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- a variant model of the closed universe in which the universe undergoes cycles of expansion and contraction.
- A model of a closed universe in which the expansion of the universe slows and reverses, thereby causing a collapse into a singularity, which then expands into a new universe, repeating the cycle. Such a model implies that our universe is just one of a long line of universes. Scientists have not been able to develop a theory of how a collapsed universe could expand into a new one.