Three mechanisms, one observational signature
Redshift is the lengthening of light waves, appearing as a shift toward red wavelengths in the spectrum. The same spectral signature can arise from three entirely different physical causes. Doppler redshift occurs when a light source moves away from the observer; the relative motion stretches the waves. Cosmological redshift occurs because space itself is expanding; photons emitted in the early universe are stretched as the universe expands between emission and arrival. Gravitational redshift occurs when light escapes a gravitational field; the photon loses energy climbing out against gravity.
These three mechanisms are indistinguishable from the wavelength shift alone. Astronomers must use context to infer which is operating. A nearby galaxy shows Doppler redshift from its motion through space; a distant galaxy shows cosmological redshift because space between us is expanding; a photon escaping the vicinity of a neutron star shows gravitational redshift.
Why the distinction matters for cosmology
Early astronomers misidentified cosmological redshift as Doppler motion, leading to incorrect distance estimates. Hubble's Law (v = H0 * d) assumes redshift arises from recession velocity caused by expanding space, not local motion. For distant galaxies, the velocity becomes so high that relativistic effects matter; the relationship between redshift and distance is nonlinear. Modern cosmology uses redshift to infer both distances and the universe's expansion rate (the Hubble constant), but only if the mechanism is correctly identified.
Gravitational redshift, predicted by Einstein's general relativity, has been measured from stars and confirmed by precision atomic clocks at different altitudes. It is a key test of gravitational theory and affects the operation of GPS satellites.