Canuto, V.M., and I. Goldman, 1983: The opacity of the universe and the strong equivalence principle. Phys. Lett. A, 95, 65-68, doi:10.1016/0375-9601(83)90784-3.
A possible explanation of why the advanced (i.e. acausal) solutions of Maxwell's equations are not observed in nature is by way of absorption by an opaque universe. As Davies has shown, the ever expanding, general relativistic cosmological models fail to provide the needed absorption. This is perhaps not surprising. In fact, the absorption mechanism calling for an interplay between local physics and cosmology, is usually developed adopting the strong equivalence principle, SEP, which precludes such interplay. We show that complete absorption of electromagnetic radiation by ionized intergalactic plasma is obtained provided a violation of the SEP, of the order of the Hubble's constant, is allowed to occur. The same degree of violation was previously found to be compatible with a large body of observational data.