Hubble's law

Hubble's law:
In the 1930's, Edwin Hubble discoveried that all galaxies have a positive redshift. In other words,
all galaxies were receding from the Milky Way. By the Copernican principle (we are not at a
special place in the Universe), we deduce that all galaxies are receding from each other, or we
live in a dynamic, expanding Universe.

The expansion of the Universe is described by a very simple equation called Hubble's law; the
velocity of the recession of a galaxy is equal to a constant times its distance (v=Hd). Where the
constant is called Hubble's constant and relates distance to velocity in units of light-years.