Redshift: How the Stretching of Light Reveals the Universe's Expansion

Hold a stretched rubber band in front of a light source and the shadows cast by marks on the band spread apart as you pull the ends. Now imagine doing this with light itself — with the wavelengths of electromagnetic radiation emitted by a galaxy billions of light-years away, stretched by the expansion of the universe during the billions of years the light has been traveling. This stretching is redshift, and it is one of the most powerful tools in astronomy.

The concept has its roots in a familiar everyday experience: the Doppler effect. When an ambulance moves toward you, its siren sounds higher-pitched than its actual frequency because the sound waves are compressed by the motion. When it moves away, the siren sounds lower-pitched as the waves are stretched. The same principle applies to light: a source moving toward an observer has its emitted light compressed to shorter, bluer wavelengths (blueshift); a source moving away has its light stretched to longer, redder wavelengths (redshift).

Two Types of Redshift

The redshift astronomers observe in distant galaxies has two distinct components, which are conceptually important to separate. Doppler redshift arises from a galaxy's peculiar velocity — its actual motion through space relative to us. This is what causes the Andromeda Galaxy, which is moving toward the Milky Way, to show blueshift rather than redshift: it is genuinely approaching us through space.

Cosmological redshift is different. It arises not from a galaxy's motion through space but from the expansion of space itself. As the universe expands, the wavelengths of light traveling through it are stretched proportionally to the expansion that occurs during the light's journey. A photon emitted when the universe was half its current size will arrive with twice its original wavelength. This is not a Doppler effect in the classical sense — the galaxy was not "moving" when it emitted the light in the way a moving vehicle is moving — but rather a consequence of the fabric of space expanding while the light was in transit.

How Redshift Revealed the Big Bang

In the 1920s, astronomer Edwin Hubble used redshift measurements to demonstrate that virtually all galaxies are moving away from us, and that more distant galaxies are receding faster. This relationship — now called Hubble's Law — was immediately interpretable as evidence for an expanding universe: if everything is moving apart uniformly, then running the clock backward implies that everything was once in the same place. The universe had a beginning.

The redshifts Hubble measured were obtained by analyzing the spectra of galaxy light — the distribution of light across different wavelengths. Elements like hydrogen and calcium emit and absorb light at specific, known wavelengths called spectral lines. When a galaxy's spectrum is measured and its lines are found at longer wavelengths than laboratory measurements, the shift can be calculated precisely, giving the galaxy's redshift. From the redshift, using Hubble's Law, the galaxy's distance can be estimated.

The Cosmic Distance Ladder's Top Rung

Today, redshift is used routinely to measure distances across the entire observable universe. For nearby galaxies, other distance indicators — Cepheid variable stars, Type Ia supernovae — provide more precise measurements. But for the most distant objects, where these indicators are too faint to measure, redshift is often the primary distance estimator. The most distant quasars and galaxies have redshifts above 10, meaning their light has been stretched to more than 11 times its original wavelength during the 13-plus billion years of its journey.

The precise redshift measurements of thousands of galaxies, conducted by surveys like the Sloan Digital Sky Survey, have produced three-dimensional maps of the universe's large-scale structure — the network of filaments, sheets, and voids that reveals how matter has been distributed and has evolved over cosmic time. Each redshift measurement is a distance measurement, and each distance measurement is a step in reconstructing the history of the universe from the stretched signatures of ancient light.