Big Crunch
From Academic Kids

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.
Gravitational_lensing.png
Lensing due to gravity
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 meaningless 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 completely counter inertia, 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 wellresolved 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, suggesting that the universe will not end with a "Big Crunch", but will instead expand forever. (The evidence of an accelerating universe is considered conclusive by most cosmologists since 2002.)
In the framework of the field equations of the General Theory of Relativity, the simplest model of an accelerating expansion corresponds to a positive value of the cosmological constant, which is often attributed to the quantum vacuum itself exerting a negative pressure that repels gravitationally on large scales. More generally, the accelerating expansion is attributed to dark energy, which could be the cosmological constant, or a dynamical field with negative pressure, leading to an effective cosmological constant that could be timevarying. In such cases, it is theoretically possible that the cosmological constant need not remain positive, leaving open the possibility of a Big Crunch in a cosmic doomsday scenario. A Big Crunch is also still theoretically possible if Einstein's theory of general relativity turns out to be incorrect on large scales. The current evidence neither favors nor rules out dark energy, or modifications of general relativity, of a form that could halt or reverse an eternal expansion; it does, however set lower bounds on the soonest the universe could collapse (~ 42 billion years from now, or more than 24 billion years at the 95% confidence level, according to one group led by Andrei Linde).
In fiction
The computer game Marathon uses the Big Crunch as a basis for the actions of a main character. This character believes that, if one leaves the universe as it is being 'crunched', that person would reach a form of godhood  either by existing in a reality without the limits imposed by the universe itself, or by being the first sentient creature in a new universe (following another Big Bang).
The Big Crunch is also referred to as the Gnab Gib ("Big Bang" read backwards).
The first documentation of a cyclical expanding and contracting universe comes from the poetic writings of Erasmus Darwin in 1791. See full text passage: Erasmus Darwin.
See also
External links
 "Cosmic doomsday delayed" (http://www.nature.com/news/2004/041101/full/04110115.html) (news@nature.com)
 "Current Observational Constraints on Cosmic Doomsday" (http://arXiv.org/abs/astroph/0409264) (Wang, Kratochvil, Linde, and Shmakova)da:Big Crunch