What if the universe stopped expanding?

The universe is everything: all the stars, planets and galaxies. Space is slowly stretching, so faraway galaxies drift apart.

What if the stretching stopped?

Look far out into space and you find something astonishing: distant galaxies are all moving away from us, and the farther away they are, the faster they go. Astronomers explain this as space itself stretching. That stretching has a name: the expansion of the universe.

So what if the universe stopped expanding? This is a thought experiment at the very edge of science. We can't stop the universe, but we can follow how scientists measure the expansion and what their models say about its future.

Cosmic expansion is among the best-established results in astronomy, yet its future is one of the largest open questions. Expansion is not galaxies flying through space but the stretching of space itself.

This thought experiment asks what would change if the expansion halted: for bound systems like our galaxy, and for the universe's long-term fate. Each step separates measurements (fact) from models and open questions (idea).

Imagine raisins in bread dough. As the dough rises, every raisin moves away from every other raisin.

Galaxies are like the raisins. The raisins don't grow, and galaxies don't grow either.

In 1929, Edwin Hubble confirmed that the universe is expanding, using measurements by his colleague Milton Humason. They found that the farther away a galaxy is, the faster it is moving away from us.

OpenStax compares this to raisin bread rising in an oven: as the dough swells, every raisin moves away from every other one, and distant raisins move apart fastest. The raisins themselves don't get bigger. In the same way, gravity holds galaxies and clusters of galaxies together, so they don't stretch; the space between them does.

Hubble and Humason (1929) compared galaxy redshifts with Cepheid-based distances and found recession speed proportional to distance: Hubble's law, now often called the Hubble-Lemaître law. OpenStax's raisin-bread and stretching-ruler analogies show why every observer sees the same pattern without any center.

Crucially, OpenStax notes that expansion does not imply galaxies or galaxy clusters themselves are expanding: gravity binds them, and they move apart without changing size. The solar system and the Milky Way are not stretching.

Our Sun, Earth and Milky Way would not change. Gravity already holds them together.

Faraway galaxies would stop drifting apart. What happens after that? Scientists can only make careful guesses.

First, the surprise: here at home, nothing would change. Gravity already holds Earth, the solar system and the Milky Way together, so they aren't being stretched now and wouldn't notice the stretching stop.

Far away, galaxies would stop moving apart. Then what? In the models OpenStax describes, if nothing else pushed space outward, gravity could slowly pull everything back together. But because of dark energy, scientists don't currently expect that. A universe that just stops is not one of the main models, so these are ideas, not predictions.

Bound structures, from the solar system to galaxy clusters, are not participating in the expansion, so a halt would leave them unaffected. On large scales, galaxies' separations would stop growing.

In matter-only models, a universe denser than the critical density stops expanding and recollapses into a 'big crunch'. OpenStax notes that with dark energy this no longer appears to be the outcome. A sudden, permanent halt is not a standard model, so its consequences are speculative ideas, not forecasts.

Light from faraway galaxies gets stretched, so it looks redder. Scientists call it redshift.

The more stretched the light, the faster the galaxy is moving away.

Astronomers use two tools. The first is redshift: light from a galaxy moving away is stretched toward the red end of the rainbow. Vesto Slipher first measured how fast galaxies were moving this way. The second is distance: in 1912 Henrietta Swan Leavitt found that certain pulsing stars, called Cepheids, reveal how far away their galaxies are.

In 1998 two teams studying exploding stars far away found they were dimmer than expected. The universe's expansion was speeding up, not slowing down. Adam Riess, Saul Perlmutter and Brian Schmidt won the 2011 Nobel Prize in Physics for the discovery.

Redshift measures recession (more precisely, the stretching of light by expanding space, as OpenStax explains); Leavitt's 1912 period-luminosity relation for Cepheid variables supplied distances. Friedmann (1922) and Lemaître (1927) showed that general relativity allows an expanding universe.

In 1998 two teams found distant Type Ia supernovae fainter than expected, implying accelerating expansion, which NASA says began about nine billion years after the big bang. Riess, Perlmutter and Schmidt shared the 2011 Nobel Prize. The cause was named dark energy, though no one knows what it is.

Something unknown is making space stretch faster and faster. Scientists call it dark energy.

Nobody knows what it is yet. New telescopes are looking for clues.

NASA says dark energy makes up roughly 68 to 70 percent of the universe, yet no one knows what it is. It might be energy belonging to empty space itself, a kind of field that changes over time, or a sign that our theory of gravity needs fixing.

In 2025 the DESI project, which mapped nearly 15 million galaxies and quasars, reported hints that dark energy's effect may be weakening over time. If so, the universe's future could be different from what scientists expected. New missions, such as Euclid and the Roman Space Telescope, are designed to find out.

Candidate explanations listed by NASA include vacuum energy (a cosmological constant), a dynamical field sometimes called quintessence, and modified gravity. Dark energy accounts for about 68.3 to 70 percent of the universe's content.

DESI's March 2025 analysis of nearly 15 million galaxies and quasars, combined with other data, gave mounting indications that dark energy's influence may be weakening, which would mean the standard model needs updating. Whether expansion could ever slow, stop or reverse remains open; Euclid, Roman and other surveys aim to measure how dark energy has changed over time.