People are generally familiar with the DopplerEffect whereby waves emitted from a moving object appear compressed to an observer towards whom the object is moving, and expanded to an observer away from whom the object is moving.  The change in pitch of a train's whistle as the train approaches and then passes an observer is the classic example.  The DopplerEffect also holds true for light.  When a light source is moving towards an observer, the frequency of the light appears to increase.  For VisibleLight this makes the light appear to be bluer.  Likewise when a light source is receding from an observer the frequency of the light emitted appears to decrease.  For VisibleLight this makes the light appear to be redder.  Hence measuring a light source's RedShift is a measure of how quickly that light source is moving away from the observer.  The RedShift of distant objects is a crucial parameter for cosmology.

It's important to note that there is another cause of redshift in astronomy and cosmology that is caused by the expansion of space.  As light travels from a distant object, the expansion of the universe stretches out the wavelength of light.  The longer light travels, the more it gets stretched out.  It is this redshift that allows the determination of the distance to far off objects.

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See also: ExpansionOfTheUniverse, SpeedOfLight

CategoryPhysics