The Electromagnetic Energy-Momentum Tensor in Expanding Universes

Abstract

In two previous papers the authors have investigated the characteristics of freely propagating cosmic photons in expanding homogenous flat universes from particle and wave points of view, respectively. The result was that the energy density of these freely propagating photons scales with 1/S3 instead with the generally accepted and redshift-related 1/S4, with S being the cosmic scale factor. This is a surprising result at first glance but finally very understandable if energy conservation has to be required also for cosmic photons without creating a conflict with observations. In this paper we treat freely propagating cosmic photons with a totally different approach, namely with the electromagnetic energy-momentum tensor (from now on abbreviated with EMT). The implementation of the EMT, when correctly interpreted for freely propagating photons, into Einstein’s field equations then automatically leads to the same 1/S3-scaling law for the energy density of cosmic photons, herewith confirming the results found earlier for the particle and wave view. We conclude that the observed cosmological redshift of photons (energy density ∝ 1/S4) is compatible with the law of energy conservation, i.e energy density ∝ 1/S3 if interpreted on a new physical basis.