How Do We Know the Universe Is Expanding? Redshift, Distance and a Stretching Space

Every chemical element absorbs and emits light at a precise set of wavelengths, producing a barcode of dark or bright lines in the spectrum of a star or galaxy that identifies what it is made of. When the same recognisable pattern appears shifted towards longer, redder wavelengths, something has stretched the light on its way here. The everyday analogy is the falling pitch of a passing siren, and for nearby objects moving through space the comparison is exact. For distant galaxies the mechanism is different and more interesting: the light is not shifted because the galaxy is travelling through space but because the space the light crosses has expanded while the light was in transit, stretching the wave along with it. The measured quantity, the redshift, therefore records how much the universe has grown since the light set out, and it is the single most important number in observational cosmology.

A redshift alone proves nothing without a distance to compare it with, and distance is the harder measurement. The technique rests on standard candles, objects whose true brightness is known, so that how faint they appear gives their distance:

• •Parallax, the apparent shift of a nearby star as the Earth moves around the sun, which gives direct geometric distances and calibrates everything else
• •Cepheid variables, stars that pulse with a period directly tied to their true brightness, a relationship found by Henrietta Swan Leavitt in 1912 that made the whole field possible
• •Type Ia supernovae, exploding white dwarfs that reach nearly the same peak brightness every time and are visible across billions of light years
• •The relationship between a spiral galaxy's rotation speed and its luminosity, used at intermediate distances
• •Each rung is calibrated against the one below it, which is why the whole structure is called the distance ladder and why an error low down propagates all the way up

Vesto Slipher measured the spectra of spiral nebulae from 1912 and found almost all of them receding, at speeds far greater than anything in our galaxy. Edwin Hubble, using Cepheids observed with the hundred-inch telescope at Mount Wilson, first showed in 1924 that these objects lie far outside the Milky Way and are galaxies in their own right, and then in 1929 plotted their distances against Slipher's velocities and found them proportional: twice as far means twice as fast. The Belgian priest and physicist Georges Lemaitre had derived the same relationship theoretically from general relativity and published it, in a little-read Belgian journal, two years earlier, which is why the constant is now often given both names. The proportionality is the key point, because it is exactly what uniform stretching produces. Mark dots on a rubber band and pull: every dot sees every other receding, and the distant ones recede fastest, with no dot at the centre.

Expansion implies a hotter, denser past, and that past left two fingerprints that have nothing to do with redshift measurements. The first is the cosmic microwave background, the light released when the universe cooled enough for atoms to form about 380,000 years after the beginning, discovered accidentally by Arno Penzias and Robert Wilson in 1965 as a persistent hiss in a radio antenna they tried to eliminate by cleaning out pigeon droppings. It fills the sky at 2.7 degrees above absolute zero, matches the predicted spectrum with extraordinary precision, and its tiny temperature variations encode the composition and geometry of the universe. The second is the abundance of the lightest elements: the theory predicts that in the first few minutes, nuclear reactions produced about a quarter of the ordinary matter as helium, together with specific traces of deuterium and lithium, and those proportions are what is measured in the oldest and least processed material we can find. Three independent lines agreeing is why the picture is not seriously disputed.

Two live problems are worth knowing. The first is acceleration: in 1998 two teams measuring distant supernovae found them fainter, and so further away, than a steadily expanding universe predicts, meaning the expansion is speeding up rather than slowing under gravity. The cause was named dark energy, which is a label for an unexplained effect making up roughly seventy percent of the energy content of the universe, and the discovery took the Nobel Prize in 2011. The second is a discrepancy in the rate itself. Measurements built from the distance ladder give a value around 73 kilometres per second per megaparsec, while the value inferred from the microwave background and the standard model gives about 67, and the gap has widened rather than closed as both have grown more precise. It is called the Hubble tension, and it is either a subtle error in one method or a sign that something in the standard cosmological model is wrong, which is why a great deal of effort is going into it.

Distant galaxies show their light stretched to longer wavelengths, and the amount of stretching is proportional to their distance, which is exactly the pattern uniform expansion produces and implies no centre. Distances come from a ladder of standard candles calibrated from parallax through Cepheid variables to supernovae, and the relationship was derived by Lemaitre and measured by Hubble in 1929. Two independent confirmations, the microwave background and the abundance of light elements, support it, while the acceleration found in 1998 and a persistent disagreement over the expansion rate remain unexplained.