Redshift

In <a href="/facts/Physics/a7aH2y08">physics</a>, a redshift is an increase in the <a href="/facts/Wavelength/kcJU0ymz">wavelength</a>, and corresponding decrease in the <a href="/facts/Frequency/QUSbPaW8">frequency</a> and <a href="/facts/Photon_energy/Gy4zEAaE">photon energy</a>, of <a href="/facts/Electromagnetic_radiation/Ynx7ujCW">electromagnetic radiation</a> (such as <a href="/facts/Light/jeofpMn4">light</a>). The opposite change, a decrease in wavelength and increase in frequency and energy, is known as a blueshift. The terms derive from the colours <a href="/facts/Red/ETkA7QeX">red</a> and <a href="/facts/Blue/DHWb8MUa">blue</a> which form the extremes of the <a href="/facts/Visible_spectrum/frPBh8o5">visible light spectrum</a>. Three forms of redshift occur in <a href="/facts/Astronomy/0ArJVM1p">astronomy</a> and <a href="/facts/Cosmology/yM19zM7Y">cosmology</a>: <a href="/facts/Doppler_effect/bzrK1BKf">Doppler</a> redshifts due to the relative motions of radiation sources, <a href="/facts/Gravitational_redshift/cEFgB3Ns">gravitational redshift</a> as radiation escapes from gravitational potentials, and <a href="/facts/Cosmological_redshift/FJYfELCP">cosmological redshifts</a> of all light sources proportional to their distances from Earth, a fact known as <a href="/facts/Hubble%2527s_law/FJYfELCP">Hubble's law</a> that implies the <a href="/facts/Expansion_of_the_universe/f6Uvt79B">universe is expanding</a>.
All redshifts can be understood under the umbrella of <a href="/facts/Frame_of_reference/15yYfB9M">frame transformation laws</a>. <a href="/facts/Gravitational_wave/4XWN3NtG">Gravitational waves</a>, which also travel at <a href="/facts/Speed_of_light/sKSQwdNx">the speed of light</a>, are subject to the same redshift phenomena. The value of a redshift is often denoted by the letter z, corresponding to the fractional change in wavelength (positive for redshifts, negative for blueshifts), and by the wavelength ratio 1 + z (which is greater than 1 for redshifts and less than 1 for blueshifts).
Examples of strong redshifting are a <a href="/facts/Gamma_ray/5Drf913J">gamma ray</a> perceived as an <a href="/facts/X-ray/wseP79cN">X-ray</a>, or initially visible light perceived as <a href="/facts/Radio_wave/LzrVhgbn">radio waves</a>. The initial heat from the <a href="/facts/Big_Bang/I5T1vTnv">Big Bang</a> has redshifted far down to become the <a href="/facts/Cosmic_microwave_background/ThB2HkLS">cosmic microwave background</a>. Subtler redshifts are seen in the <a href="/facts/Astronomical_spectroscopy/NDGQ7mmM">spectroscopic</a> observations of <a href="/facts/Astronomical/0ArJVM1p">astronomical</a> objects, and are used in terrestrial technologies such as <a href="/facts/Doppler_radar/4a6nL2lA">Doppler radar</a> and <a href="/facts/Radar_gun/UNB925gj">radar guns</a>.
Other physical processes exist that can lead to a shift in the frequency of electromagnetic radiation, including <a href="/facts/Scattering/nMou7lEH">scattering</a> and <a href="/facts/Physical_optics/Al5EFMyy">optical effects</a>; however, the resulting changes are distinguishable from (astronomical) redshift and are not generally referred to as such (see section on physical optics and radiative transfer).

Redshift open-in-new

Redshift