Big Crunch
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In physical cosmology, the Big Crunch is the hypothesis that the universe will collapse upon itself after its expansion eventually stops — a counterpart to the Big Bang. This hypothesis is the subject of much heated discussion.
[edit] Overview
If the gravitational attraction of all the matter within the observable horizon were high enough, the expansion of the universe would slow, and then reverse eventually. 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.
However, recent experimental evidence (namely the observation of distant supernova as standard candles, and the well-resolved mapping of the cosmic microwave background) has—to 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.[1] (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).
[edit] See also
- Big Rip
- Heat death of the universe
- Omega point (Tipler)
- Ultimate fate of the universe
- Arrow of time
- Entropy (arrow of time)
[edit] External links
- "Cosmic doomsday delayed" (news@nature.com)
- "Current Observational Constraints on Cosmic Doomsday" (Wang, Kratochvil, Linde, and Shmakova)
- "How to destroy the earth @ Things of Interest"
