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 causing the cosmic scale factor to reach zero or causing a reformation of the universe starting with another Big Bang.
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 idea behind the theory is that the expansion of the universe was linked to the energy generated by Big Bang, thus the speed of the matter would decrease as matter traveled away from the explosion point, simply by the attraction or gravity, of other celestial bodies traveling along. This would act as ballast and would eventually lead to a halt of the expansion. Then as bodies attract and there is no matter beyond the maximum expansion point the matter would reverse the path, traveling back to the original explosion site.
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 take in opposite order. 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.
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). 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.
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.
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- McSween, Stephen A. "Dark Energy and the Red Shift in a Contracting Universe."