What is Big Crunch theory?

This theory says the universe will one day stop expanding. Then, as gravity pulls on matter, the universe will begin to contract, falling inward until it has collapsed into a super-hot, super-dense singularity. The universe is like a giant souffle. It starts out small, expands as it heats up, and eventually, as the souffle cools, begins to collapse. Big Crunch is the consequence
of the Big Bang Theory.

The ultimate fate of the universe is a topic in physical cosmology. Many possible fates are predicted by rival scientific theories, including futures of both finite and infinite duration.

Once the notion that the universe started with a Big Bang became accepted by a consensus of scientists, the ultimate fate of the universe became a valid cosmological question, one depending upon the physical properties of the mass/energy in the universe, its average density, and the rate of expansion.

The theoretical scientific exploration of the ultimate fate of the universe became possible with Albert Einstein's 1916 theory of general relativity. General relativity can be employed to describe the universe on the largest possible scale. There are many possible solutions to the equations of general relativity, and each solution implies a possible ultimate fate of the universe. Alexander Friedman proposed a number of such solutions in 1922. In some of these the universe has been expanding from an initial singularity; this is, essentially, the Big Bang.

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