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(red -shift) A displacement of spectral lines toward longer wavelength values; for an optical line, the shift would be toward the red end of the visible spectrum. The redshift parameter, z, is given by the ratio δλ/λ, where δλ is the observed increase in wavelength of the radiation and λ is the wavelength of the spectral line at the time of emission from a source, i.e. the wavelength in the ‘normal’ terrestrial spectrum.

The redshifts of astronomical objects within the Galaxy are interpreted as Doppler shifts (see Doppler effect) caused by movement of the source away from the observer. The value of z is then v /c , where v is the relative radial velocity and c is the speed of light. The redshifts of extragalactic sources, including quasars, are also interpreted in terms of the Doppler effect, which for these objects results from the expansion of the Universe. The redshift parameter of a distant galaxy thus gives its velocity of recession; since recessional velocities can be very great, the relativistic expression for redshift must be used:

z = [(c + v )/(c v )]½ – 1

From measurements of galactic redshifts it has been possible to calculate the distances of galaxies, using Hubble's law (see distance determination).

The redshifts described above represent a loss of energy by the photons of radiation in overcoming the effects of recession or expansion. There is another mechanism, however, by which redshifts can be produced, i.e. by which photons can lose energy – the presence of a strong gravitational field. This gravitational redshift was predicted by Einstein in his general theory of relativity. Although the redshifts of galaxies are often interpreted as being caused by the relativistic Doppler effect alone, both the expansion and the gravitational field of the Universe are involved. See also cosmological redshift.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006


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.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
References in periodicals archive ?
But like all distant sources of light, the 1.4 GHz signal is redshifted by cosmic expansion.
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The galaxy is so distant that it doesn't even show up in visible light, presumably because the expansion of the universe has redshifted its visible light into the infrared part of the spectrum, and because much of its light is absorbed by neutral hydrogen gas en route to Earth.
Viewed from Earth, infrared light emitted by a galaxy some 13 billion light-years distant gets redshifted to much longer wavelengths -- into the radio region of the electromagnetic spectrum.
The first step in locating it was to identify faint candidate objects behind the gravitational lens that had the right infrared colors for a very highly redshifted object.
That discrepancy could have an intriguing explanation: The red galaxies have a structure identical to that of nearby ellipticals, yet they may lie so far away that the visible light they emit has been stretched, or redshifted, to the infrared.