Science › Big Bang & cosmos
The universe is expanding
1929· Discovery / experiment
ProvenTested many times; part of working physics.
Space itself is stretching, so distant galaxies drift apart, faster the farther away they are.
See it on the Science map →How it works
Galaxies are not flying through space away from us; the space between them is growing, like raisins in rising dough. Every galaxy sees the others receding, so there is no center. The rate today is about 70 kilometers per second for every 3.26 million light-years of distance. Light crossing that stretching space gets stretched too, shifting toward red. That 'redshift' is how astronomers read the expansion.
Why it matters
It means the universe has a history: it was smaller, hotter and denser in the past. That single fact led to the Big Bang theory.
Think of it like
Like a rent roll indexed to distance: every galaxy's recession speed scales with how far away it sits, by one universal rate.
How we know
Predicted from general relativity by Friedmann (1922) and Lemaître (1927); measured by Hubble (1929). Confirmed by redshifts of millions of galaxies, supernova distances and the cosmic microwave background.
Still a mystery
The exact rate is disputed: early-universe data give about 67, nearby measurements about 73. Either one method has a hidden error or new physics is involved.
Connections (8)
▸General relativity: gravity is curved spacetime allows The universe is expandingProven
The relationship
Einstein's equations say matter and energy curve spacetime. Applied to a universe evenly filled with matter, they forbid it from standing still: space must grow or shrink. Einstein disliked this and in 1917 added a 'cosmological constant' to hold it static. Others took the equations at face value and found expanding solutions. Expansion is a natural outcome of the theory.
How the link was found
Alexander Friedmann in 1922 and Georges Lemaître in 1927 independently found expanding solutions. Einstein accepted expansion around 1931 after Hubble's data.
Why it's proven
Hubble's 1929 measurements and modern surveys match the Friedmann-Lemaître equations, which remain the basis of all cosmological calculations.
Open questions and mysteries
Einstein's discarded cosmological constant returned in 1998 as dark energy. Why it has its tiny value is unexplained.
▸Georges Lemaître predicted The universe is expandingProven
The relationship
Lemaître solved Einstein's equations for an expanding universe and predicted that galaxies should recede at speeds proportional to their distance. He then used existing redshift and distance data to estimate the rate, about 625 kilometers per second per megaparsec. That was the expansion law, two years before Hubble published it.
How the link was found
Lemaître, 1927, in a French-language Belgian journal few read. His own 1931 English translation left out the rate estimate, as historians confirmed in 2011.
Why it's proven
Hubble's 1929 data matched the predicted pattern. In 2018 the International Astronomical Union recommended calling it the Hubble-Lemaître law.
Open questions and mysteries
The law's exact rate is still disputed today, the 'Hubble tension'.
▸Edwin Hubble measured The universe is expandingProven
The relationship
Hubble paired galaxy distances, from stars of known brightness, with speeds, from redshift. The result: speed rises in proportion to distance. That straight-line relation is the observable fingerprint of uniform expansion. If every gap in space grows at the same rate, galaxies twice as far away recede twice as fast.
How the link was found
Edwin Hubble in 1929, using 24 galaxies with distances and redshifts largely measured by Vesto Slipher; extended with Milton Humason in 1931.
Why it's proven
Every later survey, now millions of galaxies and supernovae, shows the same relation far beyond Hubble's reach.
Open questions and mysteries
His rate was about seven times too large due to miscalibrated distances. Today's two best methods still disagree by about 9%.
▸The universe is expanding run backwards implies The Big BangProven
The relationship
If galaxies are moving apart now, they were closer in the past. Rewind far enough and all matter was packed together, extremely hot and dense. Dividing distances by speeds gives a rough age of about 14 billion years; full models refine it to 13.8 billion. Expansion alone does not prove a hot beginning, but it makes one the natural reading.
How the link was found
Lemaître made the argument in 1931. George Gamow and colleagues developed the hot version in the late 1940s.
