Big Crunch

The Big Crunch is the ending in which the universe stops expanding, turns around and falls back in on itself, ending in a state as hot and dense as the one it began in. For most of the twentieth century it was one of the two obvious futures. Then it was all but ruled out. Since 2024 it has been back in the discussion.

The old question

In 1922 Alexander Friedmann’s equations showed that the fate of an expanding universe depends on how much is in it. With enough matter, gravity would one day halt the expansion and pull everything back together. With less, the expansion would go on forever. For decades astronomers tried to weigh the universe to find out which.

In 1998 two teams studying distant supernovae found something nobody expected: the expansion is not slowing down but speeding up. Something now called dark energy, about 68% of the universe’s mass and energy, is pushing it apart. If dark energy stays constant, there is no crunch. The universe expands forever and slowly goes cold, the ending described on the heat death page.

What DESI found

The Dark Energy Spectroscopic Instrument maps millions of galaxies to measure how the expansion has changed over time. Its first results in 2024, and a larger set in March 2025 from more than 14 million galaxies and quasars, hinted that dark energy may be weakening. Combined with supernova data, the preference reached 2.8 to 4.2 sigma, short of the 5 sigma physicists require for a discovery.

A weakening dark energy alone does not cause a collapse. For that it would have to fall far enough to turn negative and pull instead of push. Some models do exactly that. In 2025 Luu, Qiu and Tye fitted the new data with a negative cosmological constant and a hypothetical ultralight particle, and found a universe that lives for 33 billion years: about 11 billion more years of expansion, then a crunch about 20 billion years from now. A 2022 model by Andrei, Ijjas and Steinhardt allowed contraction to begin as soon as 100 million years from now.

The evidence is also weakening. In November 2025 the Dark Energy Survey recalibrated its supernovae, and its preference for evolving dark energy fell from 4.2 to 3.2 sigma, “only a weak preference”. Efstathiou has argued that a calibration offset of about 0.04 magnitudes in the supernova data could explain the whole signal.

What it would look like

As the collapse went on, galaxies would draw closer and merge more often. The faint background radiation left from the Big Bang would stop cooling and start to heat up. In Katie Mack’s description, starlight and that background glow would be squeezed to ever shorter, bluer wavelengths until the radiation itself set the surfaces of stars alight, before the stars ever collided. In the last moments, temperatures and densities would go beyond any known physics.