How we know the universe is expanding

Nobody has read the source code of the universe. All we have is output: light arriving at telescopes. Cosmology is a long exercise in reverse engineering that output. If the light looks like this, what must the machine behind it be doing? The claim that the universe is expanding sounds like a wild guess, but it rests on independent lines of evidence. This post follows them in the order they were found, and ends with a number that two good methods still disagree on.

Every chemical element absorbs and emits light at its own exact wavelengths, a fingerprint that physicists mapped in the 19th century by studying sunlight and flames. If a distant galaxy shows the familiar pattern of calcium, hydrogen, magnesium and sodium lines but everything sits at slightly longer wavelengths, its light has been stretched. The fractional stretch is the redshift z: z equals (observed wavelength minus emitted wavelength) divided by emitted wavelength.

From 1912 Vesto Slipher at Lowell Observatory measured these shifts for the "spiral nebulae" and found that nearly all of them were redshifted. The missing ingredient was distance. In 1912 Henrietta Leavitt had found that a class of pulsating stars, the Cepheids, pulse more slowly the brighter they truly are. Compare true brightness with apparent brightness and you have a distance. In 1923 to 1924 Edwin Hubble found Cepheids in the Andromeda "nebula" and showed it lies far outside our Milky Way: it is another galaxy.

In 1927 the Belgian priest and physicist Georges Lemaitre showed that Einstein's equations allow an expanding universe and, from the data then available, estimated the rate. In 1929 Hubble published his own plot: the farther the galaxy, the faster it recedes.

The straight line is the Hubble-Lemaitre law, v = H0 x d (the IAU recommended that name in 2018). Hubble's first value for H0 was about 500 km/s per Mpc, roughly seven times too high; today it is near 70. The error mattered: 1/H0 is a rough age, and 500 gives only 2 billion years, younger than rocks on Earth. Fixing the distances cured the problem.

The standard picture is not of galaxies flying outward through space, but of space itself stretching. A redshift of z means that the universe has grown by a factor of 1 + z since the light left.

Think of raisins in rising dough. Every gap doubles, so raisin E moves four times as far as raisin B. Seen from any raisin, the others recede at speeds proportional to distance, so Hubble's law needs no centre. Blow up a balloon with dots on it and you get the same thing:

Both pictures have limits, and the limits matter:

No single method measures distances all the way out, so astronomers stack them, each rung calibrating the next:

Parallax is plain triangulation (Friedrich Bessel did the first in 1838), and the Gaia satellite has measured it for well over a billion stars. Cepheids reach other galaxies. Type Ia supernovae, exploding white dwarfs that all peak at nearly the same brightness once corrected for how fast they fade, are visible across billions of light-years. The catch is that errors accumulate from rung to rung. Hold that thought.

In 1948 Ralph Alpher and Robert Herman predicted that a hot early universe should leave a faint cool afterglow, about 5 K. In 1964 two radio engineers, Arno Penzias and Robert Wilson, were testing a horn antenna at Bell Labs in New Jersey and found a stubborn hiss of microwave noise coming equally from every direction. They checked the equipment, even cleaned out pigeon droppings, and the hiss stayed. Not far away, at Princeton, a group around Robert Dicke was building a detector to look for exactly this. Back-to-back papers appeared in 1965, and Penzias and Wilson received the 1978 Nobel Prize in Physics.

This cosmic microwave background (CMB) is light released about 380,000 years after the Big Bang, when the universe cooled to about 3,000 K and electrons and protons first combined into atoms, so light could travel freely. Since then space has stretched by a factor of about 1,100, so that glow now arrives as 2.725 K microwaves. About 1 percent of the "snow" on an old analogue TV tuned between channels is this ancient light.

In 1990 the COBE satellite showed that the spectrum is a blackbody to extraordinary precision (Nobel Prize 2006, John Mather and George Smoot). Later missions, WMAP and Planck, mapped temperature differences of one part in 100,000, the seeds of galaxies. A third independent check is the recipe for light elements: a hot early universe predicts about 75 percent hydrogen and 25 percent helium by mass, which is what the oldest gas clouds contain.

Putting these together gives the standard timeline. (The name "Big Bang" came from Fred Hoyle, who favoured a rival theory, in a BBC radio talk in 1949.)

In the 1990s two teams, led by Saul Perlmutter and by Brian Schmidt and Adam Riess, used distant Type Ia supernovae to measure how much the expansion had slowed down, as gravity should do. They found the opposite: the supernovae were fainter, so farther away, than a slowing universe predicts. The expansion is accelerating. (Nobel Prize 2011.) The cause is called dark energy. The simplest candidate is a cosmological constant, a constant energy of empty space that Einstein once introduced and then dismissed. In the standard model dark energy makes up about 68 percent of today's energy budget, dark matter 27 percent and ordinary matter 5 percent. We do not know what it is.

Recent surveys, notably DESI in 2024 and 2025, hint that dark energy might change over time. The statistical evidence is intriguing but not decisive.

Two good ways of measuring H0 disagree. The distance ladder (the SH0ES team) gives about 73 km/s per Mpc, with an uncertainty near 1. Inferring H0 from the CMB with the standard model (Planck satellite) gives about 67.4, with an uncertainty near 0.5. That is an 8 percent gap, at the level of five standard deviations. Newer JWST observations of the Cepheids support the ladder's number, while another team using different stars gets about 70, in between.

Either an error lurks somewhere in a long chain of calibrations, or the standard model of cosmology is missing something. Nobody knows yet, and the numbers shift with each new paper, so treat these as the state of play in 2025.

Try the arithmetic yourself. The Coma galaxy cluster has z = 0.0231 and lies roughly 100 Mpc (320 million light-years) away:

That is 69, between the two camps, and 1/H0 comes out near 14 billion years, close to the true age of 13.8. One cluster settles nothing, though: its own motion and its distance are uncertain, which is why the real work uses thousands of objects. Cosmic expansion rests on the same general relativity as Black holes, simply, and the Doppler idea behind redshift also finds planets, as in Exoplanets: how we find other worlds. For the local scale this all sits on, see A tour of the solar system.