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In the classical Doppler effect, the frequency of the source is not modified, but the recessional motion causes the illusion of a lower frequency. A more complete treatment of the Doppler redshift requires considering relativistic effects associated with motion of sources close to the speed of light.
Redshift-space distortions (RSDs) manifest in two particular ways. The Fingers of God effect is where the galaxy distribution is elongated in redshift space, with an axis of elongation pointed toward the observer. [1] It is caused by a Doppler shift associated with the random peculiar velocities of galaxies bound in structures such as clusters.
Gravitational redshift can be interpreted as a consequence of the equivalence principle (that gravity and acceleration are equivalent and the redshift is caused by the Doppler effect) [5] or as a consequence of the mass–energy equivalence and conservation of energy ('falling' photons gain energy), [6] [7] though there are numerous subtleties ...
The relativistic Doppler effect is the change in frequency, wavelength and amplitude [1] of light, caused by the relative motion of the source and the observer (as in the classical Doppler effect, first proposed by Christian Doppler in 1842 [2]), when taking into account effects described by the special theory of relativity.
The Doppler effect (also Doppler shift) is the change in the frequency of a wave in relation to an observer who is moving relative to the source of the wave. [ 1 ] [ 2 ] [ 3 ] The Doppler effect is named after the physicist Christian Doppler , who described the phenomenon in 1842.
The redshift z is often described as a redshift velocity, which is the recessional velocity that would produce the same redshift if it were caused by a linear Doppler effect (which, however, is not the case, as the velocities involved are too large to use a non-relativistic formula for Doppler shift). This redshift velocity can easily exceed ...
For photons, this leads to the cosmological redshift. While the cosmological redshift is often explained as the stretching of photon wavelengths due to "expansion of space", it is more naturally viewed as a consequence of the Doppler effect. [3]
Several attempts to measure the effect were negative or inconclusive. The first generally accepted claim to have measured gravitational redshift was W.S. Adams's 1925 measurement of shifts in the spectral lines of the white dwarf star Sirius B. [2] However, even Adams's measurements have since been brought into question for various reasons. [3]