We learned before about the Doppler shift, that is the change in the wavelength of a spectral line due to the motion of its source. When the source is moving with a very high velocity, the exact expression for the Doppler shift becomes more complicated. We call this the relativistic Doppler formula.
Since galaxies are moving away from us, their doppler shifts are always positive, that is, the wavelengths of the spectral lines are redshifted. Astronomers use the expression redshift, denoted usually by z, to indicate the magnitude of the doppler shift.
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