In physics and astronomy, redshift occurs when the electromagnetic radiation, usually visible light, that is emitted from or reflected off of an object is shifted towards the red end of the electromagnetic spectrum. More generally, redshift is defined as an increase in the wavelength of electromagnetic radiation] received by a detector compared with the wavelength emitted by the source. This increase in wavelength corresponds to a decrease in the frequency of the electromagnetic radiation. Conversely, a decrease in wavelength is called blue shift.
Any increase in wavelength is called “redshift”, even if it occurs in electromagnetic radiation of non-optical wavelengths, such as gamma rays, x-rays and ultraviolet. This nomenclature might be confusing since, at wavelengths longer than red (e.g., infrared, microwaves, and radio waves), redshifts shift the radiation away from the red wavelengths.
Known redshift mechanisms
A redshift can occur when a light source moves away from an observer, corresponding to the Doppler shift that changes the frequency of sound waves. Although observing such redshifts, or complementary blue shifts, has several terrestrial applications (e.g., Doppler radar and radar guns), spectroscopic astrophysics uses Doppler redshifts to determine the movement of distant astronomical objects. This phenomenon was first predicted and observed in the 19th century as scientists began to consider the dynamical implications of the wave-nature of light.
Another type of redshift, the gravitational redshift (also known as the Einstein effect), is a result of the time dilation that occurs near massive objects, according to general relativity.
The Wolf effect is phenomenon that occurs in several closely related phenomena in radiation physics, with analogous effects occurring in the scattering of light. It was first predicted by Emil Wolf in 1987   and subsequently confirmed in the laboratory in acoustic sources by Mark F. Bocko, David H. Douglass, and Robert S. Knox, and a year later in optic souces by Dean Faklis and George Morris in 1988 . Wolf and James noted:
- “Under certain conditions the changes in the spectrum of light scattered on random media may imitate the Doppler effect, even though the source, the medium and the observer are all at rest with respect to one another.
Wolf, Daniel James and Sisir Roy et al. suggested that the Wolf Effect may explain discordant redshift in certain quasars .
Hypothetical and theoretical redshift mechanisms
The “Cosmological redshift” or expansion of the universe, is one of the most well-known redshift mechanisms, which explains the famous observation that the spectral redshifts of distant galaxies, quasars, and intergalactic gas clouds increase in proportion to their distance from the observer. This mechanism is a key feature of the Big Bang model of physical cosmology.
Alternative redshift theories
Alternative redshift theories are those that are presumed to have not been caused by one of three generally accepted causes of redshift (i.e. 1. Cosmological, 2. Doppler and 3. Gravitational). In 1981, French cosmologist and astrophysicist Henri Reboul discovered that over thirty categories of redshift theories have been proposed over the years in over 200 papers Template:Ref. Alternative theories still appear in peer-reviewed literature (see examples below), and alternative redshift theories are discussed in conferences  .
Such theories have been proposed for many reasons, such as dissatisfaction with the explanation of cosmological redshift , and as an alternative interpretation of astromomical and laboratory observational data. The purpose of this article is not to judge the merit of each suggested redshift theory, nor even to judge whether they meet the criteria of a theory, model, hypothesis or other.
Critics argue that all alternative redshift theories have been contested over the years, as they produce distorted redshifted spectra. These distinctive spectral signatures also often provides a means of distinguishing the actual mechanisms involved. For example, Brillouin scattering will shift spectral lines  (also called a redshift, ), but also produce a characteristic triplet .
Several different names have been given to redshifts that are presumed due to alternative theories; It is noted that some of these may also be applied to accepted redshift causes. These include:
|Redshift term||Examples papers using term|
|in title||in abstract|
|Intrinsic redshift||Show (5)||Show (48)|
|Non-cosmological redshift||Show (7)||Show (16)|
or Non-Doppler Redshift
or Discordant redshifts
Summary of alternative redshift theories
This is a selection of redshift theories that have been published over the years, that claim a cause that is not due to either Cosmological redshift (Friedmann), Doppler redshift, nor Gravitational redshift (Schwarzschild).
- 1909 John Evershed‘s “Evershed Effect” in the penumbra of sunspots  .
- 1923 Compton scattering is Arthur Compton‘s Nobel Prize-winning theory which causes spectral shifts. However, critics note that it also causes blurring which is not seen in the redshifts of distant objects.    .
- 1929 Tired light is Fritz Zwicky‘s theory that as photons move through space, they lose energy . Critics note several problems with tired light models in explaining the Hubble Law. It is not accepted by mainstream cosmologists as a mechanism. 
- 1955 M. A. Melvin’s photon radiation density and path length 
- 1972 Dror Sadeh et al, “Effect of Mass on Frequency” 
- 1972 Daniel M. Greenberger’s theory of “variable mass particles” which proposes a “decay redshift” 
- 1972 D.K. Ross’s “New Red-Shift Mechanism for Quasars” using the variation of particle rest mass 
- 1972 J.C. Pecker, et al photon-photon interaction (in Pecker, J. C., Roberts, A. P., and Vigier, J. P., 1972, Non-velocity redshifts and photon-photon interactions: Nature, v. 237, p. 227-229). But see also 
- 1972 S. Urbanovich’s “external influences” 
- 1974 Halton Arp suggests that the redshift of some quasars and galaxies may be non-velocity , and non-cosmological  (see also 1997 below).
