The Big Crunch is one possible scenario for the ultimate fate of the Universe. Just like many others, it is based on Einstein’s Theory of General Relativity. That is, if the Big Bang describes how the Universe most possibly began, the Big Crunch describes how it will end as a consequence of that beginning. In this theory, the metric expansion of space eventually reverses and the Universe recollapses, ultimately causing the cosmic scale factor to reach zero or causing a reformation of the Universe starting with another Big Bang.
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