Luminosity-distance from electromagnetism in curved spacetime

In many cosmology textbooks, the relation between the observed flux of a distant galaxy compared to the intrinsic luminosity of the galaxy (and thus, the luminosity distance relation) is established by using a model based on photons: the energy and power of the emitted photons is compared to the energy and power of the received photons, taking in account that the wave length of the photons is changing through time, time is expanding and when it is received, the emitted photons are now distributed in a expanded sphere.

My question is the following: is it possible to derive this relation without using a photons-based model, but rather by considering an electromagnetic wave propagating in a FLRW metric? I didn’t find this derivation anywhere! The good thing is that with a simple change of time variable the metric reduces to a metric conformal to the Minkowski metric, and thus the solutions of Maxwell equations are the same as usual. I really got troubles in the derivation of the emitted and received power, for I don’t really know which formula I have to use for the Poynting vector and for the average value of its norm…

If never you have any clue 😉

Thanks!

Jeremy