The Big Crunch is a cosmological model concerning the ultimate fate of the universe, in which the expansion of the universe eventually reverses and the universe re-collapses, ultimately causing the cosmic scale factor to reach zero, an event potentially followed by a reformation of the universe starting with another Big Bang.
An estimation is that the Big Crunch happens about 100 billion years from the present. The contracting universe would evolve roughly like the expanding phase in reverse. First, galaxy clusters, and then galaxies, would merge, and the temperature of the cosmic microwave background (CMB) would begin to rise. Stars would collide, and eventually cook themselves until they evaporate. In the last minutes, the temperature of the universe would be so great that atoms and atomic nuclei would break up and get sucked up into already coalescing black holes. At the time of the Big Crunch, all the matter in the universe would be crushed into an infinitely hot, infinitely dense singularity similar to the Big Bang. The Big Crunch may be followed by another Big Bang, creating a new universe.