Expansion of the universe

In 1912, Vesto Slipher, discovered the redshift of light from distant galaxies. This was later to be interpreted as galaxies receding from Earth. 

In 1922, Alexander Friedmann, using the field equations of Albert Einstein, provided theoretical evidence for the expanding universe. 

In 1927, Georges Lemaitre, reached a conclusion similar to Friemann, however, on a theoretical basis. Lemaitre also proposed observational evidence for Hubble's law: The linear relationship between distance to galaxies and their recessional velocity. 

Lastly, in 1929, Edwin Hubble, confirmed Lemaitre's findings. 

If we assume the cosmological principle, that the spatial distribution of matter is homogeneous and isotropic, we find that galaxies are moving away from each other.