Evolution of magnetic fields through cosmological perturbation theory

TL;DR

This paper investigates how primordial magnetic fields evolve in the early universe using cosmological perturbation theory, deriving equations for different initial conditions and comparing their dynamo-like behavior.

Contribution

It derives second-order gauge-invariant perturbed Maxwell's equations and compares magnetic field evolution in magnetized and non-magnetized FLRW backgrounds.

Findings

Derived perturbed Maxwell's equations to second order.

Compared magnetic field evolution in different cosmological scenarios.

Identified differences in dynamo-like behavior based on initial conditions.

Abstract

The origin of galactic and extra-galactic magnetic fields is an unsolved problem in modern cosmology. A possible scenario comes from the idea of these fields emerged from a small field, a seed, which was produced in the early universe (phase transitions, inflation, ...) and it evolves in time. Cosmological perturbation theory offers a natural way to study the evolution of primordial magnetic fields. The dynamics for this field in the cosmological context is described by a cosmic dynamo like equation, through the dynamo term. In this paper we get the perturbed Maxwell's equations and compute the energy momentum tensor to second order in perturbation theory in terms of gauge invariant quantities. Two possible scenarios are discussed, first we consider a FLRW background without magnetic field and we study the perturbation theory introducing the magnetic field as a perturbation. The second scenario, we consider a magnetized FLRW and build up the perturbation theory from this background. We compare the cosmological dynamo like equation in both scenarios.