Primordial Density Perturbations from Magnetic Fields

TL;DR

This paper re-derives the power spectrum of density perturbations caused by primordial magnetic fields, clarifying previous ambiguities and providing a simplified, modernized calculation.

Contribution

It offers a clear, updated derivation of the primordial density perturbation power spectrum induced by magnetic fields, resolving past discrepancies.

Findings

Revised power spectrum derivation using modern conventions

Identification of discrepancies in previous literature

Simplified expression for density perturbation spectrum

Abstract

Perturbations to the cosmic baryon density - and thus to the total-matter density - can be induced by magnetohydronamic forces if there are primordial magnetic fields. The power spectrum for these density perturbations was first provided in 1996, but without much in the way of detail in the derivation, and there has been confusion in the intervening years about this calculation. In this brief note, we re-derive this power spectrum using modern conventions, provide a simplified result, and identify some of the discrepancies in the literature.