Big Crunch

The Big Crunch is the ending where gravity wins. In the Friedmann models that have carried cosmology since the 1920s, a universe dense enough expands, slows, stops and falls back: galaxies converge, the background radiation blueshifts and reheats, and everything compresses into a hot, dense state that mirrors the Big Bang run backwards. For most of the twentieth century this was the respectable alternative to expanding forever, and 'open or closed?' was cosmology's central measurement problem.

The measurement came in, and it did not go the crunch's way. The 1998 supernova results showed expansion accelerating rather than slowing, and the matter density settled near 30 percent of the value that would close the universe on its own. A crunch now needs something extra: dark energy that decays, or a scalar field rolling into a negative potential. Such models exist and are not crazy — Andrei Linde and collaborators have written down quintessence potentials that turn attractive and end the universe within tens of billions of years — but they are additions to the picture rather than readings of it.

The crunch's real afterlife is cyclic cosmology: if a collapse can bounce, the universe need not have had a beginning. Richard Tolman raised the standing objection in 1934 — entropy accumulates across cycles, so each one runs longer than the last, and the sequence still has a first term. The ekpyrotic and cyclic models of Paul Steinhardt and Neil Turok answer by putting the bounce in a higher-dimensional setting where expansion between cycles dilutes the entropy. Whether any of it survives contact with a real theory of quantum gravity is exactly the open question.

On this site, Ω is the dial that chooses. Push the density up and the universe never gets old enough for chemistry to matter — it turns around and collapses while the gas is still hot. It is the cleanest demonstration in the tuner that one number decides both whether structure forms and how the story ends.