually referred to as the big bang theory, the rate of deceleration of a sufficiently large shell of matter in the universe is determined by the total mass contained within the shell.