The Universe is expanding. Other galaxies are moving away from us, and the speed of their recession is generally proportion to their distance from us. This relationship between distance and velocity is known as Hubble's Law.
- Recessional Velocity = Hubble's constant times distance
- V = H o D
- 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. He made large errors, underestimating the size of the Universe and the distance to the nearest galaxies. 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 deviations from the line predicted by Hubble's Law. The diagram shows a typical plot of distance versus recessional velocity, with each point showing the relationship for an individual galaxy. About in the middle of the diagram, there are a bunch of galaxies that appear to be at about the same distance. They may be gravitationally bound, orbiting a common centre. Some within this cluster will be moving towards us relative to the average velocity with the cluster and some moving away. Because clusters of galaxies are very massive, this orbital velocity can be very large, more than 1000 km/s. Therefore in the vicinity of nearby clusters of galaxies, we cannot use Hubble's law to determine accurately the distance to the galaxy.