Not able to really find an already existing thread where my question could fit in very well, I am starting this new thread which I hope is not too much confusing. Conservation of energy of photons in an expanding vacuum part of the universe? The wavelength of a photon travelling through the expanding universe increases linear with the expansion. The energy of the photon goes inversely with its wavelength, so the radiation energy density goes down with the fourth power of the expansion. Now I am confronted with the following paradox. The decrease of the energy density of a universe only containing particles (baryons) goes down with the third power since stars etc don’t take part in the expansion. Let’s assume an “elongated” photon hits an atom and brings it to a long term exited state. If after a while the universe starts shrinking (a possibility in an FRLW universe I suppose), then the travelling photons increase their energy density with the fourth power again and the energy density of the particles goes up with the third power again. But then at a certain moment our exited atom falls back to its original state and a photon will leave it at a frequency different from the frequency of the photons that never excited an atom. It looks like as if energy is not fully conserved. What is wrong in my thinking, is it just a bad way of energy book keeping?