Hubble's law is the relationship between the speed and the distance of galaxies and other remote objects; it shows that the universe is expanding.
In the 1920s, when Edwin Hubble (1889-1953) analysed the spectra of galaxies for the first time (using the 2.5 m Mount Wilson telescope), he discovered a surprising and unexpected phenomenon. He found that in the spectra of galaxies all the lines were systematically shifted towards the red; this phenomenon was given the name red shift.
Interpreted according to the laws of physics, red shift showed that all the galaxies are moving away from us at speeds that increase with distance according to the formula: c . z = H . d, where c is the speed of light, z is the spectral shift caused by the Doppler-Fizeau effect, d is the distance of the galaxy and H is the Hubble constant. It is at the origin of the theory of the expansion of the universe.