Hubble's constant: Definition from Answers.com

Constant used to relate the velocities of remote galaxies to their distances from Earth. Denoted H₀ 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 H₀, 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 = H₀ ´ 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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	Hubble's law (in astronomy)
	Doppler Effect
	List of scientific constants named after people

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