Big Crunch

The Big Crunch describes a possible end to the universe where its expansion reverses, causing everything to collapse back together. In this scenario, the cosmic scale factor shrinks to zero, which might be followed by a new Big Bang and a fresh universe. The idea goes back to 1922, when Russian physicist Alexander Friedmann developed equations showing that the universe’s fate hinges on its density—it could keep expanding or start contracting instead of remaining stable. If there is enough matter, gravity could halt the expansion and pull everything inward. This collapse would resemble a black hole. As the universe shrinks, radiation from stars and high-energy particles would be compressed and blueshifted to higher energies, becoming intense enough to ignite the surfaces of stars before they crash together. In the final moments, the universe would turn into a single fireball with near-infinite temperature, and at the very end, neither time nor space would remain.

Most evidence today suggests this hypothesis is wrong. Astronomical observations show the universe’s expansion is speeding up, making a Big Chill—a slow cooling and fading—more likely. Still, some physicists propose that a Big Crunch-like event could happen if dark energy fluctuates.

The Big Crunch scenario assumes the universe’s matter density is high enough for gravity to overcome the expansion from the Big Bang. The FLRW cosmology can predict whether expansion will stop based on average energy density, the Hubble parameter, and the cosmological constant. If expansion halts, contraction follows, accelerating over time and ending with the universe turning into a black hole.

Experimental evidence from the late 1990s and early 2000s—like observations of distant supernovas as standard candles and detailed maps of the cosmic microwave background—showed that gravity isn’t slowing expansion but accelerating it. The 2011 Nobel Prize in Physics went to researchers behind this discovery.

The Big Crunch also leads to the Big Bounce hypothesis, where the collapse destroys the universe, then triggers a new expansionary epoch—another Big Bang. This could repeat forever in a cyclic universe.

Historically, Richard Bentley, a churchman and scholar, wrote to Isaac Newton before a lecture on Newton’s theories and atheism. This question, known as Bentley’s paradox, was an early precursor to the Big Crunch. It’s now known that stars move and aren’t static.

Albert Einstein preferred an unchanging universe. In 1917, he worked with Dutch astronomer Willem de Sitter to show that general relativity could describe a static model. Willem demonstrated his equations could describe a very simple universe. Initially, no problems appeared, and scientists adapted the model. But they encountered a different form of Bentley’s paradox. General relativity also described a restless universe. Einstein realized that for a static universe—what was observed at the time—an anti-gravity force was needed to counter gravity’s pull. This extra force, called the cosmological constant, was added to relativity.

Edwin Hubble, at Mount Wilson Observatory, measured galaxy distances and paired them with Vesto Slipher and Milton Humason’s redshift measurements. He found a rough proportionality between an object’s redshift and its distance. Plotting a trend line from 46 galaxies, he calculated the Hubble Constant as 500 km/s/Mpc—nearly seven times today’s value—but still proving the universe was expanding, not static.

After Hubble’s discovery, Einstein abandoned the cosmological constant. In their simplest form, the equations produced a universe that expanded or contracted, contradicting what was observed, which led to the constant. Once expansion was confirmed, Einstein called his static-universe assumption his “biggest mistake.” In 1931, he visited Hubble to thank him for “providing the basis of modern cosmology.” After this, both Einstein’s and Newton’s models of a contracting, static universe were dropped for the expanding universe model.

The Big Bounce hypothesis suggests the universe could collapse back to its starting state and then initiate another Big Bang, allowing the universe to last forever through phases of expansion (Big Bang) and contraction (Big Crunch). This means there might be a universe in constant cycles of Big Bangs and Big Crunches. Albert Einstein briefly considered cyclic universes in 1931, hypothesizing that a universe existed before the Big Bang, which ends.