Cosmological redshift

Cosmological redshift is the increase in a photon’s wavelength as the universe expands during its travel. It links observed light to the universe’s expansion history.

Scale factor (cosmology): A dimensionless measure of how the universe’s size changes over time in cosmological models. Its growth between emission and observation determines the wavelength stretch.
Redshift survey: An astronomical survey that measures galaxy redshifts to map their distribution in three dimensions. It uses cosmological redshift to place galaxies along the line of sight.
Doppler effect: The change in a wave’s observed frequency caused by relative motion between source and observer. Unlike cosmological redshift, it arises from relative motion through local space.
Cosmic distance ladder: A sequence of calibrated methods for measuring distances from nearby stars to distant galaxies. Redshift-distance comparisons help calibrate and test the ladder at cosmic scales.
Wavelength: The spatial period of a wave, measured between successive points of equal phase. Cosmological redshift is specifically an increase in this property of light.
Friedmann–Lemaître–Robertson–Walker metric: A spacetime metric describing a homogeneous, isotropic universe that may expand or contract. Its changing spatial scale mathematically describes the expansion that stretches photons.
Hubble–Lemaître law: The empirical relation between a galaxy’s recession velocity and its distance at sufficiently small distances. Redshift provides the observable used to estimate recession and infer distance.
Gravitational redshift: The decrease in photon frequency as light climbs out of a gravitational potential. It shifts light through gravity rather than the universe’s changing scale.
Lookback time: The time light has traveled from an astronomical source to an observer. A measured redshift helps infer how long the universe expanded during the photon’s journey.
Scale invariance: A property in which a system’s description retains its form under changes of scale. The wavelength-to-scale-factor relation reflects uniform stretching in an expanding homogeneous universe.

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