A property of waves when the distance between the observer and the source of the waves is increasing, and by extension a property of the source object itself (which assumes a standard position for the observer, namely Earth). The relative motion causes an increase in wavelength, or, another way of describing the same thing, a decrease in frequency, due to the Doppler effect.
It is called “red” shift because light at the longer wavelength end of the spectrum of visible light is red. Visible light that is redshifted moves through the spectrum in the direction of that end. (For emitters moving toward the observer there is a corresponding “blueshift;” blue, because light at the short wavelength end of the visible spectrum is violet.)
Redshift is a dimensionless ratio. Symbol, z.
where λ (lambda) is the observed wavelength of the radiation from an emitter moving away from the observer, and λo (lambda sub oh) is the wavelength of the same radiation measured in the laboratory with the observer stationary with respect to the emitter.
Astronomers make extensive use of the Doppler effect through spectroscopy, the measurement of the wavelengths of light. Atoms emit or absorb light at particular wavelengths which are characteristic of the isotope. A spectroscope creates a spectrum in which each of these wavelengths is visible as a line. A given isotope creates a pattern of lines which is recognizable even if it is shifted to a different part of the spectrum. measuring the blue shift of light from the side rotating toward us and the redshift of light from the side rotating away from us, astronomers are able to determine the speed of rotation of the galaxy.
However, the most important use of redshift, and the form usually encountered, is the cosmological redshift, which is caused by the expansion of the universe. It is a measure applied to distant objects based on the effect of the expansion of the universe on the wavelengths of light from them that reaches us.
The cosmological redshift becomes, by proxy, a measure of distance, since the greater the distance the greater the redshift.
v = c·ln(1 + z)
where c is the velocity of light and v is the velocity with which the object is receding from us.
|Velocity at which object is receding from us
as a percentage of the velocity of light
|3C273 (quasar)||mid 60s||0.158||480|
|3C48 (quasar)||mid 60's||0.367||1100|
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Last revised: 16 April 2004.