= Solution
For successive wavecrests from the same comoving source, $1+z=a(t_0)/a(t_s)$. Therefore
$$
\Delta z=\frac{a(t_0+\Delta t_0)}{a(t_s+\Delta t_s)}
-\frac{a(t_0)}{a(t_s)}.
$$
The same radial null path implies equality of the elapsed <conformal time>,
$$
\frac{\Delta t_s}{a(t_s)}=\frac{\Delta t_0}{a(t_0)},
\qquad
\Delta t_s=\frac{\Delta t_0}{1+z_1}.
$$
Expanding both scale factors to first order gives the <redshift drift>
$$
\boxed{\Delta z
=\left[(1+z_1)H_0-H(z_1)\right]\Delta t_0
=H_0\Delta t_0\left[1+z_1-E(z_1)\right]}.
$$
Back to article page
