The redshift of a given object is a measure of the amount that the wavelength of its emitted or reflected electromagnetic radiation has increased. This increase in wavelength is due to the relative motion of a source with respect to an observer; a light emitting source moving away from an observer will have its spectrum shifted toward longer (in optical light, redder) wavelengths. In practice this is analogous to the Doppler shifting of sound waves emitted from a moving source. Light emitted from a source moving towards an observer will experience a blueshift. In the case of redshift, however, most of the redshift is believed to be due not to the object moving through space, but to the expansion of space itself.
Support for the Big Bang theory
The Hubble redshift in absorption and emission lines in the spectra of distant cosmic objects, discovered by Edwin Hubble in 1929, is claimed as a source of evidence for the Big Bang theory. The redshift, the expanded wavelength of light, suggests that these objects are moving away from Earth due to the expansion of the universe. It also suggests that the universe is undergoing accelerating expansion for an unknown reason.
Actually the reason is known and confirmed by Supernova Cosmology Project team in 1998 with accuracy to one standard deviation (the second term of Taylor series of Hubble "constant" as function of time is causing the illusion of accelerating expansion of space with
where is apparent acceleration of expansion of space,
is Hubble constant, is speed of light in vacuum, and is "Einstein radius", or radius of curvature of space, about 13 billion light years).
- Edwin Hubble (1929). "A relation between distance and radial velocity among extra-galactic nebulae". Proceedings of the National Academy of Sciences 15: 168–173.