Hi all!
I've got a question about the cosmological redshift. We're given the metric
ds^2 = c^2\,dt^2 - a(t)^2 \left[ dr^2 + r^2\,d\theta^2 + r^2\sin^2 \theta\,d\varphi^2 \right]
Now light moves on null geodesics, so c^2\,dt^2 - a(t)^2\,dr^2 for radially moving light. For a GR exercise, we are...