Definition
Redshift is the shift of a spectral feature toward longer (redder) wavelengths.
Three distinct mechanisms produce it: the classical Doppler effect of a source receding through space; gravitational redshift as light climbs out of a gravitational potential well; and cosmological redshift, the stretching of light's wavelength by the expansion of the universe itself between emission and observation.
Why it matters
How it works
The redshift parameter is defined as z = (λ_obs − λ_emit) / λ_emit. A redshift of 1 means the wavelength has doubled; a redshift of 7 (some of the most distant galaxies known) means it has stretched by a factor of eight.
The Doppler interpretation works in flat spacetime: a source moving away with velocity v shifts spectral lines by approximately z ≈ v/c at non-relativistic speeds, and by the full relativistic formula 1 + z = √((1+β)/(1−β)) at speeds approaching c. This is the mechanism for stellar radial velocity, exoplanet detection, and binary-star analysis.
The gravitational interpretation is a consequence of general relativity. Light leaving a gravitational potential well loses energy, lengthening its wavelength by a factor of approximately (1 + GM/rc²). Pound and Rebka measured the ~2.5×10⁻¹⁵ shift over 22.5 metres of vertical drop in a Harvard tower in 1959 — confirmation of general relativity in a basement experiment.
The cosmological interpretation is distinct and dominates at large distances. Light emitted by a galaxy when the cosmic scale factor was a_emit and observed today when it is a_today is stretched by exactly the same factor that space has stretched: 1 + z = a_today / a_emit. There is no need to invoke peculiar motion through space — the photons simply travel through an expanding metric. This is the redshift used in H₀ d = cz (for small z) and in the cosmological distance ladder.