Hubble's law
Although widely attributed to Edwin Hubble, the law was first derived from the General Relativity equations by Georges Lemaître in a 1927 article. There he proposed that the Universe is expanding, and suggested a value for the rate of expansion, now called the Hubble constant. Two years later Edwin Hubble confirmed the existence of that law and got a more accurate value for the constant that now bears his name. The recession velocity of the objects was inferred from their redshifts, as measured earlier by Vesto Slipher in 1917 and related to velocity by him.