Big Crunch

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An animation of the supposed behaviour of a Big Crunch

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The Big Crunch is one possible scenario for the ultimate fate of the universe, in which the metric expansion of
spaceeventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero or
causing a reformation of the universe starting with another Big Bang.
Recent experimental evidence suggests 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,
a Big Crunch is still possible, although not observationally supported as of today. [citation needed]

Overview[edit]
If the universe's expansion speed does not exceed the escape velocity, then the mutual gravitational attraction of all
its matterwill 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. [1] Eventually all matter would
collapse into black holes, which would then coalesce, producing a unified black hole or Big Crunch singularity.
The idea behind the theory is that the expansion of the universe is linked to the energy released in the Big Bang,
therefore the outward speed of the matter would decrease over time due to gravity (mutual attraction). This would act as
ballast and would eventually lead to a halt of the expansion. As matter attracts and there is no matter beyond the
maximum expansion point, eventually all matter would begin to travel inwards again, accelerating as time passes.
The exact details of the events that would take place before such final collapse depend on the length of both the
expansion phase as well as the previous contraction phase; the longer both lasted, the more events expected to take
place in an ever-expanding universe would happen; nonetheless it's expected that the contraction phase would not
immediately be noticed by hypothetical observers because of the delay caused by the speed of light, that the
temperature of the cosmic microwave background would rise during contraction symmetrically compared to the previous
expansion phase, and that the events that took place during the Big Bang would occur in opposite order. [2] For a
contracting Universe similar to ours in composition it's expected that superclusters would merge among themselves
followed by galaxy clusters and later galaxies. By the time stars were so close together that collisions among them were
frequent, the temperature of the cosmic microwave background would have increased so much that stars would be
unable to expel their internal heat, slowly cooking until they exploded, leaving behind a hot and highly heterogeneous
gas, whose atoms would break down into their constituent subatomic particles because of the increasing temperature,
that would be absorbed by the already coalescing black holes before the Big Crunch itself. [2]
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 [1]—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 heat death of the universe, where the universe reaches the maximum state of entropy that
is thermodynamic equilibrium. In the state of thermodynamic equilibrium energy in the universe is evenly distributed
so heat transfer or any other energy transfer is impossible so no reactions can happen in such universe making it
"dead".[3][not in citation given] One theory proposes that the universe could collapse to the state where it began and then initiate
another Big Bang,[1] so in this way the universe would last forever, but would pass through phases of expansion (Big
Bang) and contraction (Big Crunch).[4] Another scenario results in a flat universe which occurs when the critical density
is just right. In this state the universe would always be slowing down, and eventually come to a stop in an interminable
amount of time. Although, it is now understood that the critical density has been measured and determined to be a flat
universe.[5]
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. [6][7]