Cyclic model

A cosmological theory in which the universe passes through recurring phases, often cycles of expansion and contraction. Different versions propose different mechanisms for beginning and ending each cycle.

Big Bounce: A cosmological scenario in which a contracting universe transitions into expansion without reaching a singularity. It replaces the big bang singularity with a transition between successive cosmic phases.
Alexander Friedmann: A Russian physicist whose solutions to Einstein's equations described expanding and contracting universes. His relativistic models established early mathematical possibilities for non-static cosmic histories.
Big Bang: The early hot, dense state from which the observable universe expanded. A standard Big Bang history need not include a preceding contraction or future recurrence.
Cosmic microwave background: Relic electromagnetic radiation released when the early universe became transparent, now observed across the sky. Its measured temperature patterns constrain proposed signatures of earlier cosmic cycles.
Ekpyrotic universe: A cosmological model in which a collision between higher-dimensional branes produces a hot, expanding universe. Its brane collisions provide one proposed trigger for repeated cosmic cycles.
Richard Tolman: An American physicist who analyzed relativistic cosmology and the thermodynamics of oscillating universes. His work exposed how entropy accumulation changes cycle lengths and sizes.
Big Crunch: A hypothetical end in which cosmic expansion reverses and the universe collapses to extreme density. A Big Crunch is one possible ending, but by itself does not establish another cycle.
Gravitational waves: Ripples in spacetime produced by accelerating masses and propagating outward at the speed of light. A primordial background could distinguish some cyclic scenarios from conventional early-universe histories.
Conformal cyclic cosmology: Roger Penrose's cosmological proposal that the remote future of one universe can be identified with the Big Bang of the next. It links successive cosmic eras through conformal geometry rather than contraction and bounce.
Georges Lemaître: A Belgian priest and physicist who proposed an expanding universe originating from a primeval atom. His expanding-universe picture became a key alternative against which cyclic histories developed.

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