"Plane" electromagnetic wave in spatially flat Friedman universe

TL;DR

This paper explores how electromagnetic waves behave in a spatially flat Friedman universe, showing that their frequency and wave vector depend on the universe's expansion, while their phase velocity remains constant.

Contribution

It derives the properties of plane electromagnetic waves in a Friedman universe, highlighting the dependence of wave characteristics on the scale factor without affecting phase velocity.

Findings

Wave frequency and wave covector depend on the scale factor.

Phase velocity of electromagnetic waves remains constant.

Uniform electromagnetic fields can be sustained in a flat Friedman universe.

Abstract

The electromagnetic theory is, to a large extend, metric independent. Before the metric is introduced, it is called premetric electrodynamics. Metric enters the constitutive relation. We consider this relation for the Friedman model of an expanding Universe and find that the magnitudes of the field quantities depend on the scale factor. This factor, however, does not enter the permeability and permittivity of the vacuum. A spatially uniform electromagnetic field is obtained for spatially flat metric. Then plane electromagnetic wave is found with uniform field playing the role of amplitudes. It turns out that the magnitudes of the frequency and the wave covector depend on the scale factor determining the redshift, but the phase velocity of the wave is constant.