The Big Crunch Theory
The Ultimate Fate of the Universe?

The Big Crunch Theory
The Ultimate Fate of the Universe?

The Big Crunch Theory is basically the shrinking of the universe. Our universe currently is expanding but the theory of the big crunch is that space will shrink in time. This theory would be the ultimate fate of the universe.

Speculation that the universe will slow down not because of gravity but because of lack of acceleration and then collapse into black holes creating one large black hole or big crunch singularity. This is a possible theory. 

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.

The Big Crunch Theory is a model of the future of the universe in which it stops expanding and ultimately collapses on itself due to the force of gravity of its constituent parts.

If the universe is finite in extent and the cosmological principle (not to be confused with the cosmological constant) does not apply, and the expansion speed does not exceed the escape velocity, then the mutual gravitational attraction of all its matter will eventually cause it to contract.

Because entropy continues to increase in the contracting phase, 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 the matter and in the universe, or in other words, the critical density of the universe.

The Big Crunch Theory

The Big Crunch theory is a symmetric view of the ultimate fate of the Universe. Just as the Big Bang started a cosmological expansion, this theory postulates that the average density of the universe is enough to stop its expansion and begin contracting.

The end result is unknown; a simple extrapolation would have all the matter and space-time in the universe collapse into a dimensionless singularity, but at these scales unknown quantum effects need to be considered.

This scenario allows the Big Bang to have been immediately preceded by the Big Crunch of a preceding universe. If this occurs repeatedly, we have an oscillatory universe. The universe could then consist of an infinite sequence of finite universes, each finite universe ending with a Big Crunch that is also the Big Bang of the next universe.

Theoretically, the oscillating universe could not be reconciled with the second law of thermodynamics: entropy would build up from oscillation to oscillation and cause heat death.

Other measurements suggested the universe is not closed. These arguments caused cosmologists to abandon the oscillating universe model. A somewhat similar idea is embraced by the cyclic model, but this idea evades heat death, because of an expansion of the branes that dilutes entropy accumulated in the previous cycle.

Birth and Death 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.

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.

Some theorize 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 supernova as standard candles, and the well-resolved mapping of the cosmic microwave background) have 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 drives the acceleration is unknown, it is still possible that it might eventually reverse sign and cause a collapse.