Redshift is the displacement of the spectrum of an astronomical object toward longer (red) wavelengths. It is generally attributed to the Doppler effect, a change in wavelength that results when a given source of waves (e.g., light or radio waves) and an observer are in rapid motion with respect to each other.
The American astronomer Edwin Powell Hubble reported in 1929 that the distant galaxies were receding from the Milky Way system, in which the Earth is located, and that their redshifts increase proportionally with their increasing distance. This generalization became the basis for what is called Hubble's law, which correlates the recessional velocity of a galaxy with its distance from the Earth. That is to say, the greater the redshift manifested by light emanating from such an object, the greater the distance of the object and the larger its recessional velocity (see also Hubble's constant). This law of redshifts has been confirmed by subsequent research and provides the cornerstone of modern relativistic cosmological theories that postulate that the universe is expanding.
Since the early 1960s astronomers have discovered cosmic objects known as quasars that exhibit larger redshifts than any of the remotest galaxies previously observed. The extremely large redshifts of various quasars suggest that they are moving away from the Earth at tremendous velocities (i.e., approximately 90 percent the speed of light) and thereby constitute some of the most distant objects in the universe.
Excerpt from the Encyclopedia Britannica without permission.