Hi, I figured out a thought experiment/problem, here it is: A friend of mine is living in a galaxy far away. He has a machine which can make individual photons with a definite frequency (lets say blue light) and can emit them toward me. Here I have machine which can detect these photons and can tell their frequency. While they get here their wavelength increase because of the Doppler effect and the cosmological redshift. So I detect red light instead of blue. If I calculate the energy of the photon (E=hf) when it was emitted and when I detected it, there will be a difference between the results. Where is the missing energy?