Hubble expansion is the expansion of the universe, space itself expanding, which is currently thought to have occurred over the life of the universe. At the scale of galaxies, being bound by gravity, the expansion is resisted, but at very large scales, it is evident, i.e., the distance between galaxy clusters is growing. In the early 20th century, Edwin Hubble established that the luminosity distance (determined from variable stars of known luminosity) of galaxies is proportional to their redshift, the latter of which indicates a receding radial velocity, i.e., everything moving away from us (illustrated with his Hubble diagram). This result is now termed Hubble's law, which is thought to be approximately correct over the interval we've been able to observe, but the expansion is now thought to have increased over time, giving rise to the concept of dark energy, to provide a cause for such acceleration. The Hubble constant (H0) is the current rate of expansion and a reasonable approximation of the expansion for much of the age of the universe, and the Hubble parameter is the rate of expansion as a function of time, i.e., accommodating the acceleration. Actual determination of distance from redshift requires integration involving the Hubble parameter, and the mapping of the universe's expansion over time is of continuing interest. The apparent movement (increasing distances between distant objects) is termed the Hubble flow, to distinguish it from the peculiar velocity of the objects, e.g., the influence on their motion due to nearby massive objects.