Constant used to relate the velocities of remote galaxies to their distances from Earth. Denoted &math.H;0 and named in honour of Edwin Hubble, it expresses the rate of expansion of the universe.
Its actual value has been debated for decades; improved measurements have narrowed the range in which it falls to about 13.3–14.5 miles/second (21.5–23.4 km/second) per million light-years. The reciprocal of &math.H;0, which is between 13 billion and 14 billion years, gives the time since the galaxies began to separate from each other—i.e., the approximate age of the universe. Hubble used the redshifts of distant galaxies measured by Vesto Slipher (1875–1969) and his own distance estimates of those galaxies to establish the cosmological velocity-distance law (Hubble law): velocity = &math.H;0 × distance, according to which the greater a galaxy's distance, the faster it is receding. Derived from theoretical considerations and confirmed by observations, the Hubble law has made secure the concept of an expanding universe.
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