The constant H giving the rate of recession of distant astronomical objects per unit distance away. The fact that more distant objects are receding more rapidly than closer ones is interpreted as implying expansion of the universe, and is the main observation which led to the Big Bang theory. The Hubble constant changes as a function of time depending on the precise cosmological models as the expansion of the universe slows due to gravitational attraction of the matter within it. Most models give an age of the universe of order (which does indeed have units of time).
The current value of the Hubble constant, denoted , is hotly debated, with two opposing camps generally getting values near the high and low ends of 50 and 100 km s^-1/Mpc(where Mpc is a megaparsec, equal to 10⁶ parsecs ). Using infrared observations of Cepheid variables, Madore (1992) obtained km s^-1/Mpc. Using HST observations of Cepheid variables in the Coma cluster of galaxies, van den Bergh (1995) obtained km s^-1/Mpc. Sandage et al. (1996) hold out for a small value of 57 ± 4 km s^-1/Mpc using supernovae.