In my last Cosmology class, the professor explained how the expansion of the universe increase the wavelength of the light. But his explanation was a non-local one, I mean, you need to compare the time beween two wavecrests to determine that the light was redshifted. The explanation was that the expansion of the universe stretch the space between the two wavecrests. But what is the local explanation. What if I observe just one photon? I supposed that it should be redshifted two, but then the explanation given by him does not apply. What's the QM explanation in terms of the energy of the photon?