Most astronomical observations made to date support big bang theories. According to these theories, the universe is still expanding. Two kinds of observations strongly support the idea of an expanding universe. These observations indicate that all galaxies are moving away from one another and that the galaxies farthest from the Milky Way are moving away most rapidly. The law was named after American astronomer Edwin P. Hubble, who reported it in 1929.
Astronomers estimate the speed at which a galaxy is moving away by measuring the galaxy's red shift.
The red shift is an apparent lengthening of electromagnetic waves emitted by an object moving away from the observer. A red shift can be measured when light from a galaxy is broken up and spread out into a band of colors called a spectrum. The spectrum of a galaxy contains bright and dark lines that are determined by the galaxy's temperature, density, and chemical composition. These lines are shifted toward the red end of the spectrum if the galaxy is moving away. The greater the amount of red shift, the more rapid the movement.