Redshift

Redshift is an increase in light’s wavelength, shifting its spectral features toward the red end of the spectrum. It can result from relative motion, gravity, or the expansion of space.

Doppler effect: A change in a wave’s observed frequency caused by relative motion between its source and observer. Radial motion between source and observer changes light’s wavelength through the Doppler effect.
Hubble's law: The empirical relation that a galaxy’s recession speed increases with its distance at sufficiently large scales. Galaxy redshifts provide the recession-speed measurements used to establish this relation.
Blueshift: A decrease in a wave’s wavelength, shifting its spectral features toward the blue end of the spectrum. It is the opposite wavelength change and commonly signals approach in the Doppler interpretation.
Christian Doppler: An Austrian physicist who described the frequency shift caused by relative motion in 1842. His principle supplied the motion-based explanation later applied to shifts in astronomical spectra.
Cosmological redshift: The increase in wavelength of light as the universe expands during its journey to an observer. It explains redshift accumulated across expanding space, even without ordinary motion through space.
Hubble–Lemaître law: The relation between a galaxy’s distance and its recession rate in an expanding universe. It connects cosmological redshift measurements with the expansion rate of the universe.
Proper motion: The angular change in an object's apparent position across the sky over time. It measures transverse motion, unlike redshift’s sensitivity to motion along the line of sight.
Vesto Slipher: An American astronomer who measured radial velocities of spiral nebulae from their spectra in the early twentieth century. His observations established that many galaxies showed large spectral shifts, mostly toward longer wavelengths.
Gravitational redshift: The increase in a photon’s wavelength as it moves from a stronger gravitational field to a weaker one. It is a distinct redshift caused by gravity rather than source-observer motion.
Cosmic microwave background: The relic radiation released when the early universe became transparent, now observed as nearly uniform microwave radiation. Cosmic expansion redshifted this radiation from visible and infrared wavelengths into microwaves.

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