Redshift

(astrophysics) A systematic displacement toward longer wavelengths of lines in the spectra of distant galaxies and also of the continuous portion of the spectrum; increases with distance from the observer. Also known as Hubble effect.

A systematic displacement toward longer wavelengths of lines in the spectra of distant galaxies, and also of the continuous part of the spectrum. First studied systematically by E.Hubble, redshift is central to observational cosmology, in which it provides the basis for the modern picture of an expanding universe.

There are two fundamental properties of redshifts. First, the fractional redshift Δ&lgr:/&lgr: is independent of wavelength. This rule has been verified from 21 cm (radio radiation from neutral hydrogen atoms) to about 6 × 10⁻⁵ cm (the visible region of the electromagnetic spectrum) and leads to the interpretation of redshift as resulting from a recession of distant galaxies. Though this interpretation has been questioned, no other mechanism is known that would explain the observed effect.

Second, redshift is correlated with apparent magnitude in such a way that when redshift is translated into recession speed and apparent magnitude into distance, the recession speed is found to be nearly proportional to the distance. This rule was formulated by Hubble in 1929, and the constant of proportionality bears his name. See also Hubble constant; Magnitude (astronomy).