Phenomena · Universe scale
Redshift
The stretching of light waves from a source moving away from an observer — or from the expansion of space itself. In cosmology, redshift is the primary tool for measuring cosmic distances and the rate at which the universe expands.
The formula
z = (λobs − λrest) / λrest
z = 0 → no shift · z = 1 → universe was half its current size
Source: ESA Science — What is redshift?
Why redshift matters for cosmology
Edwin Hubble's 1929 discovery — that the more distant a galaxy, the faster it recedes — revealed that the universe is expanding. The relationship between redshift and distance (Hubble's Law) is the foundation of cosmological distance measurements and the basis for determining the age and size of the observable universe.
Redshift also acts as a cosmic time machine. When we observe a galaxy at z = 1, we see it as it was when the universe was about half its current age. At z = 6, we see a galaxy as it was just one billion years after the Big Bang. The James Webb Space Telescope has observed galaxies at z > 13, pushing our view back to within ~300 million years of the Big Bang.
Redshift and the Cosmic Microwave Background
The Cosmic Microwave Background (CMB) — the afterglow of the Big Bang — has a redshift of z ≈ 1,100. The photons we detect today were emitted when the universe was 380,000 years old and have been travelling (and stretching) ever since. The CMB's measured temperature (2.725 K) and its tiny temperature fluctuations are the primary source of our knowledge of the universe's composition. The ESA Planck mission produced the most precise CMB maps, revealing the proportions of ordinary matter, dark matter, and dark energy.
Frequently asked questions
- Doppler redshift occurs when a source is physically moving away from the observer through space — like an ambulance siren dropping in pitch as it drives past. The faster the recession, the greater the redshift. Cosmological redshift is different: the light wave is stretched because space itself expanded while the light was travelling. The photon's wavelength grows with the universe. For nearby galaxies the distinction is minor; for very distant galaxies the cosmological effect dominates. The ESA's explanation of redshift covers both clearly.
- Redshift (symbol z) is measured by comparing the observed wavelength of a spectral line to its known rest-frame wavelength: z = (λ_obs − λ_rest) / λ_rest. A spectrum containing hydrogen absorption lines at known wavelengths can be compared against the observed positions of those same lines in a galaxy's spectrum. If they appear at longer (redder) wavelengths, the galaxy is receding. The DESI survey is currently measuring redshifts for tens of millions of galaxies to map the large-scale structure of the universe.
- A high redshift (z > 1) means we are looking at a galaxy as it was when the universe was less than half its current age. The most distant confirmed galaxies have redshifts above z = 13, meaning their light left when the universe was less than 400 million years old — only about 3% of its current age. The James Webb Space Telescope is designed to observe these early galaxies. Our Webb deep field news article covers the latest record-setting observations.
- Yes. Blueshift occurs when a source is moving toward the observer, compressing the light waves to shorter (bluer) wavelengths. The Andromeda Galaxy (M31) is one of the few large galaxies that is blueshifted — it is approaching the Milky Way at about 110 km/s due to their mutual gravitational attraction, and they are expected to merge in about 4.5 billion years. Most other galaxies are redshifted because the universe is expanding. NASA's page on the Milky Way–Andromeda merger has detail.
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