In cosmology, the Big Crunch is a hypothesis that states the universe will stop expanding and start to collapse upon itself; a counterpart to the Big Bang.
If the gravitational attraction of all the matter in the observable horizon is high enough, then it could stop the expansion of the universe, and then reverse it. The universe would then contract, in about the same time as the expansion took. Eventually, all matter and energy would be compressed back into a gravitational singularity. It is impossible to ask what would happen after this, as time would stop in this singularity as well.
For this to happen, the average density of matter in the Universe has to be so high, that the overall spatial curvature of the Universe is positive, like the surface of a sphere. If the matter density is less than a certain value, called the critical density, the curvature is negative (like a hyperbolic surface, which is a mathematical manifold often compared to the form of a saddle) and gravitation will be too feeble to completly counter gravity, so that expansion will continue to slow down but never come to an end. These two cases, and the limiting case in between in which space is flat, are called the 3 Friedmann models. They assume the cosmological constant to be zero.
However, recent experimental evidence (namely the observation of distant supernovae as standard candles, and the well-resolved mapping of the cosmic microwave background) have - to most scientists' considerable surprise - shown that the expansion of the universe is not being slowed down by gravity, but instead, accelerating. (This evidence is considered conclusive by most cosmologists since 2002.)
In the framework of the field equations of the General Theory of Relativity this corresponds to a non-zero value of the cosmological constant, which again means the existence of a mysterious substance or field or force or property of the vacuum itself exerting a negative pressure that counters gravity on large scales.
The Big Crunch is also referred to as the Gnab Gib ("Big Bang" read backwards).
