Big Crunch

In physical cosmology, the Big Crunch is a hypothesized collapse of the universe upon itself after its expansion eventually stops — a counterpart to the Big Bang.

If the gravitational attraction of all the matter within the observable horizon is high enough, it could slow the expansion of the universe, and then reverse it. The universe would then contract, with about the same duration as the expansion. Eventually, all matter and energy would be compressed back into a gravitational singularity. It is meaningless to ask what would happen after this, because time, as we know it, would end in this singularity.
Lensing due to gravity.

For this to occur, the average density of matter in the universe has to be sufficient so 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 three 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) has—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, though some scientists have contested this theory. (The evidence of an accelerating universe has been 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 can be attributed to the quantum vacuum itself exerting a force 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 time-varying. In such cases, it is theoretically possible that the cosmological "constant" need not remain positive, leaving open the possibility of a Big Crunch as the ultimate fate of the universe. A Big Crunch is also still theoretically possible if Einstein's theory of general relativity were found not to apply 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 duration collapse (approximately 42 billion years from now, or more than 24 billion years at the 95% confidence level, according to one group led by Andrei Linde).
The Big Crunch in popular culture
The computer game Marathon uses the Big Crunch as a basis for the actions of a main character, the rampant AI Durandal. 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). Another possibility is that Durandal intends to stop the Big Crunch and stabilise the universe. If Durandal removes enough mass from the universe, it will no longer have sufficient gravitational energy to collapse. This is hinted at by Tycho (another AI): "...we have both begun to realise how it may be cheated (though the price may number in the tens of thousands of stars)".
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.
Mantrid succeeds, but not without causing the universe to collapse in on itself – which results from the rapid convergence of remaining matter to its center – and sending the starship Lexx through a singularity to an alternate "dark" universe.
In the The Simpsons episode "Treehouse of Horror XVI", the universe collapses into a singularity when Kang and Kodos activate an accelator-beam at a baseball game. As the acceleration continues, the baseball stadium is drawn in to a specific point where the gravity is high. Afterwards the stadium's surroundings are drawn into this point and then the whole city, then Earth's oceans, the whole Earth, the solar system, numerous galaxies and finally God is drawn into this point. When this happens, Kang leaves a note saying: "Treehouse of Horror XVI".
The Marvel Comics character Galactus, a being of cosmic powers, is a reincarnation of "Galan", an intrepid space explorer of the planet Taa and the last survivor of the previous universe. Upon that universe's Big Crunch, Galan was transformed and then released into the newborn universe upon its Big Bang. After spending a lengthy but indeterminate time in gestation, Galactus emerged and began his relentless feeding upon the life energy of numerous planets, destroying them in the process.
In the movie K-PAX, the lead character, an alleged alien named Prot, says that his race has long ago figured that the universe will one day contract into a singularity, then expand again, and repeat the process eternally. He also states that each cycle will be exactly the same, including everyone's actions.
The band The Electrons wrote a song called "Big Crunch" that uses the concept as a metaphor for a relationship that has gone sour.
Poul Anderson's novel Tau Zero involves a starship that ultimately survives the Big Crunch at the end of the novel.
Although the true meaning of the song Big Bang is debatable, it's lyrics make reference to both the Big Bang and Big Crunch theories.