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, and the further the astronomical object, the greater the speed, as illustrated with his Hubble diagram. Given the increased speed with distance, this implies everything is spreading out, and an observer in some other galaxy would see everything moving away from them as well. 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) reflects the current rate of expansion, which is a reasonable approximation of the expansion for much of the age of the universe, and the Hubble parameter reflects 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 the course of 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 (the local flow).