Red Shift

Red Shift, shift towards longer wavelengths observed in the lines of spectra of celestial objects. The American astronomer Edwin Powell Hubble, in 1929, linked the red shift observed in spectra of galaxies to the expansion of the universe. Hubble suggested that this red shift, called the cosmological red shift, is caused by the Doppler effect and hence indicates the speed of recession of the galaxies—and, by using Hubble's law, the distances of the galaxies.

A second mechanism for the red shift is the gravitational red shift, also called the Einstein shift. It was predicted by Albert Einstein's general relativity theory, according to which periodic processes are slowed down in a gravitational field. The Einstein shift is noticeable in the spectra of compact massive stars, such as white dwarfs.

The large red shifts observed in quasars are generally believed to be cosmological. Some scientists, however, believe that the red shifts in quasars are caused by the Einstein shift or by another unknown mechanism.