Hubble constant; H0 (E.P. Hubble; 1925):
The constant that determines the relationship among the distance to a galaxy and its velocity of recession because of the expansion of the Universe. As the Universe is self-gravitating, it is not constant truly. In cosmology, it is described as
H = (da/dt)/a,
where a refer to the 4-radius of the Universe. While evaluated for the present, it is written
H0 == H(t = now).
The Hubble constant is not known to great correctness (only about a factor of 2), however is supposed to lie somewhere among 50 and 100 km/s/Mpc.