If the density of the universe is equal to a specified critical value, expansion will gradually slow down until it stops completely at the end of an infinite time period. This is the flat universe model. On the other hand, if the density of the universe exceeds the critical value, gravitational force will stop the expansion. When expansion stops, the universe will collapse into itself, and return to its initial state in a form of reverse Big Bang, called the Big Crunch. This is the closed universe model.
