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In physical cosmology, the Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity or causing a reformation of the universe starting with another big bang. Sudden singularities and crunch or rip singularities at late times occur only for hypothetical matter with implausible physical properties.
If the universe's expansion speed does not exceed the escape velocity, then the mutual gravitational attraction of all its matter will eventually cause it to contract. If entropy continues to increase in the contracting phase (see Ergodic hypothesis), the contraction would appear very different from the time reversal of the expansion. While the early universe was highly uniform, a contracting universe would become increasingly clumped. Eventually all matter would collapse into black holes, which would then coalesce producing a unified black hole or Big Crunch singularity.
The Hubble Constant measures the current state of expansion in the universe, and the strength of the gravitational force depends on the density and pressure of matter in the universe, or in other words, the critical density of the universe. If the density of the universe is greater than the critical density, then the strength of the gravitational force will stop the universe from expanding and the universe will collapse back on itself—assuming that there is no repulsive force such as a cosmological constant. Conversely, if the density of the universe is less than the critical density, the universe will continue to expand and the gravitational pull will not be enough to stop the universe from expanding. This scenario would result in the "Big Freeze", where the universe cools as it expands and reaches a state of entropy. One theory proposes that the universe could collapse to the state where it began and then initiate another Big Bang, so in this way the universe would last forever, but would pass through phases of expansion (Big Bang) and contraction (Big Crunch).
Recent experimental evidence (namely the observation of distant supernovae as standard candles, and the well-resolved mapping of the cosmic microwave background) has led to speculation that the expansion of the universe is not being slowed down by gravity but rather accelerating. However, since the nature of the dark energy that is postulated to drive the acceleration is unknown, it is still possible (though not observationally supported as of today) that it might eventually reverse its developmental path and cause a collapse.
This topic is being rigorously debated among scientists. While lack of further data casts doubts on the notion, it appears both expansion theories, the Big Rip and the Big Freeze, as well as the Big Crunch have had recent publicity in several notable, prominent, scientific publications. An article regarding the contraction of the universe was posted to accredited Phys.org and another on popular science blog IFLScience. A month later a newer article was posted in favor of its expansion without any reference to the new study regarding the contraction.
- Arrow of time
- Bentley's paradox
- Big Bounce
- Big Rip
- Chronology of the universe
- Cyclic model
- Entropy (arrow of time)
- Eternal return
- Gravitational collapse
- Heat death of the universe
- Timeline of the far future
- Timeline of the formation of the Universe
- Ellis, George F. R., R. Maartens, and M. A. H. MacCallum. Relativistic Cosmology. Cambridge: Cambridge UP, 2012. 146–47. Print.
- How the Universe Works 3. End of the Universe. Discovery Channel. 2014.
- Dr. Gary F. Hinshaw, WMAP Introduction to Cosmology. NASA (2008)
- Jennifer Bergman, The Big Crunch, Windows to the Universe (2003)
- Y Wang, J M Kratochvil, A Linde, and M Shmakova, Current Observational Constraints on Cosmic Doomsday. JCAP 0412 (2004) 006, astro-ph/0409264