redshift

Redshift is the effect in which light from a celestial object appears at longer wavelengths than it should, shifted toward the red end of the spectrum. For nearby stars this shift is usually explained by the Doppler effect, that is, the source's motion through space. The cosmological redshift seen in distant galaxies is instead caused by the expansion of space between galaxies stretching the wavelength of light, and it is the main evidence that the universe is expanding.