Editing Redshift (section)

{{short description|Change of wavelength in photons during travel}}
{{About|the astronomical phenomenon||}}
[[File:Redshift.svg|thumb|upright|[[Spectral line|Absorption lines]] in the [[visible spectrum]] of a [[supercluster]] of distant galaxies (right), as compared to absorption lines in the visible spectrum of the [[Sun]] (left). Arrows indicate redshift. Wavelength increases up towards the red and beyond (frequency decreases).]]
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In [[physics]], a '''redshift''' is an increase in the [[wavelength]], and corresponding decrease in the [[frequency]] and [[photon energy]], of [[electromagnetic radiation]] (such as [[light]]). The opposite change, a decrease in wavelength and increase in frequency and energy, is known as a [[#Blueshift|blueshift]]. The terms derive from the colours [[red]] and [[blue]] which form the extremes of the [[Visible spectrum|visible light spectrum]]. Three forms of redshift occur in [[astronomy]] and [[cosmology]]: [[Doppler effect|Doppler]] redshifts due to the relative motions of radiation sources, [[gravitational redshift]] as radiation escapes from gravitational potentials, and [[cosmological redshift]]s of all light sources proportional to their distances from Earth, a fact known as [[Hubble's law]] that implies the [[expansion of the universe|universe is expanding]].

All redshifts can be understood under the umbrella of [[Frame of reference|frame transformation laws]]. [[Gravitational wave]]s, which also travel at [[Speed of light|the speed of light]], are subject to the same redshift phenomena.<ref>{{cite journal | title=Detectability of primordial black hole binaries at high redshift | last=Ding | first=Qianhang | journal=Physical Review D | volume=104 | issue=4 | at=id. 043527 | date=August 2021 | doi=10.1103/PhysRevD.104.043527 | arxiv=2011.13643 | bibcode=2021PhRvD.104d3527D }}</ref> The value of a redshift is often denoted by the letter {{math|''z''}}, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio {{math|1 + ''z''}} (which is greater than 1 for redshifts and less than 1 for blueshifts).

Examples of strong redshifting are a [[gamma ray]] perceived as an [[X-ray]], or initially visible light perceived as [[radio wave]]s. The initial heat from the [[Big Bang]] has redshifted far down to become the [[cosmic microwave background]]. Subtler redshifts are seen in the [[astronomical spectroscopy|spectroscopic]] observations of [[astronomical]] objects, and are used in terrestrial technologies such as [[Doppler radar]] and [[radar gun]]s.

Other physical processes exist that can lead to a shift in the frequency of electromagnetic radiation, including [[scattering]] and [[physical optics|optical effects]]; however, the resulting changes are distinguishable from (astronomical) redshift and are not generally referred to as such (see section on [[#Effects from physical optics or radiative transfer|physical optics and radiative transfer]]).