What is the cause of the redshifts of galaxies? Strictly speaking, this is still a question in dispute. One can, however, adopt the most obvious hypothesis namely, that they originate from the motion of the galaxies, speci cally dueto a motion of recession from the observer. That is, the physical phenomenon responsible for the observed spectral shifts is the so-called Doppler e ect. Redshifts are usually represented by the letter z. For z<0.1, the recession velocity is then given by v=cz, where c is the speed of light in vacuum
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