As applied to the universe as a whole, critical mass refers to the total required mass of matter in the universe which will allow the effects of gravity to overcome its continued outward expansion. If the universe contains more than the critical mass of matter, its gravity will eventually reverse the expansion, causing the universe to collapse back to what has become known as the Big Crunch. If, however, it contains insufficient matter, it will go on expanding forever. In the same way, critical density is that overall density of the matter in the universe which will just allow continued expansion. In other contexts, critical mass is also used to refer to the amount of fissile material needed to sustain nuclear fission.