Hubble's law states that the velocity, v, of a distant object such as a galaxy is proportional to its distance, d:
The proportionality constant, H, is called the Hubble constant. Hubble's Law was formulated to describe the measurements of galaxies Edwin Hubble made in the 1920s. These measurements were revolutionary because they provided concrete evidence that the universe is expanding, rather than static. Today we understand Hubble's Law as the result of the stretching of light as the universe expands rather than the motion of individual galaxies. This cosmological redshift can be expressed in the usual velocity units of kilometers per second, but its origin is in the overall expansion of the universe.