Hello all, I have a question on cosmic redshift and I am really hoping that one of you can solve this for me, as you have many times in the past. Consider a quasar that is emitting a steady stream of X-ray photons many billions of years into the Universe's past. When the X-rays finally reach the earth now, they appear to be visible light radiation due to cosmic redshift. If we were to consider just one photon that started out at the quasar and reached the earth, we clearly find that the frequency of this photon is vastly reduced by the redshift. However, Since the energy of the photon is proportional to the frequency, the photon has apparently lost energy due to cosmic redshift - without interacting with anything at all on the way. How can we explain the change in energy of the photon by cosmic redshift alone? Thanks!