Red shift is an apparent increase of the wavelength of light toward the red end of the electromagnetic spectrum caused by relative motion between the light-emitting object and an observer. The red shift of light occurs due to relative velocities of source and observer, cosmological expansion, and gravity. It is an instance of the Doppler effect, which is commonly observed with sound waves. Waves emitted by a moving object will be blue shifted (compressed) if approaching an observer and red shifted (elongated) if receding. This occurs with both sound and light, but the Doppler effect in light is different than that of sound because light waves do not need a medium to travel in.

As a source of waves speeds away from an observer, the crest of each wave spread from each other; that is, the waves arrive with longer wavelengths. In light, longer wavelengths are...