Hubble Constant
H = \frac{v}{d}, the proportionality constant relating the recession velocity of a galaxy to its distance, crucial for measuring the expansion rate.
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The statement of the theorem
Define the Hubble parameter as the proportionality constant relating the recession velocity of a galaxy to its distance via Hubble's Law:\n \nWhere is the recession velocity, and is the scale factor of the universe.
Source: Wikipedia