Redshift

Definition

The redshift is a quantity, generally noted z, that measures the Doppler effect of a celestial object which is going away from the observer, thus whose light exhibits an apparent shift toward a longer wavelength. The term is especially suited for distant objects in the expanding universe (galaxies, quasars, clusters of galaxies), since it supposes that the change of wavelength is positive (the apparent longer wavelength makes the object redder than it is in its restframe). The term blueshift is sometimes used for the reverse situation of an object which is approaching the observer. The redshift gives a direct information on the recession velocity v of the object. For a velocity which is much lower than the speed of light c, the Galilean approximation z = v/c is valid. For velocity approaching the speed of light, a relativistic formulation must be used. Note that there is also a redshift, i.e., a positive change in wavelength, that could be due to a very strong gravitational...