Big Crunch

In cosmology, the Big Crunch is a hypothesis that states the universe will stop expanding and start to collapse upon itself; a counterpart to the Big Bang.

If the gravitational attraction of all the matter in the observable horizon is high enough, then it could stop the expansion of the universe, and then reverse it. The universe would then contract, in about the same time as the expansion took. Eventually, all matter and energy would be compressed back into a gravitational singularity. It is impossible to ask what would happen after this, as time would stop in this singularity as well.

Some theorists say they have proven this theory incorrect, and claim the universe will continue to expand forever.

See also the Big Rip.