Portal:Physics/Selected article/May 2007

In physics and astronomy, redshift occurs when the electromagnetic radiation, usually visible light, that is emitted from or reflected off of an object is shifted towards the red end of the electromagnetic spectrum. More generally, redshift is defined as an increase in the wavelength of electromagnetic radiation received by a detector compared with the wavelength emitted by the source. This increase in wavelength corresponds to a decrease in the frequency of the electromagnetic radiation. Conversely, a decrease in wavelength is called blue shift.

A redshift can occur when a light source moves away from an observer, corresponding to the Doppler shift that changes the frequency of sound waves. Although observing such redshifts, or complementary blue shifts, has several terrestrial applications (e.g., Doppler radar and radar guns), spectroscopic astrophysics uses Doppler redshifts to determine the movement of distant astronomical objects. This phenomenon was first predicted and observed in the 19th century as scientists began to consider the dynamical implications of the wave-nature of light.

Another redshift mechanism is the expansion of the universe, which explains the famous observation that the spectral redshifts of distant galaxies, quasars, and intergalactic gas clouds increase in proportion to their distance from the observer. This mechanism is a key feature of the Big Bang model of physical cosmology. Yet a third type of redshift, the gravitational redshift (also known as the Einstein effect), is a result of the time dilation that occurs near massive objects, according to general relativity.

Image: Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared with that of the Sun (left). Wavelength increases up towards the red and beyond (frequency decreases).