Hubble's law was found first by a Belgian priest who edited himself out
Two years before Edwin Hubble's famous 1929 paper, Georges Lemaître, a Belgian priest and astronomer, showed that distant galaxies recede at speeds proportional to their distance. He published in French, in an obscure journal. When the paper was translated into English in 1931, the key equation was dropped, and the change was made by Lemaître himself.
The rule is simple: the farther away a galaxy lies, the faster it moves off. Speed is inferred from redshift, the stretching of a galaxy's light toward longer wavelengths, and distance from how bright certain objects appear. Divide one by the other and you get the Hubble constant, now around 70 kilometres per second for every megaparsec of distance, a unit of roughly 31 million million million kilometres. Put another way, space is growing by roughly 7 percent every billion years.
Many people contributed. Vesto Slipher measured the first redshift of a spiral nebula in 1912 and found nearly all were receding, without grasping why. Alexander Friedmann showed in 1922 that Einstein's equations allowed an expanding universe, and Carl Wirtz noticed that smaller, dimmer galaxies had larger redshifts. Hubble, working at Mount Wilson with the most powerful telescope of the day, used Cepheid variable stars and Henrietta Swan Leavitt's method to prove the spiral nebulae were far outside the Milky Way. Plotting 46 galaxies, he got a constant of 500, badly inflated by errors in his distances.
The discovery upset Einstein, who had added a cosmological constant to his equations purely to keep the universe static. He later called the static assumption his greatest mistake, and in 1931 travelled to Mount Wilson to thank Hubble. Ironically, the constant has returned in recent decades as a possible explanation for dark energy.
The name misleads in another way too. The Hubble constant is the same everywhere at a given moment but changes over cosmic time. Beyond a boundary called the Hubble sphere, galaxies recede faster than light. Current evidence also suggests the expansion is speeding up. Fittingly, the rule's official name is now the Hubble–Lemaître law.
Source: Hubble's law
