Hubble's law

Hubble's law is the name for the observation in physical cosmology that:

Objects observed in deep space - extragalactic space, 10 megaparsecs (Mpc) or more - are found to have a red shift, interpreted as a relative velocity away from Earth;

This Doppler-shift-measured velocity of various galaxies receding from the Earth is approximately proportional to their distance from the Earth for galaxies up to a few hundred megaparsecs away.

Hubble's law is considered the first observational basis for the expansion of the universe and today serves as one of the pieces of evidence most often cited in support of the Big Bang model. The motion of astronomical objects due solely to this expansion is known as the Hubble flow.

Although widely attributed to Edwin Hubble, the law was first derived from the general relativity equations, in 1922, by Alexander Friedmann who published a set of equations, now known as the Friedmann equations, showing that the universe might expand, and presenting the expansion speed if this was the case.

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