Solution
= Solution
A <comoving coordinate> $\boldsymbol\chi$ labels a galaxy carried by the homogeneous cosmological flow. Its physical separation at time $t$ is $\mathbf r(t)=a(t)\boldsymbol\chi$, where $a$ is the <scale factor>. Thus $\mathbf v=\dot a\boldsymbol\chi=(\dot a/a)\mathbf r$, the <Hubble law>, apart from peculiar <velocities> relative to this flow. In particular
$$
\boxed{H_0=\frac{\dot a(t_0)}{a(t_0)}.}
$$