The textbook crunch. Too much matter, closed geometry, gravity wins and pulls it all back.
Classical Closed Recollapse
The original route to a Big Crunch: a universe with enough matter and positive spatial curvature halts its expansion under gravity and collapses, the closed Friedmann model from before dark energy.
Looping ambient scene for Recollapse and the Big Crunch. A recollapsing universe is the mirror image of the expanding one: gravity, or a dark energy that turns negative, eventually halts the expansion and pulls everything back together into a Big Crunch. Three routes lead here. A dynamical dark energy whose potential becomes negative can trigger a turnaround even in a flat universe. A negative cosmological constant has the same effect through different bookkeeping, giving an anti-de-Sitter-like collapse. And a closed spatial geometry, the classical pre-dark-energy route to a crunch, recollapses under its own gravity once expansion stalls. Current data, a flat universe with w near -1, disfavour an imminent crunch, but a future measurement that w is drifting negative or that space is slightly closed would put a recollapse back on the table.
§1 · The claim, in one sentence
Classical Closed Recollapse is the original Big Crunch, the closed Friedmann universe that dominated cosmology before dark energy was discovered. If the total density exceeds a critical value, space has positive curvature and is finite, and gravity eventually halts the expansion and pulls everything back into a hot, dense crunch. Current measurements of a flat universe with dark energy have made this the disfavoured route, but it remains the foundational picture of a recollapsing cosmos.
Why this was rejected
- ▸Planck and other CMB measurements constrain the spatial curvature to near zero, consistent with a flat universe, not the positively curved closed geometry this model requires
- ▸The observed accelerating expansion implies a positive dark energy that prevents recollapse even if the geometry were marginally closed
§2 · Why it might be true
In general relativity without dark energy, the fate of the universe came down to a single number: the total density relative to a critical value. Below critical the universe is open and expands forever; above critical it is closed, with positive spatial curvature, finite in extent, and destined to recollapse. This was the classic fork in twentieth-century cosmology.
A closed universe expands, slows under the gravity of its own contents, reaches a maximum size, then contracts. The contraction runs the expansion history in reverse: galaxies approach, the cosmic background blueshifts and heats, and the universe ends in a Big Crunch, a hot dense state mirroring the Big Bang. For decades this was a leading candidate for the end of everything.
The 1998 discovery of accelerating expansion changed the picture. A positive dark energy can drive eternal expansion even in a closed universe, and precision measurements show space is flat to within a percent. The classical closed recollapse is therefore no longer the expected fate. It is retained here as the historical foundation of the crunch idea and the limiting case the dark-energy-driven variants modify.
The family stance
The universe ends by falling back together. Expansion is not guaranteed to last forever; if the dark energy turns negative or the geometry is closed, gravity wins, the expansion reverses, and everything collapses into a hot, dense Big Crunch, possibly followed by a bounce.
§2.5 · Evidence
- It is the cleanest, most economical crunch: only gravity and geometry, no exotic dark energy needed
- It was the standard textbook fate for much of the twentieth century and remains mathematically exact within Friedmann cosmology
§3 · What you'd need to test it
- The total density exceeds the critical value, giving positive spatial curvature and a finite, closed universe
- Expansion reaches a maximum and reverses purely under gravity, with no dark energy required
- The contraction reverses cosmic history: blueshifted, reheating background radiation ending in a hot dense crunch
- A measurement of significantly positive spatial curvature would revive this route to a crunch
§4 · Where it breaks
- Precision cosmological measurements are consistent with a spatially flat universe, removing the positive curvature this scenario requires
- A positive dark energy, which is observed, drives eternal expansion even in a closed universe, overriding the gravitational recollapse
- On current data this is not a viable future, which is why it carries historical status
Go deeper
The closed Friedmann model follows from the Friedmann equation with positive spatial curvature and no cosmological constant. The density parameter Omega, the ratio of actual to critical density, exceeds one, the curvature term eventually overtakes the diluting matter density, and the expansion rate passes through zero and goes negative. The collapse is time-symmetric to the expansion in the idealised case, ending in a curvature singularity unless new physics intervenes.
Historically this was half of the great cosmological dichotomy, open-and-eternal versus closed-and-recollapsing, that framed the subject from Friedmann 1922 through the 1990s. The discovery of dark energy added a third axis, the vacuum energy, that decouples fate from geometry: a flat or even closed universe can expand forever if dark energy is positive. The classical recollapse is the dark-energy-free corner of that larger map.
Cross-references: the dynamical-dark-energy-turnaround and negative-lambda-recollapse variants in this family recover a crunch in a flat universe by making the dark energy negative, rather than by closing the geometry. The Standard Cosmological Model family in Chapter 5 carries the flat-universe measurements that disfavour this classical route.
Variants in this family
Compare variants▸§5 · Who built it, and when(1 source, 1 established)
- EstablishedFriedmann, A. (1922). 'Über die Krümmung des Raumes.' Z. Phys. 10, 377
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