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On the Cosmology of Electromagnetic Wave Energy in Expanding Universes
Authors: Hans-Jörg Fahr, Michael Heyl
Number of views: 363
Modern cosmology considers several different ingredients entering the energy-momentum
tensor of the GTR-field equations, amongst them normal baryonic matter, dark matter, dark energy
and photons. The photons are usually taken as negligibly influencing the present-day expansion
dynamics because they are seen as permanently loosing energy due to being cosmologically redshifted.
In an earlier study we have discussed why this view is questionable and why freely propagating
photons, while being transported in an expanding universe, do not change their energy. In this
paper, instead of using a cosmic photon view, we treat the cosmic electromagnetic radiation field as
a system of monodirectional wave modes. Single photons moving with the velocity of light can not
be described as sources of gravity, raising the question how cosmic photons hence may contribute to
the cosmic gravity field. Here we conclude that photons or the associated electromagnetic waves
can only be described as gravity sources, if they constitute a form of localized standing energy. To
represent localized energy, electromagnetic waves in an appropriate manner have to interfere with
their counter-propagating waves of an appropriate phase shift π to produce standing waves.
We describe the energy distribution of monochromatic standing waves and consider these monodirectional,
monochromatic wave modes as undamped wave modes freely extending over the dimension
of the universe. We show that they keep a constant mode energy despite the cosmic expansion
connected with the shift of their wavelengths proportional to the cosmic scale S. With these
considerations we obtain an expression for the energy density of standing electromagnetic waves and
show that their wave energy density scales with S
−3
, instead like S
−4
as expected for the cosmic
radiation energy density by the present-day cosmology. We conclude that with our present result we
confirm an earlier study carried out on the basis of freely propagating single photons, since on both
ways we find that the energy density of cosmic electromagnetic radiation scales identically to the
scaling of the cosmic matter density, namely according to S
−3
.