  According to present cosmological views the energy density of CMB (Cosmic
Microwave Background) photons, freely propagating through the expanding cosmos,
varies proportional to 1/S^4 with S being the scale factor of the universe.
This behavior is expected, because General Theory of Relativity, in application
to FLRW- (Friedmann-Lemaitre-Robertson-Walker) cosmological universes, leads to
the conclusion that the photon wavelengths increase during their free passage
through the spacetime metrics of the universe by the same factor as does the
scale factor S. This appears to be a reasonable explanation for the presently
observed Planckian CMB spectrum with its actual temperature of about 2.7 K,
while at the time of its origin after the last scattering during the
recombination phase its temperature should have been about 3000 K, at an epoch
of about 380 ky after the Big Bang, when the scale of the universe S_r was
smaller by roughly a factor of S/S_r = 1+z_r = 1100 compared to the present
scale S = S_0 of the universe. In this paper we start from putting the question
whether the scale-behavior of the CMB energy density that enters the
energy-momentum tensor of the field equations describing the expanding universe
is really falling off like 1/S^4 and, if in fact a deviation from a behavior
according to 1/S^4 would occur, why do we nevertheless presently observe a CMB
energy density which appears to be in accordance with such a 1/S^4-scaling?
This question arises from another basic and fundamental question, namely: Can
we really assume that the wavelength of the freely propagating photon during
its travel all the way along its light geodetic is permanently affected by the
expansion of the universe, i.e continuously recognizes the expansion of the
cosmic scale S? With other words: Do freely propagating photons ........
