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Hubble constantSymbol: H 0. The rate at which the expansion velocity of the Universe changes with distance; it is commonly measured in km s–1 megaparsec–1. Current estimates place H 0 between 60 and 75 km s–1 Mpc–1.
The value of H 0 is derived from the ratio of recession velocity to distance for galaxies beyond the Local Group. The velocity can be measured accurately from the redshift in the galaxy's spectrum. It must be corrected for the Sun's motion (see galactic rotation) and the Virgocentric flow. The main source of dispute over the value of H 0 comes from the uncertainty of the distances to far-flung galaxies. Refinements in distance measurements have reduced the value since Hubble first determined it (as about 500 km s–1 Mpc–1). Uncertainties in large extragalactic distances by a factor of two still leave H 0 in doubt by the same factor (see distance determination).
The inverse of H 0 has the dimensions of time and is a measure of the age of an open Universe – the Hubble time (see age of the Universe). In an evolving Universe the Hubble ‘constant’ actually changes with time at a rate dependent upon the deceleration parameter, q 0. It is independent of time only in a steady-state Universe where q 0 equals –1. In terms of the cosmic scale factor, R ,
See steady-state theory.