Large-scale magnetic fields, curvature fluctuations and the thermal history of the Universe

TL;DR

This paper investigates how large-scale magnetic fields influence the evolution of curvature perturbations in the Universe, providing constraints on the magnetic power spectrum based on thermodynamic history and gauge-invariant analysis.

Contribution

It introduces a method to constrain magnetic fields using curvature perturbations and examines the impact of different thermal histories on these constraints.

Findings

Magnetic fields affect curvature perturbations similarly to non-adiabatic pressure fluctuations.

Constraints on magnetic energy spectrum depend on the Universe's thermodynamic history.

Theoretical uncertainties are evaluated by comparing standard and alternative thermal histories.

Abstract

It is shown that gravitating magnetic fields affect the evolution of curvature perturbations in a way that is reminiscent of a pristine non-adiabatic pressure fluctuation. The gauge-invariant evolution of curvature perturbations is used to constrain the magnetic power spectrum. Depending on the essential features of the thermodynamic history of the Universe, the explicit derivation of the bound is modified. The theoretical uncertainty in the constraints on the magnetic energy spectrum is assessed by comparing the results obtained in the case of the conventional thermal history with the estimates stemming from less conventional (but phenomenologically allowed) post-inflationary evolutions.