Redshift and the expanding universe
Light stretched by expanding space — which is not the same thing as a Doppler shift, though it is usually described as one.
4 min read · Published 27 August 2026
The short answer
Light from distant galaxies is shifted toward longer wavelengths in proportion to distance, discovered in the 1920s and known as Hubble's law. It is usually explained as a Doppler shift from receding motion, but cosmological redshift is more accurately understood as light being stretched by the expansion of space during its journey. The distinction matters because the naive Doppler reading implies motion through space that is not occurring.
What redshift is
Elements absorb and emit light at fixed characteristic wavelengths, producing lines in a spectrum. Those wavelengths are laboratory constants.
In light from distant galaxies, the same recognisable pattern appears shifted toward longer, redder wavelengths. The pattern is unmistakable — the spacing is preserved — so the shift is measurable with precision.
The redshift z quantifies it. A galaxy at z = 1 has its light stretched to twice the emitted wavelength. The most distant galaxies currently known sit above z = 14, meaning their light has been stretched by a factor of more than fifteen.
The discovery
Vesto Slipher measured redshifts of spiral nebulae in the 1910s and found most receding, some at surprising speeds. At the time it was not established that these were external galaxies at all.
Edwin Hubble then measured their distances, using Cepheid variable stars calibrated by Henrietta Swan Leavitt's period-luminosity relation — see the women of Harvard's computers. Combining distance with Slipher's velocities revealed a proportional relationship: further galaxies recede faster.
Georges Lemaître had derived an expanding solution from general relativity and predicted this relationship before the observational confirmation, which is why the relation is increasingly called the Hubble–Lemaître law.
That proportionality is exactly what uniform expansion produces, and it is the foundation of the Big Bang model.
Why it is not really a Doppler shift
The standard explanation — galaxies moving away, like an ambulance siren dropping in pitch — is a useful approximation and not what is happening.
In the standard cosmological picture, distant galaxies are not moving through space away from us. Space between us is expanding, and light traversing it is stretched along with it. The wavelength grows because the space the wave occupies grows.
This distinction has consequences. Recession velocities computed naively from high redshifts exceed the speed of light, which would be impossible for motion through space. Expansion of space is not motion in that sense and no relativistic limit applies.
For nearby objects the distinction is negligible, and genuine Doppler shifts from local motion do occur — Andromeda is blueshifted because it is genuinely approaching, as covered in the Andromeda collision.
The Hubble tension
The current expansion rate, the Hubble constant, can be measured two ways.
Locally, using Cepheids and Type Ia supernovae to build a distance ladder.
From the early universe, by fitting the cosmic microwave background with the standard cosmological model and projecting forward.
The two give values that disagree by more than their stated uncertainties, and the gap has persisted as measurements improved. This is the Hubble tension.
It may indicate an unrecognised systematic error in one method. It may indicate something missing from the cosmological model. Nobody knows, and it is currently one of the most actively worked problems in the field.
It is worth citing whenever cosmology is presented as finished. A central number describing the universe's expansion is measured two ways and the answers do not match, and the field says so openly.
Common questions
- What is redshift?
- The shift of light toward longer wavelengths. Elements produce spectral lines at fixed wavelengths, and in distant galaxies that recognisable pattern appears stretched toward the red, in proportion to distance.
- Is cosmological redshift a Doppler shift?
- Not really. Distant galaxies are not moving through space away from us — the space between is expanding, stretching the light as it travels. The distinction matters because naive Doppler calculations at high redshift give recession velocities exceeding light speed.
- Who discovered the expanding universe?
- Vesto Slipher measured the redshifts, Edwin Hubble measured the distances using Cepheid variables calibrated by Henrietta Swan Leavitt, and Georges Lemaître derived an expanding solution and predicted the relationship beforehand. It is increasingly called the Hubble-Lemaître law.
- What is the Hubble tension?
- Measurements of the universe's current expansion rate from local distance indicators disagree with those derived from the cosmic microwave background by more than their stated uncertainties. The discrepancy has persisted as measurements improved and remains unresolved.