Redshift is a shift in the frequency of electromagnetic waves caused by the movement of an object. This effect is observed in astronomy, and is the result of the Doppler effect, which applies to sound and electromagnetic waves. Redshift can result in light appearing red if the source is moving away from an observer. The term redshift applies to the entire electromagnetic spectrum, and is used to mean any shift of radiation towards lower frequencies. Hubble’s law provided evidence for an expanding universe that originated in a Big Bang.
A redshift is a shift in the frequency of an electromagnetic wave caused by the movement of an object. Light from objects moving away from an observer have their light waves shifted towards 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, although 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 and electromagnetic waves and is often experienced by humans on a daily basis. The horn of an approaching train has a higher 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 according to the movement of the train. A similar effect occurs with light, with lower frequency light resulting from a source receding into space.
Within the visible spectrum of light, low-frequency light waves are perceived by humans as red. High frequency light waves are seen as blue. Thus, a redshift will result from light whose source is moving away from an observer. For example, a galaxy that is moving away from Earth at high speed may appear red. Similarly, an approaching galaxy might look blue if its velocity is within a certain range.
Although the term redshift implies a change in color, the Doppler effect applies to the entire electromagnetic spectrum. All radiation, of which visible light is one type, is moved according to the relative velocity of the radiation source. An astronomical object that is receding with sufficient velocity can “redshift” from the entire visible spectrum, even beyond red. The resulting radiation received by an observer would be in the range of infrared radiation, invisible to the naked eye. Therefore, astronomers use the term redshift to mean any shift of 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 the galaxies were, the faster they seemed to be moving away from the Earth. This trend is called Hubble’s law, and it provided some of the first evidence to support a model of an expanding universe that originated in a Big Bang. In an explosion, particles of varying speed all increase their distances from all other particles. The same is true in an “exploding” universe: all the galaxies would appear to be moving away from any observer.
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