The dominant motion in the universe is the smooth expansion
known as Hubble's Law.
- Recessional Velocity = Hubble's constant times distance
- V = Ho D
- V is the observed velocity of the galaxy away from us, usually in
- 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:
About in the middle of the diagram, there are a bunch of
galaxies that appear to be at about the same distance but are spread
out a lot in the velocity direction. This feature suggests the presence
of a large cluster of galaxies, like the Virgo
cluster. In addition to their "Hubble velocities", these galaxies
have an extra velocity caused by their orbital motion around the center
of the cluster. 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.
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".