Relativistic magnetised perturbations: magnetic pressure vs magnetic tension

TL;DR

This paper investigates how magnetic pressure and tension influence the evolution of cosmological perturbations after recombination, revealing their opposing effects on large and small scales within a relativistic framework.

Contribution

It is the first relativistic study to include magnetic tension effects alongside magnetic pressure in the evolution of cosmological perturbations.

Findings

Magnetic pressure causes oscillations and decay on small scales.

Magnetic tension induces weak growth near the Jeans length.

Large-scale perturbations are unaffected by magnetic fields.

Abstract

We study the linear evolution of magnetised cosmological perturbations in the post-recombination epoch. Using full general relativity and adopting the ideal magnetohydrodynamic approximation, we refine and extend the previous treatments. More specifically, this is the first relativistic study that accounts for the effects of the magnetic tension, in addition to those of the field's pressure. Our solutions show that on sufficiently large scales, larger than the (purely magnetic) Jeans length, the perturbations evolve essentially unaffected by the magnetic presence. The magnetic pressure dominates on small scales, where it forces the perturbations to oscillate and decay. Close to the Jeans length, however, the field's tension takes over and leads to a weak growth of the inhomogeneities. These solutions clearly demonstrate the opposing action of the aforementioned two magnetic agents, namely of the field's pressure and tension, on the linear evolution of cosmological density perturbations.