Wikipedia:Today's featured article/December 29, 2006

Redshift occurs when the visible light from an object is shifted towards the red end of the spectrum. This increase in wavelength corresponds to a decrease in the frequency of the electromagnetic radiation. 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 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 Doppler redshift phenomenon was first predicted and observed in the nineteenth century as scientists began to consider the dynamical implications of the wave-nature of light. Another redshift mechanism accounts for the famous observation that the spectral redshifts of distant galaxies, quasars, and intergalactic gas clouds are observed to increase proportionally with their distance to the observer. This relation is accounted for by models that predict the universe is expanding, seen in, for example, the Big Bang model. Yet a third type of redshift, the gravitational redshift also known as the Einstein effect, results from the time dilation that occurs in general relativity near massive objects. (more...)

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