The motion of galaxies in the universe, in relation to each other,
appears to be a smooth recession away from each other.
The relative velocity of a galaxy to 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. Also, galaxies probably have some momentum of their own that they inherited
from the clouds that formed them. Nevertheless, the overall effect is a general
expansion of the Universe.
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
(historical note).
Recessional Velocity = Hubble's Constant times Distance
- V = Ho D
- V is the observed velocity of the galaxy away from us, usually in km/sec
- Ho is
Hubble's Constant,
in km/sec/Mpc
- D is the distance to the galaxy in
Mpc
ASTRO 101