Redshift

Redshift is an increase in the wavelength of light caused by the movement of the light source which is moving away from the observer. High speeds produce a large redshift, making it possible for the radial velocity of the source to be measured. In an expanding universe, galaxies with a high redshift are at greater distances than those with a low redshift. A redshift can also be caused by the source being immersed in a high gravitational field such as that of a black hole.