Cosmological perturbations on a magnetised Bianchi I background

TL;DR

This paper develops a new approach to study cosmological perturbations in a magnetised universe using a Bianchi I background, revealing direction-dependent effects and the limits of the standard FRW approximation.

Contribution

It introduces a Bianchi I background framework to incorporate anisotropic magnetic effects, advancing understanding of magnetism and geometry coupling in cosmology.

Findings

Magneto-curvature effects are influenced by magnetic tension along field lines.

FRW approximation is accurate on large scales but less so on small scales.

Some magnetic effects are direction-dependent, especially relative to the magnetic field vector.

Abstract

Motivated by the isotropy of the CMB spectrum, all existing studies of magnetised cosmological perturbations employ FRW backgrounds. However, it is important, to know the limits of this approximation and the effects one loses by neglecting the anisotropy of the background magnetic field. We develop a new treatment, which fully incorporates the anisotropic magnetic effects by allowing for a Bianchi I background universe. The anisotropy of the unperturbed model facilitates the closer study of the coupling between magnetism and geometry. The latter leads to a curvature stress, which accelerates positively curved perturbed regions and balances the effect of magnetic pressure gradients on matter condensations. We argue that the tension carried along the magnetic force-lines is the reason behind these magneto-curvature effects. For a relatively weak field, we also compare to the results of the almost-FRW approach. We find that some of the effects identified by the FRW treatment are in fact direction dependent, where the key direction is that of the background magnetic field vector. Nevertheless, the FRW-based approach to magnetised cosmological perturbations remains an accurate approximation, particularly on large scales, when one looks at the lowest order magnetic impact on gravitational collapse. On small scales however, the accuracy of the perturbed Friedmann framework may be compromised by extra shear effects.