= Hubble parameter
{c}
{title2=$H(t)=\dot a(t)/a(t)$}
{wiki}
The <Hubble parameter> is the instantaneous fractional rate of <cosmic expansion>:
$$
H=\frac{\dot a}{a},
$$
where the dot is a <derivative> with respect to <cosmic time>. Thus a short interval $\Delta t$ changes any fixed-comoving <proper distance in cosmology> by the fraction $\Delta d/d\simeq H\Delta t$. Its dimensions are inverse time; $H>0$ describes expansion and $H<0$ contraction. The <Hubble parameter identity> gives $\dot H+H^2=\ddot a/a$.
Back to article page
