Stretched starlight revealed that the universe has a history
Light from the Big Bang started out at about 3,000 kelvin. Stretched by the expansion of space over billions of years, it now arrives as a faint 3-kelvin microwave glow. That stretching, called redshift, turned the colours of distant galaxies into a record of cosmic history.
A redshift is any lengthening of a wave's wavelength, which also lowers its frequency; the reverse is a blueshift. Astronomy knows three kinds. Motion of a source away from the observer produces a Doppler redshift, light climbing out of a gravitational well is shifted gravitationally, and the expansion of the universe stretches light cosmologically. Astronomers express the size of the shift with the letter z, the fractional change in wavelength. The same effect powers police radar guns, and gravitational waves are stretched too.
The idea begins with the Austrian mathematician Christian Doppler, who explained it in 1842 and suggested star colours might reflect their motion. Christophorus Buys Ballot confirmed it for sound in 1845, and in 1868 William Huggins became the first to measure a star's speed of retreat from its spectral shift. Starting in 1912, Vesto Slipher found the Andromeda Galaxy approaching at about 300 kilometres per second, yet of 15 spiral nebulae he measured, all but three were moving away.
By 1923 Edwin Hubble had shown these nebulae were galaxies and devised a way to gauge their distances using Cepheid variable stars. In 1929 he combined his distances with Slipher's shifts and measurements by Milton Humason to show that redshift grows with distance, the relation now known as Hubble's law. Meanwhile Alexander Friedmann in 1922 and Georges Lemaître in 1927 had found expanding solutions to Einstein's equations. By early 1930 observation and theory together showed the cosmos was evolving and could be studied as such.
Alternatives were tried. Fritz Zwicky proposed that light simply grows tired over long journeys, an idea largely ruled out by the way type Ia supernovae appear stretched in time. Arthur Eddington wrote red shift as two words in 1923; Willem de Sitter merged them in 1934. Automated redshift surveys now map the large-scale structure of the universe.
Source: Redshift
