After astronomers realised that the universe is expanding they introduced the Hubble constant as the measure of speed of this expansion as a ratio of assumed recession velocity of galaxies to their distance. The Hubble constant equals approximately, for <math>v\ll c</math>
where <math>v</math> is assumed speed of recession of a galaxy, <math>c</math>, speed of light in vacuum, combined with <math>v</math> producing "redshift" <math>Z=v/c</math>, and <math>r</math> is distance from the observer to the observed galaxy, and its exact value turned out to be
where <math>R_E</math> is Einstein's radius of universe. Present observations indicate that the Hubble constant is about 70km/s/Mpc.
The recessional velocity turned out not to be proportional to the distance though and so the Hubble constant is refered to the the nearest galaxies for which it may be considered to be relatively constant.
The Essay:Problems in Cosmology explains the possibility of the Hubble constant being a feature of universe that is stationary (neither expanding nor contracting) and the recessional velocity of galaxies being an illusion caused by the time runing slower at the deep space galaxies than in our Galaxy which mimics the accelerating expansion.
- Halton Arp, "Quasars, Redshifts and Controversies", 1987.