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In 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 would end in this singularity.
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) 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 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 in a cosmic doomsday scenario. A Big Crunch is also still theoretically possible if Einstein's theory of general relativity was 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).
Literature and entertainment
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).
The computer game Anachronox' story features the destruction and creation of subsequent universes.
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.
The British television show Red Dwarf, the episode "Backwards" deals with a parallel universe going through the Big Crunch by going reverse in time and space that would eventually descend into a singularity point in the universe before the Big Bang.
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.
The Polish science-fiction book "Paroxysm number minus one" ("Paroksyzm numer minus jeden") by Ryszard Głowacki features beings that survived the last Big Crunch (called "paroxysm") and await the next one.
See also
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