Oscillating universe theory, artwork. Also called cyclic models, these theories of the origin and nature of the universe postulate that following the Big Bang there is enough mass to reverse the expansion towards a Big Crunch. This is then followed by a rebound and a new cycle of existence. Within each cycle, there are galaxies and stars. Between each cycle is a singularity. Examples of cyclic models are the one proposed by Einstein in 1930. Modern theories rely on concepts such as dark energy and vacuum fluctuations.