Answer
$\alpha = H$
Work Step by Step
In part (d), we found that $\frac{\Delta \lambda}{\lambda} = \frac{r~\alpha}{c}$
If we assume that $\Delta \lambda$ is due to the Doppler effect, then $\frac{\Delta \lambda}{\lambda} = \frac{v}{c}$
Then:
$\frac{r~\alpha}{c} = \frac{v}{c}$
$r~\alpha = v$
$r~\alpha = H~r$
$\alpha = H$
