Will the Universe expand forever or recollapse?
This depends on the ratio of the density of the Universe to the critical density. If the density is higher than the critical density the Universe will recollapse in a Big Crunch. But current data suggest that the density is less than or equal to the critical density so the Universe will expand forever. See Part 3 of the tutorial for more information. Back to top. Does entropy prevent a Big Crunch? The issue of entropy in the Universe is subtle and not entirely settled. Theorists are still trying to work out what happens to the entropy of matter that falls into a black hole, a problem which involves both quantum mechanics and strong gravity. When a successful theory of quantum gravity is worked out, it should explain why the Universe came out of the Big Bang singularity with a very large entropy, and what happens to the entropy of the Universe if it recollapses. Entropy is related to the number of ways a system can be in a given state or condition. Thus a shuffled deck of cards has a hi