Why it's proven
A rival 'Steady State' theory (Hoyle, Bondi, Gold, 1948) kept the universe unchanging by creating new matter. It failed once the microwave background was found in 1965.
Open questions and mysteries
Rewinding all the way gives infinite density, where the theory breaks down. What happened at that point is unknown.
▸Dark energy is speeding up The universe is expandingWell supported
The relationship
Gravity from matter slows expansion. Dark energy pushes the other way, and as space grows, matter thins out while dark energy apparently does not. About six billion years ago dark energy's push overtook matter's pull, and expansion switched from slowing down to speeding up. If it stays constant, galaxies beyond our local group will eventually drift out of view.
How the link was found
Two teams measuring exploding white-dwarf stars, led by Saul Perlmutter and by Brian Schmidt with Adam Riess, announced acceleration in 1998 (Nobel 2011).
Is it proven?
Confirmed independently by patterns in galaxy clustering and by the microwave background. That expansion is accelerating is well supported; what drives it is unknown.
Open questions and mysteries
DESI survey results in 2024-2025 hint dark energy may be weakening. If so, the long-term future of expansion changes.
▸The Friedmann equations: the rules of expansion predicted The universe is expandingProven
The relationship
Given today's expansion rate and contents, the equations predict the whole past: how much space has stretched since any light was emitted (its redshift), how distance relates to redshift, and the time since everything was densely packed. Run back to the first second, they give a temperature of about 10 billion degrees, which sets element formation.
How the link was found
Friedmann's 1922 solutions came before any expansion data; Hubble's 1929 measurements then fit them. Hubble's rate, however, gave an age of only about 2 billion years, younger than geologists' age for Earth.
Why it's proven
Walter Baade's 1952 recalibration of star distances fixed the age conflict. Today supernovae and galaxy surveys trace distance against redshift over more than 10 billion years, matching the predicted curve.
Open questions and mysteries
The equations need the contents as input; they cannot say what dark matter and dark energy are. And today's rate itself is disputed, the Hubble tension.
▸The universe is expanding at a rate measured two ways, creating The Hubble tensionOpen question
The relationship
Route one builds a distance ladder: parallax, the geometry of Earth's orbit, measures nearby pulsing Cepheid stars; Cepheids calibrate Type Ia supernovae in other galaxies; supernovae reach far enough to read the expansion, giving about 73. Route two measures the sound-wave ruler in the afterglow and projects forward with ΛCDM, giving 67.4. Both look solid; they disagree by about 9%.
How the link was found
Adam Riess's SH0ES team built the modern ladder from 2005; Planck's 2013 results first exposed a clear gap, which grew to about 5σ by 2022.
Is it proven?
The gap is real at about 5σ unless one method hides an error. Methods independent of Cepheids give mixed results, from about 70 (red giant stars) to about 73 (lensed quasars).
Open questions and mysteries
Is the error in nearby distances, in the early-universe model, or nowhere? Gravitational waves from colliding neutron stars offer a third route but are not yet precise enough.
▸1998: exploding stars reveal faster expansion showed acceleration in The universe is expandingProven
The relationship
Each supernova gives two readings: brightness, which gives how far its light traveled, and redshift, which gives how much space stretched meanwhile. If expansion had always been slowing, distant supernovae would look relatively close and bright. They were dimmer, so expansion was slower in the past and has sped up. It switched from slowing to speeding up roughly 5–6 billion years ago.
How the link was found
The High-Z team (Riess and colleagues, 1998, 16 distant supernovae) and the Supernova Cosmology Project (Perlmutter and colleagues, 1999, 42). The rivals' agreement made the result credible.
Why it's proven
Hubble Space Telescope supernovae from more than 8 billion years ago (Riess, 2004) look relatively brighter, catching the earlier slowing era. Dust or evolving stars could dim supernovae, but not create that turnaround.
Open questions and mysteries
Exactly when the switch happened depends on what dark energy is. If DESI is right that dark energy is fading, acceleration itself may weaken in the far future.