What is red shift?

Red shift is the lengthening ('stretching') of the wavelength of electromagnetic radiation emitted from a body moving away from the observer. This process occurs as the EM radiation loses energy while it passes through space, thereby decreasing the frequency of the wave, and making its wavelengths longer. Longer wavelengths correspond to a shift towards the red side of the visible light spectrum (or indeed further in that direction past the visible light portion). This is analogous to the Doppler Effect (commonly found and easily observable in sound waves). Red shift occurs when the source of the EM radiation is moving away from the observer. If the source is moving towards the observer, the mechanism operates in reverse (causing blue shift). The existence and prevalence of red shift is an important piece of evidence for the big bang, as it shows that most galaxies in space are moving away from each other, and therefore that they must have been closer in the past.