Magnetised cosmological perturbations in the post-recombination era

TL;DR

This paper investigates how magnetic fields influence the evolution of density perturbations in the universe after recombination, providing new analytic solutions within a Newtonian framework that account for magnetic pressure and tension effects.

Contribution

It offers the first linear analytic solutions for magnetised cosmological perturbations including magnetic pressure and tension effects, comparing Newtonian and relativistic approaches.

Findings

Magnetic pressure inhibits density growth and defines a magnetic Jeans length.

Perturbations grow slower than in non-magnetised universes on large scales.

Wavelengths below the magnetic Jeans length oscillate with decreasing amplitude.

Abstract

We study inhomogeneous magnetised cosmologies through the post-recombination era in the framework of Newtonian gravity and the ideal-magnetohydrodynamic limit. The nonlinear kinematic and dynamic equations are derived and linearised around the Newtonian counterpart of the Einstein-de Sitter universe. This allows for a direct comparison with the earlier relativistic treatments of the issue. Focusing on the evolution of linear density perturbations, we provide new analytic solutions which include the effects of the magnetic pressure as well as those of the field's tension. We confirm that the pressure of field inhibits the growth of density distortions and can induce a purely magnetic Jeans length. On scales larger than the aforementioned characteristic length the inhomogeneities grow, though slower than in non-magnetised universes. Wavelengths smaller than the magnetic Jeans length typically oscillate with decreasing amplitude. We also identify a narrow range of scales, just below the Jeans length, where the perturbations exhibit a slower power-law decay. In all cases, the effect of the field is proportional to its strength and increases as we move to progressively smaller lengths.