Using measurements of redshifts, Hubble proposed in 1929 that almost all galaxies were moving away from the Milky Way. He postulated that recessional velocity was directly proportional to distance r,
where H is a constant known as the Hubble constant. It is given by
where a is the expansion parameter. In the Friedmann-Lemaître cosmological model,
Hubble's Law is an immediate consequence of assuming homogeneity and isotropy of the universe. Eric Weisstein's World of Astronomy However, the "roughly linear relation" claimed by Hubble shows the galactic recessional velocities to be almost uncorrelated with distance! By 1931, better data showed a linear slope was actually present and allowed its slope to be estimated (Weinberg 1988, p. 26).
References
Weinberg, S. The First Three Minutes: A Modern View of the Origin of the Universe, upd. ed. New York: Basic Books, 1988.
