hubbles-constant

Hubble's constant

noun Astronomy.
the ratio of the recessional velocity of galaxies to their distance from the sun, with current measurements of its value ranging from 50 to 100 km/sec per megaparsec.

Origin:
1950–55; after E. P. Hubble

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WordNet
hubble's constant

noun
(cosmology) the ratio of the speed of recession of a galaxy (due to the expansion of the universe) to its distance from the observer; the Hubble constant is not actually a constant, but is regarded as measuring the expansion rate today 
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