According to Hubble's Law, what is the recessional velocity ($v$) proportional to?

Answer

The distance ($d$) to the galaxy.

Hubble's Law states that a galaxy's recessional velocity ($v$) is directly proportional to its distance ($d$), often written as $v = H_0 d$.

According to Hubble's Law, what is the recessional velocity ($v$) proportional to?

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