What is redshift and how do you measure it?
Redshift is a special astronomical case of a physical phenomenon called the Doppler effect (after Johann Doppler [1803-1853]). The Doppler effect occurs when a source sending out waves (either sound or light) is moving with respect to an observer. When the source is moving toward the observer, waves arrive earlier than they would in the stationary case and the wave peaks arrive closer together (the sound is higher pitch or the light is bluer). If the source is moving away from the observer, the waves get more stretched out (the sound is lower pitch or the light is redder). The Doppler effect on sound can be clearly heard when a siren or fast train is passing by. In astronomy, most galaxies are moving away from us because the Universe is expanding, so the light from the galaxies is redshifted. The farther the galaxy is away from us, the faster it is moving, and the larger the redshift. How redshift is connected to the distance of an object depends on the expansion rate of the Universe,
