A redshift is a shift in the frequency of an electromagnetic wave caused by an object’s motion. Light from objects moving away from an observer have their light waves shifted toward the red part of the spectrum. Redshift is commonly observed in astronomy, particularly in the observation of very distant objects. The effect is not limited to electromagnetic radiation in the visible range, though the term caught on because some receding astronomical objects appeared red.
Redshift is the result of the Doppler effect. The Doppler effect applies to sound waves as well as electromagnetic waves and is often experienced by humans on a daily basis. The horn of an approaching train sounds higher in pitch than when the train is moving away, even though the horn itself produces a sound of constant pitch. This is because sound travels at a uniform speed in a given medium—it is the frequency of the sound wave that changes based on movement of the train. A similar effect occurs with light, with a lower frequency light resulting from a source that is moving away in space.
Within the visible spectrum of light, low-frequency light waves are sensed by humans as being red. High-frequency light waves are seen as blue. Therefore, a redshift will result from light whose source is moving away from an observer. A galaxy, for example, that is moving away from Earth at high velocity may appear red in color. Likewise, an approaching galaxy could look blue if its velocity was within a certain range.
Though the term redshift implies a color change, the Doppler effect applies to the entire electromagnetic spectrum. All radiation, of which visible light is one type, is shifted based on the relative velocity of the source of radiation. An astronomical object that is moving away with sufficient velocity may "redshift" out of the entire visible spectrum—even past red. The resulting radiation received by an observer would be in the range of infrared radiation, which is invisible to the naked eye. Thus, astronomers use the term redshift to indicate any shift in radiation towards lower frequencies.
In the 1920s, American astronomer Edwin Hubble and others observed that most galaxies appeared to be redshifted, with the amount of redshift proportional to their distances from Earth. The further away galaxies were, the faster they seemed to be moving away from Earth. This trend is called Hubble's law, and it provided some of the first evidence supporting an expanding universe model that originated from a Big Bang. In an explosion, particles of varying speeds are all increasing their distances from all other particles. The same holds true in an "exploding" universe—all galaxies would appear to be moving away from any observer.