A cyclic model (or oscillating model) is any of several cosmological models in which the universe undergoes infinite or indefinite self-sustaining cycles. For instance, the oscillating universe theory briefly explored by Albert Einstein in 1930 proposed a universe experiencing an eternal series of oscillations. Each cycle starts with a Big Bang, expands for a duration, and concludes with a Big Crunch. During the interim, the universe expands for a period before gravitational attraction causes it to collapse, undergo a bounce, and repeat the cycle.
The Friedmann universe is a model universe developed in 1922 by the Russian meteorologist and mathematician Aleksandr Friedmann (1888–1925). He argued that Albert Einstein’s general theory of relativity demanded a theory of a dynamic universe, in contrast to the static universe that scientists had previously postulated. Friedmann proposed a model involving a big bang followed by expansion, subsequent contraction, and an eventual big crunch. This model assumes a closed universe. However, he also suggested alternative solutions, including an open universe (expanding infinitely) or a flat universe (continuing to expand infinitely but gradually approaching a rate of zero).
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