- 1974 P. Merat et al, “Interaction between incident transverse photons and light neutral bosons” 
- 1976 Z. Maric et al, Photon-boson scattering 
- 1976 X.-Q. Li’s photon motion in the discrete space-time under the photon’s own force field 
- 1977 J. V. Narlikar’s variable mass version of general relativity  
- 1977 Susan M. Simkin’s “Simkin effect”   which is a description of one of the effects of light pollution.
- 1979 E. Schatzman’s “Ageing of photons by collisions with a hypothetical particle” 
- 1979 E. R. Harrison and T. W. Noonan’s “Interpretation of extragalactic redshifts” as “”Corrected” redshifts” 
- 1984 William G Tifft et al, “Global redshift quantization”   
- 1987 Emil Wolf‘s “Wolf effect”  , confirmed in the laboratory by Dean Faklis and George Morris in 1988 . The frequency shift is generally not disortion free. However, in 1996, Wolf and Daniel F. V James reported that “under certain circumstances the changes in the spectrum of light scattered on random media may imitate the Doppler effect”  
- 1990 Paul Marmet’s inelastic transmission of photons in gases 
- 1997 Halton Arp suggests that redshift is a measure of age, rather than distance , based on Narlikar’s variable mass version of general relativity  (resulting in Arp’s book, Seeing Red).
- 2000 Ari Brynjolfsson’s “Plasma redshift“, that the interaction of photons with hot sparse electron plasma may produce a redshift   
- 2003 CREIL (Coherent Raman Effect on Incoherent Light) has been proposed by Jacques Moret-Bailly  
- 2004 Charles Gallo’s “Neutrino redshifts”  (not a new theory, but a proposal to look for redshifts in neutrino spectra)
To this list may be added several theories based on scattering processes, such as Brillouin scattering, Compton scattering, Raman scattering and Rayleigh scattering.
Alternative redshift theory categorisation
Over the years, many peer-reviewed theories have been published attempting to explain the cause of intrinsic redshift. Most theories have been contested, and none have been accepted by mainstream science.
In 1981, Henri Reboul published a paper Untrivial redshifts – A bibliographical catalogue in which “We arbitrarily define as trivial a redshift which can be easily explained by a combination of the three following effects: Doppler, Schwarzschild, Friedmann.” (ie. Doppler, Gravitational and Cosmological reshifts). The paper includes “seven hundred and seventy-two coded references, covering 70 years of study of anomalous, i.e., untrivial redshifts (NTZ), are presented. Definitions are given for trivial Z and for the classical theories, and lists are presented for 17 classes of NTZ and 19 classes of unclassical theories”. Template:Ref
|Reboul’s categorisation of “non-trivial redshift” papers |
refered to in papers
by the paper’s author(s)
The April Meeting 2003 of The American Physics Society, a session was devoted to “Non-Doppler Redshift Mechanisms with Possible Cosmological Applications” , and in 2005 the Alternative Cosmology Group held the “1st Crisis In Cosmology Conference” that included papers on “Old massive galaxies at large redshifts” .
- Template:Note Arp, H., Anomalous redshifts in galaxies and quasars (1977)
- Template:Note Arp, Halton C., Catalogue of discordant redshift associations (2003)
- Template:Note Burbidge, Geoffrey, The Distribution of Redshifts in Quasi-Stellar Objects, N-Systems and Some Radio and Compact Galaxies (1968) Astrophysical Journal, vol. 154, p.L41
- Template:Note Narlikar, J. V., Non-Cosmological Redshifts (1977)
- Template:Note Reboul, H. J. Untrivial redshifts – A bibliographical catalogue (1981) Astronomy and Astrophysics Supplement Series, vol. 45, July 1981, p. 129-144. Full text
- Template:Note Simkins, S. M., Small “non-velocity” redshifts in galaxies, Nature, 239, 43-44
- ↑ See Feynman, Leighton and Sands (1989) or any introductory undergraduate (and many high school) physics textbooks. See Taylor (1992) for a relativistic discussion.
- ↑ See Binney and Merrifeld (1998), Carroll and Ostlie (1996), Kutner (2003) for applications in astronomy.
- ↑ See Misner, Thorne and Wheeler (1973) and Weinberg (1971).
- ↑ 4.0 4.1 James, Daniel, “The Wolf effect and the redshift of quasars” (1998) Pure Appl. Opt. 7: 959-970. (Full text, PDF)
- ↑ 5.0 5.1 Wolf, Emil “Noncosmological redshifts of spectral lines” (1987) Nature 326: 363—365.
- ↑ Wolf, Emil, “Redshifts and blueshifts of spectral lines caused by source correlations” (1987) Optics Communications 62: 12—16.
- ↑ Mark F. Bocko, David H. Douglass, and Robert S. Knox, “Observation of frequency shifts of spectral lines due to source correlations” (1987) Physical Review Letters 58: 2649—2651.
- ↑ Faklis, Dean, and Morris, George Michael, “Spectral shifts produced by source correlations” (1988) Optics Letters 13 (1): 4—6.
- ↑ Wolf, Emil, and James, Daniel F. V., “Correlation-induced spectral changes” (1996) Reports on Progress in Physics 59: 771—818. (Full text, PDF)
- ↑ Roy, Sisir, Kafatos, Menas, and Datta, Suman, “Shift of spectral lines due to dynamic multiple scattering and screening effect: implications for discordant redshifts” (2000) Astronomy and Astrophysics, v.353, p.1134-1138 353: 1134—1138.
- ↑ See Misner, Thorne and Wheeler (1973) and Weinberg (1971) or any of the physical cosmology textbooks