Cosmological perturbations on a magnetized Bianchi I background

Motivated by the isotropy of the CMB spectrum, all existing studies of magn etized cosmological perturbations employ FRW backgrounds. However, it is im portant to know the limits of this approximation and the effects one loses by neglecting the anisotropy of the background magnetic field. We develop a new treatment, which fully incorporates the anisotropic magnetic effects b y allowing for a Bianchi I background universe. The anisotropy of the unper turbed model facilitates the closer study of the coupling between magnetism and geometry. The latter leads to a curvature stress, which accelerates po sitively curved perturbed regions and balances the effect of magnetic press ure gradients on matter condensations. We argue that the tension carried al ong the magnetic force lines is the reason behind these magneto-curvature e ffects. For a relatively weak field, we also compare with the results of th e almost-FRW approach. We find that some of the effects identified by the F RW treatment are in fact direction-dependent, where the key direction is th at of the background magnetic field vector. Nevertheless, the FRW-based app roach to magnetized cosmological perturbations remains an accurate approxim ation, particularly on large scales, when one looks at the lowest-order mag netic impact on gravitational collapse. On small scales, however, the accur acy of the perturbed Friedmann framework may be compromised by extra shear effects.