Big Crunch

The name Big Crunch is used for an end to the universe that would happen if gravity eventually drew the universe back together, somewhat like an opposite to the Big Bang. This would require the density of the universe to be above the critical density and the absence of any force that would prevent such a contraction. Before it was noted that the expansion is accelerating (and the term dark energy was coined for whatever causes that), the Big Crunch was considered a significant possibility. Some other ends of the universe that have been theorized:

• heat death aka the Big Freeze - the temperature of everything in the universe settles to the same value, i.e., entropy reaches its maximum.
• the Big Rip - all objects in the universe are torn apart into particles which continue to spread toward the distance between them becoming infinite, due to dark energy, if it turns out to have certain theorized characteristics.
• the Big Bounce - basically, the Big Crunch turns out to be a Big Bang for a subsequent universe, i.e., the universe is cyclically repeating.
• vacuum decay (aka false vacuum decay aka the Big Slurp) - presumes the universe as we know it is based on a false vacuum, and a true vacuum existing or created somewhere within the universe expands, swallowing everything.