Redshift
In physics, a redshift is an increase in the wavelength, or equivalently, a 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. The terms derive from the colours red and blue which form the extremes of the visible light spectrum.
Three forms of redshift occur in astronomy and cosmology: Doppler redshifts due to the relative motions of radiation sources, gravitational redshift as radiation escapes from gravitational potentials, and cosmological redshifts caused by the universe expanding.
In astronomy, 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). Automated astronomical redshift surveys are an important tool for learning about the large scale structure of the universe.
Examples of strong redshifting are a gamma ray perceived as an X-ray, or initially visible light perceived as radio waves. The initial heat from the Big Bang has redshifted far down to become the cosmic microwave background. Subtler redshifts are seen in the spectroscopic observations of astronomical objects, and are used in terrestrial technologies such as Doppler radar and radar guns.
Gravitational waves, which also travel at the speed of light, are subject to the same redshift phenomena.
Other physical processes exist that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; 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).
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