The law that governs the expansion of the Universe. According to the law, the apparent recession velocity of galaxies is proportional to their distance from the observer. In mathematical terms, v = H0 r, where v is the velocity, r the distance, and H0 the Hubble constant. The law was put forward in 1929 by E. P. Hubble. It is also known as the Hubble–Lemaître law to recognize the contribution of G. E. Lemaître. See also hubble diagram.