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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.^[1]

Overview

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.^[2] 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^[2]—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.^[3] One theory proposes that the universe could collapse to the state where it began and then initiate another Big Bang,^[2] so in this way the universe would last forever, but would pass through phases of expansion (Big Bang) and contraction (Big Crunch).^[4]

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.^[5]

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.

References

^ Ellis, George F. R., R. Maartens, and M. A. H. MacCallum. Relativistic Cosmology. Cambridge: Cambridge UP, 2012. 146–47. Print.
^ ^a ^b ^c How the Universe Works 3. Vol. 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

[1] Ellis, George F. R., R. Maartens, and M. A. H. MacCallum. Relativistic Cosmology. Cambridge: Cambridge UP, 2012. 146–47. Print.

[HTUW-2] How the Universe Works 3. Vol. End of the Universe. Discovery Channel. 2014.

[3] Dr. Gary F. Hinshaw, WMAP Introduction to Cosmology. NASA (2008)

[4] Jennifer Bergman, The Big Crunch, Windows to the Universe (2003)

[5] Y Wang, J M Kratochvil, A Linde, and M Shmakova, Current Observational Constraints on Cosmic Doomsday. JCAP 0412 (2004) 006, astro-ph/0409264

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 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 [http://phys.org/news/2015-03-universe-brink-collapse-cosmological-timescale.html contraction] of the universe was posted to  accredited [[Phys.org]] and [http://www.iflscience.com/physics/big-crunch-back-possible-end-universe another] on popular science blog [[Elise Andrew|IFLScience]].  A month later a newer [http://earthsky.org/space/what-is-dark-energy%20expansion article] was posted in favor of its expansion without any reference to the new study regarding the contraction.
-== In popular culture ==
-* The first documentation of a cyclical expanding and contracting universe comes from the poetic writings of [[Erasmus Darwin]] in 1791.
-* In ''[[The Restaurant at the End of the Universe]]'', the big crunch is the floor show entertainment.  It is referred to as "the Gnab Gib".
-* The Polish science-fiction novel ''[[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.
-* [[Poul Anderson]]'s novel ''[[Tau Zero]]'' involves a starship that ultimately survives the Big Crunch at the end of the novel.
-* In ''[[Super Mario Galaxy]]'', [[Bowser (character)|Bowser]] builds a sun which collapses in on itself to form a [[supermassive black hole]]. The Universe is sucked into it and ultimately destroyed via being crunched into a singularity, and is soon after reconstructed in a big bang.
-* The computer game ''[[Marathon (computer game)|Marathon]]'' uses the Big Crunch as a basis for the actions of a main character, the [[rampancy|rampant]] [[artificial intelligence|AI]] [[Durandal]].
-* The computer game ''[[Anachronox]]'' features the destruction and creation of subsequent universes.
-* The animated television show ''((Futurama))'', features an episode wherein some of the main characters use a time machine that only goes forwards in time, after visiting the end of the universe. Mysteriously, it restarts ten feet lower than the last one.
-* An episode of the third series of ''[[Red Dwarf]]'', "Backwards", also (albeit comically) takes place on Earth during a big crunch, during which time runs in reverse along with the universe contracting.<ref>http://www.comedy.co.uk/guide/tv/red_dwarf/episodes/3/1/</ref>
-* In the ''[[Doctor Who]]'' television series between the years 1980 and 1989 the various opening credits follow a "Big Bang" and expansion of the universe, while the closing credits show the reverse: a contraction and eventual "Big Crunch".
-* In the second season of the television series ''[[LEXX]]'', an evil scientist called [[Mantrid]] converts all matter in the "light" universe into one-armed "Mantrid [[Robot|drones]]" which causes the universe to collapse in on itself and sends the [[starship]] ''Lexx'' through a [[Gravitational singularity|singularity]] to an [[parallel universe (fiction)|alternate "dark" universe]].
-* In the ''[[The Simpsons]]'' episode "[[Treehouse of Horror XVI]]", the entire universe (including God) collapses into a singularity when [[Kang and Kodos]] activate an accelator-beam at a [[baseball]] game.
-* 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]].
-* The Marvel Comics [[Wolverine (comic book)|Wolverine]] series included [[Mojo (comics)|Mojo]] attempting to stop the Big Crunch.
-* In the movie [[K-PAX (film)|''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.
-* Lyrics of the [[Bad Religion]] song "Big Bang" reference both the [[Big Bang]] and Big Crunch theories.
-* Big Crunch and time reversal accompanying it are central themes in the [[Mr. Nobody (film)|''Mr. Nobody'']] movie.
-* In part 6 of ''[[JoJo's Bizarre Adventure]]'' the main antagonist uses his stand's power to speed up time dramatically causing a Big Crunch to occur much earlier than intended. The result is the creation of an entirely new universe.
-* In episode 8 of ''[[Umineko no Naku Koro ni]]'', Bernkastel and Lambdadelta create and destroy megaverse when they are fighting in City of Books, the highest dimensional space of the whole series. A Big Bang and Big Crunch take place between the two witches.
-* In ''[[The Zero Theorem]]'', Qohen is tasked to prove life is meaningless, unknowingly, by the character known as Management, using the Big Crunch.
-* Lyrics from [[Franz Ferdinand (band)|Franz Ferdinand]]'s Universe Expanded implicitly references the Big Crunch.
-*In [[Philip José Farmer]]'s novel [[Dark Is the Sun]] is set on Earth fifteen billion years in the future when the universe is approaching the big crunch.
 == See also ==