Gravito-magnetic instabilities in anisotropically expanding fluids

TL;DR

This paper investigates how magnetic fields influence gravitational instabilities in anisotropically expanding cosmological fluids, revealing that resistivity can enable sub-horizon condensations, expanding the scope of instability conditions.

Contribution

It extends the analysis of gravito-magnetic instabilities to anisotropic models using general-relativistic magnetohydrodynamics, including resistivity effects, which was not previously explored.

Findings

Magnetized condensations can form on sub-horizon scales in anisotropic models.

Resistivity reduces Jeans lengths, allowing smaller scale instabilities.

Most unstable perturbations are concentrated near the horizon in non-resistive cases.

Abstract

Gravitational instabilities in a magnetized Friedman - Robertson - Walker (FRW) Universe, in which the magnetic field was assumed to be too weak to destroy the isotropy of the model, are known and have been studied in the past. Accordingly, it became evident that the external magnetic field disfavors the perturbations' growth, suppressing the corresponding rate by an amount proportional to its strength. However, the spatial isotropy of the FRW Universe is not compatible with the presence of large-scale magnetic fields. Therefore, in this article we use the general-relativistic (GR) version of the (linearized) perturbed magnetohydrodynamic equations with and without resistivity, to discuss a generalized Jeans criterion and the potential formation of density condensations within a class of homogeneous and anisotropically expanding, self-gravitating, magnetized fluids in curved space-time. We find that, for a wide variety of anisotropic cosmological models, gravito-magnetic instabilities can lead to sub-horizonal, magnetized condensations. In the non-resistive case, the power spectrum of the unstable cosmological perturbations suggests that most of the power is concentrated on large scales (small k), very close to the horizon. On the other hand, in a resistive medium, the critical wave-numbers so obtained, exhibit a delicate dependence on resistivity, resulting in the reduction of the corresponding Jeans lengths to smaller scales (well bellow the horizon) than the non-resistive ones, while increasing the range of cosmological models which admit such an instability.