Big Crunch

The Big Crunch is a hypothetical cosmic ending in which expansion reverses and matter and energy collapse into an extremely dense state.

Friedmann equations: Equations in cosmology that describe how the universe’s expansion changes with its matter, energy, and curvature. They determine whether expansion can slow, stop, and reverse under a given cosmic model.
Big Freeze: A proposed cosmic fate in which expansion continues and the universe gradually approaches maximum entropy. It contrasts with the Big Crunch’s reversal by allowing expansion to continue indefinitely.
Alexander Friedmann: A Russian mathematician who derived dynamic, non-static solutions to Einstein’s field equations for the universe. His equations made expanding and contracting universes mathematically possible.
Equation of state of dark energy: The relation between dark energy’s pressure and density, often summarized by the parameter w. A changing equation of state could alter whether cosmic acceleration persists or permits a turnaround.
Cosmic expansion: The increase over time of distances between sufficiently separated galaxies as space expands. The Big Crunch requires this expansion to halt before collapse begins.
Big Rip: A hypothetical cosmic ending in which accelerated expansion eventually tears apart gravitationally bound structures. It predicts expansion becoming destructive rather than reversing into collapse.
Georges Lemaître: A Belgian priest and physicist who developed an expanding-universe model and proposed the primeval atom. His work helped establish the dynamical cosmologies in which recollapse could be considered.
Dark energy survey: An astronomical survey that mapped galaxies and supernovae to study cosmic expansion and structure formation. Its measurements constrain expansion histories relevant to whether recollapse remains possible.
Critical density: The density of cosmic matter and energy that separates spatially closed, flat, and open models in standard cosmology. It once served as a key test of whether gravity could eventually reverse expansion.
Bounce cosmology: Cosmological models in which a contracting universe reaches a minimum size and begins expanding again. A bounce can follow contraction without ending in a final dense state.

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