red shift

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red shift (z) Lengthening of the wavelength of light or other electromagnetic radiation from a source, caused either by the source moving away (the Doppler effect) or by the expansion of the universe (cosmological red shifts). It is defined as the change in the wavelength of a particular spectral line, divided by the rest wavelength of that line. The Doppler effect results from motion through space; cosmological red shifts are caused by the expansion of space itself stretching the wavelengths of light travelling towards Earth.