redshift

redshift (symbol z) The amount by which the wavelength of light from a receding object is lengthened (i.e. moved to the red) by the Doppler shift. Redshift is calculated from the formulaz = Δ λ / λ,where λ is the original wavelength (as measured in the laboratory) and Δ λ the observed change in wavelength. A redshift of 0.1, for example, means that the light has been redshifted by 10% in wavelength, whereas a redshift of 1 means a change of 100% (i.e. a doubling in wavelength). All galaxies at large distances from our own have redshifts resulting from the expansion of the Universe. At redshifts less than about 0.1, z is related to the velocity of the object, v, by the simple expression z = v/c, where c is the velocity of light. At larger redshifts, this relationship is no longer true, and it is better to think of the redshift as being caused by the expansion of space rather than a Doppler shift. While light is on its way to us from a distant object the Universe is expanding, and this expansion ‘stretches’ the wavelength of the light. For example, when the light left a galaxy with a redshift of 1 the Universe was only half its present size; during the time taken for the light to travel from the galaxy to our telescopes the Universe has doubled in size, causing the wavelength of the light to increase by the same factor.

A galaxy's redshift can be measured quite easily from its spectrum, unlike its distance, which is very hard to measure directly. At small redshifts, astronomers can calculate the velocity of the galaxy from its redshift and then use Hubble's law, which relates velocity and distance, to estimate the distance of the galaxy. At larger redshifts, the expansion of the Universe means that distance becomes an ambiguous quantity, but astronomers can use a galaxy's redshift to estimate the age of the Universe at the time the light was emitted, which is the most important method they have for investigating the history of the Universe (see galaxy evolution). The largest redshifts currently known are over 6, meaning that the light has been shifted in wavelength by 600% so that ultraviolet lines appear in the red part of the spectrum. See also gravitational redshift; Redshift Survey.