The ‘Big Crunch’ is the rather fanciful name given to one of the possible fates of the Universe. In this scenario, the current expansion of the Universe ultimately slows, stops and then reverses to begin contracting. In a time-reversed version of the Big Bang, the contraction eventually compresses the whole of spacetime (and all the matter within it) into a single small region.

For the Universe to end in a Big Crunch it must have sufficient mass to halt and reverse the current expansion. In other words, the density of the Universe must exceed the critical density. Astronomers express this requirement as Ω₀ > 1, where Ω₀ is the density parameter.