This might be a tired topic but please help me to understand. Assume a photon moving in vacuum, ignore potential interstellar medium absorption and re-emission since it is not relevant to discussion of space expansion in this context. Redshifted photons will undergo energy loss between *right after* emission at source and *right before* absorption at destination. Since photons always travel at c and experience no acceleration, increase in wavelength must occur in one and same inertial frame. Therefore we can apply special relativity. In special relativity, energy is conserved as long as reference frame remains static. If we were to consider the photon right before absorption, it should be already redshifted and lost energy through unknown means, yet we can not know since as soon as we observe SR no longer applies. So either I'm misunderstanding SR, or energy is being lost can not be explained by SR. So where does this energy go? How can a photon experience a change in total energy during one inertial frame when it does not experience time in the instance of that inertial frame?