# Why does the Hubble constant have a subscript zero?

The title says it all. Why is the Hubble constant written $H_0$ and not just $H$? What is the purpose of that subscript $_0$?

The Hubble parameter is defined as $\frac{\dot{a}}{a}$, where $a$ is the scale factor in the Friedmann-Robertson-Walker metric: $$g_{\mu\nu} \, \mathrm{d}x^{\mu} \, \mathrm{d}x^{\nu}=c^2 \, \mathrm{d}t^2-a^2 \left(\frac{\mathrm{d}r^2}{1-kr^2}+r^2\mathrm{d}\Omega ^2 \right) \,.$$
When you put this metric into Einstein Field Equation, you will get an equation that describes how $a$ evolves with time $t$. Thus in general the Hubble parameter is a function of time.
$H_0$ is used to indicate the actual value of the Hubble parameter, measured at our time.
The value of the Hubble parameter at the present epoch is represented with $H_0$.