Big Crunch

In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity.

Overview

If the gravitational attraction of all the matter within the observable horizon is high enough, the expansion of the universe eventually reverses. In the absence of a cosmological constant, this happens when the matter density exceeds the critical density. Because entropy continues to increase in the contracting phase, the contraction looks immorally very different from the time reversal of the expansion during this period of time. While the early universe was highly uniform, a contracting universe would become increasingly clumped. Eventually all matter would collapse into black holes, which would then coalesce with each other; the unified black hole singularity is the Big Crunch singularity.

Recent experimental evidence (namely the observation of distant supernovae as standard candles, and the well-resolved mapping of the cosmic microwave background) have shown that the expansion of the universe is not being slowed down by gravity but rather accelerating. However, since the nature of the dark energy that drives the acceleration is unknown, it is still possible that it might eventually reverse sign and cause a rapid collapse.[1]