It says clearly at the wiki article regarding the CMB that: "The photons that existed at the time of photon decoupling have been propagating ever since, though growing fainter and less energetic, since exactly the same photons fill a larger and larger universe." I don't see how that's true. As I understand things when an electron descends from a higher orbit it releases a photon. When electrons are in a lower orbit they have less energy and that loss of energy is transferred to the photons so that the amount of energy in the universe is conserved. It appears to me then that photons are created from electrons and later they are absorbed by electrons when they ascend to a higher orbit. It seems like I've unwittingly answered my question. Still, I want it to be sure. Is it accurate to say that any time an electron releases a photon by descending an orbit that same photon must later be absorbed by a different electron ascending to a higher orbit?