Cosmological Ohm's law and dynamics of non-minimal electromagnetism

TL;DR

This paper investigates how non-minimal electromagnetic couplings influence the evolution of cosmic magnetic fields, concluding that such effects are highly constrained and do not significantly alter flux freezing at large scales.

Contribution

It derives a generalized Ohm's law in a cosmological context with non-minimal couplings, providing a comprehensive framework for understanding plasma currents and magnetic field evolution.

Findings

Non-minimal couplings have limited impact due to high plasma conductivity.

Modifications to magnetic field evolution are strongly constrained by observations.

Flux freezing remains valid at scales beyond a Mpc despite non-minimal effects.

Abstract

The origin of large-scale magnetic fields in cosmic structures and the intergalactic medium is still poorly understood. We explore the effects of non-minimal couplings of electromagnetism on the cosmological evolution of currents and magnetic fields. In this context, we revisit the mildly non-linear plasma dynamics around recombination that are known to generate weak magnetic fields. We use the covariant approach to obtain a fully general and non-linear evolution equation for the plasma currents and derive a generalised Ohm law valid on large scales as well as in the presence of non-minimal couplings to cosmological (pseudo-)scalar fields. Due to the sizeable conductivity of the plasma and the stringent observational bounds on such couplings, we conclude that modifications of the standard (adiabatic) evolution of magnetic fields are severely limited in these scenarios. Even at scales well beyond a Mpc, any departure from flux freezing behaviour is inhibited.