Red+Shift

In physics and astronomy, **redshift** occurs when electromagnetic radiation—usually visible light—emitted or reflected by an object is shifted towards the less energetic ("red") end of the electromagnetic spectrum due to the Doppler effect or other gravitationally-induced effects. 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 drop 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 visible light (e.g., infrared, microwaves, and radio waves), redshifts shift the radiation //away// from the red wavelengths. An observed redshift due to the Doppler effect occurs whenever a light source moves away from the observer, corresponding to the Doppler shift that changes the perceived 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. A special relativistic redshift formula (and its Newtonian approximation) is used when spacetime is flat. Where gravitational effects are important, redshift must be calculated using general relativity. Two important special-case formulae are the so-called gravitational redshift formula which applies to any stationary (that is, unchanging with time) gravitational field, and the cosmological redshift formula which applies to the expanding universe of Big Bang cosmology. Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. There exist other physical processes that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects; however, the resulting changes are distinguishable from true redshift and not generally referred as such. (See section on physical optics and radiative transfer).