A model of the universe that can collapse in on itself. Alexander Friedmann 1888-1925 discovered an error in Einstein’s work on cosmology, from which he showed that expanding universes and oscillating universes were possible in General Relativity. This work 1922 and 1924 was completely ignored until rediscovered and presented as the Lema”tre universe. The Friedmann universe can be closed if the density of matter is sufficient to reverse the expansion, and this has led to searches for the so-called missing mass. See also: open universe, closed universe, Robertson-Walker metric.