Glossary · Thiên văn học
Redshift
The observation of light from an object at a longer wavelength than it had when emitted, shifted toward the red end of the spectrum. For distant galaxies, the main cause is the expansion of the universe stretching the light's wavelength.
Redshift can have more than one cause. When a light source moves away from an observer, its spectral lines appear at longer wavelengths (the Doppler effect); when it moves closer, the lines shift to shorter wavelengths, which is called blueshift. The redshift of distant galaxies is not strictly a Doppler effect. It comes mainly from the expansion of space itself stretching light as it travels, and this part is called cosmological redshift.
Redshift is usually written as z: the observed wavelength minus the emitted wavelength, divided by the emitted wavelength. Edwin Hubble found that more distant galaxies tend to have more strongly redshifted spectra, so for very distant galaxies redshift serves as a primary indicator of distance.
Because light takes time to reach us, the higher an object's redshift, the further back in time we are seeing it.