Big Bounce

A cosmological model in which a contracting universe reaches a minimum size and rebounds into expansion, replacing an initial singularity with a transition between cosmic phases.

Cosmic singularity: A boundary in a spacetime model where quantities such as density or curvature become unbounded or the theory ceases to apply. The bounce is proposed as an alternative to the singular beginning predicted by classical cosmology.
Big Bang: The hot, dense early state from which the observable universe expanded. The Big Bounce replaces an absolute beginning with a preceding contracting phase.
Steady-state theory: A cosmological model proposing that the universe has no beginning and maintains constant large-scale properties as it expands. It avoids a beginning without using a prior contracting phase or rebound.
Bouncing cosmology perturbations: Small fluctuations in matter and spacetime evolved through a contracting phase and cosmological bounce. Their survival and evolution determine whether a bounce can produce the patterns observed today.
Loop quantum cosmology: A symmetry-reduced application of loop quantum gravity to cosmology that predicts quantum corrections to early-universe dynamics. Its effective equations can make high-density contraction turn into expansion.
General relativity: Einstein’s theory describing gravity as the curvature of spacetime caused by matter and energy. Classical relativistic cosmology supplies the equations whose singular solutions bounce proposals seek to modify.
Eternal inflation: A regime in some inflationary models where inflation continues in parts of the universe indefinitely. It offers a different route to a past-extending cosmology, through continuing inflation rather than repeated contraction.
Primordial gravitational waves: Hypothetical gravitational waves generated in the early universe before or during the formation of cosmic structure. Their predicted spectrum could distinguish some bounce models from inflationary cosmology.
Ekpyrotic universe: A cosmological scenario in which a slow contracting phase precedes the hot expanding universe. Some versions connect contraction to a bounce, though the ekpyrotic phase is not itself the bounce.
Friedmann equations: Equations derived from general relativity that govern the expansion of a homogeneous, isotropic universe. They describe cosmic contraction and expansion, and reveal where standard assumptions prevent a bounce.

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