Cosmological Electromagnetic Fields due to Gravitational Wave Perturbations

TL;DR

This paper investigates how gravitational wave perturbations in the early universe can generate electromagnetic waves through coupling with magnetic fields, with implications for cosmological observations.

Contribution

It demonstrates the coupling mechanism between gravitational waves and magnetic fields in a cosmological setting, leading to electromagnetic wave generation, using a covariant gauge-invariant approach.

Findings

Coupling produces an initial electromagnetic pulse with specific width and amplitude.

The effect's magnitude is bounded by CMB quadrupole anisotropy limits.

Electromagnetic wave generation depends on wavelengths of gravitational waves and magnetic fields.

Abstract

We consider the dynamics of electromagnetic fields in an almost-Friedmann-Robertson-Walker universe using the covariant and gauge-invariant approach of Ellis and Bruni. Focusing on the situation where deviations from the background model are generated by tensor perturbations only, we demonstrate that the coupling between gravitational waves and a weak magnetic test field can generate electromagnetic waves. We show that this coupling leads to an initial pulse of electromagnetic waves whose width and amplitude is determined by the wavelengths of the magnetic field and gravitational waves. A number of implications for cosmology are discussed, in particular we calculate an upper bound of the magnitude of this effect using limits on the quadrapole anisotropy of the Cosmic Microwave Background.