Spanel Planetarium

Hubble's Law

V = Ho d

Edwin Hubble 1889-1953
Edwin Hubble 1889-1953
The motion of galaxies in the universe appears to be a smooth recession away from each other. Also, the velocity of a galaxy away from any observer is proportional to the distance from that observer.

Some galaxies that are in close proximity to each other, such as Andromeda and the Milky Way, are actually moving towards one another because gravity at a close distance is a greater factor than the recession. So the galaxies are in groups bound by gravity, and all of those groups other than our own are moving away from us. Galaxies also have some momentum of their own that they inherited from the clouds that formed them (the cosmic scatter). Nevertheless, the overall effect is a general expansion of the Universe as a whole.

An observational effect of the expansion is that the farther a galaxy is from you, the faster it appears to be receding. This creates a relationship between the recessional velocity and distance. This phenomenon is known as Hubble's Law.

            Velocity = Hubble's constant multiplied by distance

            V is the observed velocity of the galaxy away from us (kilometers/second).

            Ho is Hubble's Constant, the rate of expansion (kilometers/second/Megaparsec).

            d is the distance to the galaxy (Megaparsecs).

Current measurements give an estimate for Hubble's constant of 73.8 km/sec/Mpc.