Vorticity production and survival in viscous and magnetized cosmologies

TL;DR

This paper investigates how viscosity and magnetic fields influence vorticity in cosmological fluids, revealing that viscosity can generate vorticity locally while magnetic fields tend to sustain cosmic rotation, with implications for universe evolution.

Contribution

It provides a detailed analysis of vorticity production mechanisms in viscous and magnetized cosmologies using Newtonian and MHD approximations, highlighting the contrasting roles of viscosity and magnetic tension.

Findings

Viscosity can generate vorticity in inhomogeneous environments.

Magnetic tension can produce vorticity even without pressure or density gradients.

Magnetic fields slow down the decay of cosmic vortices, increasing residual rotation.

Abstract

We study the role of viscosity and the effects of a magnetic field on a rotating, self-gravitating fluid, using Newtonian theory and adopting the ideal magnetohydrodynamic approximation. Our results confirm that viscosity can generate vorticity in inhomogeneous environments, while the magnetic tension can produce vorticity even in the absence of fluid pressure and density gradients. Linearizing our equations around an Einstein-de Sitter cosmology, we find that viscosity adds to the diluting effect of the universal expansion. Typically, however, the dissipative viscous effects are confined to relatively small scales. We also identify the characteristic length bellow which the viscous dissipation is strong and beyond which viscosity is essentially negligible. In contrast, magnetism seems to favor cosmic rotation. The magnetic presence is found to slow down the standard decay-rate of linear vortices, thus leading to universes with more residual rotation than generally anticipated.