I know that QM and GR have not net been combined, so perhaps this is a foolish question, but I'll try anyhow. A photon travelling in the vacuum, has energy h/λ. That energy is fixed. It never varies in any circumstances (true?) except one. That one is the expansion of space time; i.e. the scale factor. That is why high energy primordial photons appear low energy today (CMB 3 degrees K). The only way I can think of that loss of energy is when the wavelength λ increases with the scale factor a. We could express λ in giga light years instead of meters as a reminder that we are talking about cosmological scale distances. What I would like to find is a quantum expression for the energy of a photon to decrease by one increment as the result of scale factor increasing. A non-energy conserving quantum event. If such exists, where might I go to study it?