Redshift

When a light source recedes from the observer, the observed wavelength is longer than the emitted wavelength. For visible light, that shift moves spectral lines toward the red end of the spectrum — hence the name.

Redshift is quantified by the dimensionless z = (λ_obs − λ_src) / λ_src. For small recession velocities, z ≈ v/c. Most galaxies in the universe are redshifted; the amount of redshift correlates with distance, which is the basis of Hubble's law and the evidence for cosmic expansion.

First measured for a galaxy (Andromeda, which is actually blueshifted) by Vesto Slipher in 1912. The pattern of galactic redshifts — nearly all positive — became the foundation of observational cosmology.