Redshift
A redshift is an apparent shift toward longer wavelengths of spectral lines in the radiation emitted by an object caused by the emitting object moving away from the observer. When the light an object emits is displaced toward the red end of the spectrum it is said to be redshifted. In general, photons of light that are emitted at a source at one energy and detected by an observer at a lower energy are redshifted.
Often, the redshift of an object can be measured by examining atomic absorption or emission lines in its spectrum. Redshifts can be caused by the motion of a source away from an observer. For distant objects, redshifts can be caused by the expansion of the Universe.
Redshift is part of Doppler effect that is present with other waves like sound waves.
