What are possible fates of the universe?

Answer 1

The Big Crunch

The matter within our Universe causes the expansion of space to slow down, stop, and then reverse itself. Eventually space returns to a much smaller size, taking matter with it. All matter eventually becomes as dense as it was at the time of the Big Bang.

The Big Freeze

Stars and galaxies become so separated from each other that stars can no longer re-form from the remnants of super-nova. All stars end up as white dwarfs, neutron stars, and black holes. Eventually these all die out, leaving a Universe as nothing but collections of inert hydrogen and helium.

The Big Rip

The expansion of space speeds up until all matter -- even protons and electrons -- are torn apart from each other. Every single sub-atomic particle in our Universe becomes separate from every other one; in effect existing only by itself.

Multiverse Collisions

This hypothesis depends on the existence of an infinite number of universes other than our own, each expanding within a multi-verse. Since some universes are matter and some are anti-matter; the inevitable collision of such universes will release enough energy to start a new Big Bang an new universe.

Answer 2

Possible fates of the universe include the "Big Freeze," where the universe continues to expand and cool until it becomes a cold and lifeless place; the "Big Crunch," where the universe collapses under its own gravity; the "Big Rip," where the expansion of the universe accelerates until everything is torn apart; or the possibility of an eternal, stable universe.