Why Astronomers Think the Universe Is Expanding — And Speeding Up

The Raisin Bread Model That Actually Works

Picture a loaf of raisin bread rising in an oven. The raisins don't move on their own — but as the dough expands, every raisin gets farther from every other raisin. A raisin close to another sees it drift away slowly; a raisin far away appears to recede much faster, simply because there's more dough between them to stretch. That, in essence, is how astronomers understand the expanding universe.

The universe isn't galaxies flying outward through empty space from some central explosion. It's space itself — the "dough" — growing in all directions simultaneously. There is no center, no edge, and no outside. Every galaxy cluster observes every other moving away, and the recession speed increases with distance. This point trips up many people new to cosmology — see common myths about the Big Bang for a closer look at why the explosion analogy breaks down.

Redshift: The Universe's Speedometer

The key tool astronomers use to measure expansion is redshift. Light travels as waves, and when a source moves away from an observer, those waves get stretched — shifting toward the red end of the spectrum. The greater the stretch, the faster the recession. In 1929, Edwin Hubble combined measurements of galaxy distances with their redshifts and found a remarkably consistent pattern: distant galaxies were receding proportionally to how far away they were. This relationship, now called Hubble's Law, became the observational foundation of modern cosmology.

Measuring distances precisely across billions of light-years is genuinely difficult — new astronomers are often surprised by how complex the relationship between brightness and distance really is. Astronomers use "standard candles" — objects whose intrinsic brightness is known — to anchor the cosmic distance ladder. Cepheid variable stars work for relatively nearby galaxies; for farther reaches, Type Ia supernovae serve as more powerful yardsticks.

The Surprise of 1998: Expansion Is Speeding Up

For most of the 20th century, physicists assumed the expansion was gradually slowing down. Gravity, after all, should be tugging galaxies back toward one another. Two independent research teams studied distant Type Ia supernovae in the late 1990s expecting to measure that deceleration. Instead, both teams found the supernovae were dimmer than expected — meaning they were farther away than a decelerating expansion would predict. The universe wasn't slowing. It was accelerating.

This was not a minor revision. It overturned decades of assumptions and earned the 2011 Nobel Prize in Physics for the team leaders Saul Perlmutter, Brian Schmidt, and Adam Riess. The agent driving this acceleration was named dark energy — a placeholder label for whatever property of space is pushing galaxies apart with increasing speed. To understand the full picture of what's filling the universe and driving its behavior, see our explainer on dark matter and dark energy.

“The most incomprehensible thing about the universe is that it is comprehensible.”

— Albert Einstein, Theoretical physicist and developer of the general theory of relativity

What This Means for the Universe's Future

If expansion continues accelerating, the long-term future of the universe looks profoundly lonely. Galaxies beyond our local group will eventually recede faster than light can cross the growing gap — making them permanently invisible and unreachable. The observable universe will effectively shrink over cosmic time, not because the universe itself gets smaller, but because more of it passes beyond our horizon. Stars will burn out, and if the acceleration persists, the universe may trend toward a cold, dark, and diffuse state sometimes called the Big Freeze.

None of this affects structures held together by gravity. The Milky Way, the Solar System, Earth — these remain bound by forces that dwarf the influence of dark energy at local scales. Cosmic expansion is a phenomenon of the very large and the very distant. It's worth noting that the space between stars isn't truly empty either — gas, dust, and fields fill those gaps in ways that matter for how galaxies evolve even as they drift apart.

The Hubble tension — a persistent disagreement between two methods of measuring the expansion rate — remains unresolved. Some researchers suspect it points to new physics beyond the current model. Telescopes like the James Webb Space Telescope are providing ever-sharper data on distant galaxies; Webb sees the universe in ways Hubble couldn't, potentially helping to resolve this tension in coming years.