Cosmological perturbations on a magnetized Bianchi I background.

*(English)*Zbl 0957.83044The implications of primordial magnetic fields for the formation and the evolution of the observed structure have been the subject of continuous theoretical investigation. All existing studies of magnetized cosmological perturbations employ FRW background. In this paper the authors develop a new approach which incorporates the anisotropic magnetic effects by allowing a Bianchi I background universe. The coupling between magnetism and geometry leads to a curvature stress, which accelerates positively curved perturbed regions and balances the effect of magnetic pressure gradients on matter condensations. They argue that the magneto-curvature effects are due to the tension carried along the magnetic force lines. For weak fields they compare with the results of the almost-FRW approach. They find that some of the effects identified by the FRW treatment are in fact direction-dependent, where the key direction is that of the background magnetic field vector. The FRW-based approach to magnetized cosmological perturbations remains an accurate approximations, particularly on large scales, when one looks at the lowest-order magnetic impact on gravitational collapse. On small scales, the accuracy of the perturbed Friedmann framework may be improved by the consideration of extra shear effects.

Reviewer: Varsha Daftardar-Gejji (Pune)

##### MSC:

83F05 | Cosmology |

83C25 | Approximation procedures, weak fields in general relativity and gravitational theory |

83E05 | Geometrodynamics and the holographic principle |