Redshift

In astronomy, when matter moves away from an observation point, its light spectrum displays a redshift. A redshift is one type of Doppler effect. Named for Austrian physicist Christian Johann Doppler (1803–1853), this principle states that if a light (or sound) source is moving away from a given point, its wavelengths (distance between two peaks of a wave) will be lengthened. Conversely, if an object emitting light or sound is moving toward that point, its wavelengths will be shortened.

With light, longer wavelengths stretch to the red end of the color spectrum while shorter wavelengths bunch up at the blue end. The shortening of wavelengths of an approaching object is called a blueshift.

The first astronomer to observe a space object's Doppler shift was American astronomer Vesto Melvin Slipher (1875–1969) in 1912. His subject was the Andromeda galaxy, which was then believed to be a nebula, or a cloud of dust and gas (at that time it was not known there were other galaxies beyond the Milky Way). Slipher discovered that the spectrum of Andromeda was shifted toward the blue end, meaning that it was approaching Earth.

Two years later, Slipher analyzed the spectra of fourteen other spiral nebula and found that only two were blueshifted, while twelve were redshifted. The redshifts he observed for some spirals implied they were moving at enormous speeds.

Words to Know

Blueshift: The Doppler shift observed when a celestial object is moving closer to Earth.

Doppler effect: The change in wavelength and frequency (number of vibrations per second) of either light or sound as the source is moving either towards or away from the observer.

Redshift: The Doppler shift observed when a celestial object is moving farther away from Earth.

Spectrum: Range of individual wavelengths of radiation produced when light is broken down into its component colors.

Speed of light: Speed at which light travels in a vacuum: approximately 186,000 miles (299,000 kilometers) per second.

Wavelength: The distance between two peaks in any wave.

Hubble and the expanding universe

An extremely important finding relating to redshifts was made in 1929 by Edwin Hubble (1889–1953), the American astronomer who first proved the existence of other galaxies. Together with his colleague Milton Humason, Hubble photographed distant galaxies and discovered that their spectra were all shifted toward the red wavelengths of light. Further study showed a relationship between the degree of redshift and that object's distance from Earth. In other words, the greater an object's redshift, the more distant it is and the faster it is moving away from Earth.

The large degree of redshift in the spectra of these galaxies suggested that they were moving away from Earth at a phenomenal rate. Humason found some galaxies moving at one-seventh the speed of light.

Hubble and Humason's research on redshifts led to two important conclusions: every galaxy is moving away from every other galaxy and, therefore, the universe is expanding.

[See also Binary star; Doppler effect; Electromagnetic spectrum; Galaxy; Star ]

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.