In 1929 American astronomer, astrophysicist and cosmologist Edwin Hubble of the Mount Wilson Observatory near Pasadena, California published "A Relation Between Distance and Radial Velocity among Extra-Galactic Nebulae," Proceedings National Academy of Sciences, 15 (1929) 168-173.
This was Hubble’s first paper on his discovery that the degree of redshift observed in light coming from a galaxy increased in proportion to the distance of that galaxy from the Milky Way. This became known as Hubble's law on the proportionality of distance and radial velocity of galaxies, indicating an expanding universe.
“Though only six pages in length, Hubble’s first paper on the velocity-distance relation represented a giant step in modern cosmology. . . . In place of a static picture of the cosmos, it seemed to many that the universe must be regarded as expanding, the rate of the mutual recession of its parts increasing with their relative distance” (Christianson, Edwin Hubble, Mariner of the Nebulae, 191, 188-192).
It has been said that Hubble’s discovery made as great a change in man’s conception of the universe as the Copernican revolution 400 years before.