Hubble's constant is the constant of proportionality in the relation between the velocities of remote galaxies and their distances. It expresses the rate at which the universe is expanding. It is denoted by the symbol H and named in honour of Edwin Hubble, the American astronomer who attempted in 1929 to measure its value. With redshifts of distant galaxies measured by Vesto Slipher, also of the United States, and with his own distance estimates of these galaxies, Hubble established the cosmological velocity-distance law.
According to this law, known as the Hubble law, the greater the distance of a galaxy, the faster it recedes. Derived from theoretical considerations and confirmed by observations, the velocity-distance law has made secure the concept of an expanding universe. Hubble's original value for H was 150 km (93 miles) per second per 1,000,000 light-years. Modern estimates, using more precise distance measurements, place the value of H at between 15 and 30 km (9.3 and 18.6 miles) per second per 1,000,000 light-years. The reciprocal of Hubble's constant lies between 10 billion and 20 billion years, and this cosmic time scale serves as an approximate measure of the age of the universe.
Excerpt from the Encyclopedia Britannica without permission.