Hubble's Law

Recessional Velocity = Hubble's constant times distance

V = H_o D

where

V is the observed velocity of the galaxy away from us, usually in km/sec

H is Hubble's "constant", in km/sec/Mpc

D is the distance to the galaxy in Mpc

In 1929, Hubble estimated the value of the expansion factor, now called the Hubble constant, to be about 500 km/sec/Mpc. Today the value is still rather uncertain, but is generally believed to be in the range of 45-90 km/sec/Mpc.

While in general galaxies follow the smooth expansion, the more distant ones moving faster away from us, other motions cause slight deviations from the line predicted by Hubble's Law. This diagram shows a typical plot of distance versus recessional velocity, with each point showing the relationship for an individual galaxy. In the example shown here, two things should be apparent:

Few of the points fall exactly on the line. This is because all galaxies have some additional residual motion in addition to the pure expansion. This is referred to as the "cosmic velocity dispersion" or "cosmic scatter" and is probably due to the fact that the gas clouds that formed the galaxies all had some small additional motion of their own. The recessional velocity of a galaxy at a particular distance inferred from Hubble's law is called the "Hubble velocity".