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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.


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.
References in periodicals archive ?
The zCosmos-deep galactic survey [3] consists of a collection of visible galaxies with respective redshifts obtained for a given spectroscopic area in the sky.
Hubble found enough early galaxies, spread out over different redshifts, to suggest that they formed slowly.
When galaxy redshifts are corrected for the motion of our own galaxy, they come in distinct multiples of 37.
Tifft expanded his database to other clusters of galaxies to see if a relationship between redshifts and magnitudes was present.
The light travel time versus redshift is computed as follows (derived from dt = da/[?
Astronomers have used the bright light of supernovae with redshifts up to about 1.
We have found several hundred galaxies at redshift 4.
In the present study a redshift adjustment to the distance modulus was introduced.
Using the Anglo-Australian Telescope near Coonabarabran, Australia, the astronomers surveyed galaxies' redshifts, the amount by which cosmic expansion stretches light toward longer wavelengths.
A histogram of the galactic counts versus redshifts is obtained by counting the set of objects contained within each redshift bucket.
Some of the galaxies mapped had previously-measured redshifts, and Huchra started painstakingly measuring redshifts for the others in the late 1990s using mainly two telescopes: one at the Fred Lawrence Whipple Observatory on Mt.
The previous record-holding early clusters were between redshifts 4 and 5.