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The electromagnetic perturbation equations are solved in a closed Robertson-Walker space-time; the frequency spectrum and the proper modes are found. A simple quantum theory of electromagnetic waves in the background metric is then developed using the proper modes of the static metric conformally connected with the closed Robertson-Walker metric. It is shown that the number of "photons" does not change as a result of expansion (or contraction). The application of these results to the cosmic background radiation is discussed.
Bahram Mashhoon (Sat,) studied this question.