Vorticity survival in magnetised Friedmann universes

We use a general relativistic approach to investigate the effects of weak cosmological magnetic fields on linear rotational perturbations during the radiation and dust epochs of the universe. This includes ordinary kinematic vorticity, as well as vortex-like inhomogeneities in the density distribution of the cosmic medium. Our study confirms that magnetic fields can source both types of perturbations and shows that their presence also helps cosmic rotation to survive longer. In agreement with previous Newtonian studies, we find that during the dust era vorticity decays slower than in non-magnetised cosmologies. Prior to equipartition the magnetic effect is more pronounced, since it helps the aforementioned rotational distortions to maintain constant magnitude. Overall, magnetised universes seem to provide a much better environment for the survival of cosmic vorticity than their magnetic-free counterparts.