Redshift

In physics, redshift describes a lengthening of the wavelength of electromagnetic radiation, such as light, which also means its frequency drops. The reverse—shorter wavelength, higher frequency, and greater energy—is called blueshift. Within astronomy and cosmology, three distinct types of redshift are recognized: one caused by the relative motion of the source (Doppler redshift), another from radiation climbing out of a gravitational field (gravitational redshift), and a third due to the expansion of the universe (cosmological redshift). The symbol *z* typically represents the redshift value, defined as the fractional change in wavelength; the ratio of the observed to emitted wavelength is written as 1 + *z* (greater than 1 for redshift, less than 1 for blueshift). Automated surveys of astronomical redshifts are a key method for studying the universe’s large-scale structure. Redshift and blueshift also connect to photon energy and, through Planck’s law, to an equivalent blackbody temperature.

Strong redshifting can turn a gamma ray into an X-ray, or visible light into radio waves. The primordial radiation from the Big Bang, initially at 3000 K, has redshifted into the 3 K cosmic microwave background. More subtle shifts appear in spectroscopic observations of celestial objects, and terrestrial technologies like Doppler radar and radar guns also rely on the same principle. Gravitational waves, moving at light speed, undergo identical redshift effects. Other processes—such as scattering or optical effects—can alter electromagnetic frequencies, but these are distinct from astronomical redshift and are not usually called by that name.

The history of redshift began in the 1800s with classical wave mechanics and studies of the Doppler effect. Austrian mathematician Christian Doppler gave the first known physical explanation in 1842, and Dutch scientist Christophorus Buys Ballot confirmed it for sound waves in 1845. Doppler correctly predicted the effect applied to all waves and suggested that stars’ colors might reflect their motion relative to Earth. Unaware of Doppler’s work, French physicist Hippolyte Fizeau proposed in 1848 that shifts in stellar spectral lines could measure motion relative to Earth. In 1850, François-Napoléon-Marie Moigno analyzed both ideas in a publication read by James Clerk Maxwell and William Huggins; Huggins initially thought star colors came from chemistry, but by 1868 he became the first to determine a star’s recessional velocity using spectral shifts.

Optical redshift was confirmed in 1871 through observations of Fraunhofer lines during solar rotation, showing a shift of about 0.1 Å toward the red. In 1887, Hermann Carl Vogel and Julius Scheiner discovered the “annual Doppler effect”—a yearly variation in the Doppler shift of stars near the ecliptic, caused by Earth’s orbital motion. Aristarkh Belopolsky verified optical redshift in a laboratory in 1901 using rotating mirrors. Starting in 1912, Vesto Slipher observed a blueshift in the Andromeda Galaxy, indicating it was moving toward Earth. He first reported this in the inaugural volume of the *Lowell Observatory Bulletin* and later wrote a review in *Popular Astronomy*, noting that the exceptional velocity of –300 km/s for the Andromeda spiral showed the available methods could investigate both the spectra and velocities of spiral nebulae. Slipher reported velocities for 15 spiral nebulae across the sky; all but three showed positive (recessional) velocities.

Until 1923, the nature of these nebulae was unclear. That year, Edwin Hubble established they were galaxies and developed a method to measure distance using the period-luminosity relation of Cepheid variables. This allowed testing of Willem de Sitter’s 1917 prediction that redshift would correlate with distance. In 1929, Hubble combined his distance estimates with Slipher’s redshift data and measurements by Milton Humason to report an approximate relationship—now called Hubble’s law. Theoretical work on the redshift-distance relation also advanced in the 1920s. De Sitter’s solution to general relativity contained no matter, but in 1922 Alexander Friedmann derived dynamic solutions (the Friedmann equations) based on frictionless fluid models. Georges Lemaître independently derived similar equations in 1927, and his analysis became widely known around the time of Hubble’s key publication. By early 1930, the combination of redshift measurements and theoretical models marked a major breakthrough: the universe had a history, and its expansion could be studied with physical models supported by observational astronomy. Fritz Zwicky proposed an alternative “tired light” effect when cosmological redshifts were first discovered, but this model has largely been ruled out by observations of timescale stretch.