GENERATION OF DENSITY PERTURBATIONS BY PRIMORDIAL MAGNETIC-FIELDS

We study the generation and evolution of density perturbations and pec uliar velocities due to primordial magnetic fields. We assume that a r andom magnetic held was present before recombination and follow the he ld's effect on the baryon fluid starting at recombination. We find tha t magnetic fields generate growing density perturbations on length sca les larger than the magnetic Jeans length, I,, and damped oscillations for scales smaller than lambda(B). For small wavenumbers k (large len gth scales), we find that the magnetic field-induced density power spe ctrum generally scales as k(4). We derive the magnetic Jeans length ex plicitly by including the back-reaction of the velocity held onto the magnetic held and by decomposing the magnetic field into a force-free background held and perturbations about it. Depending on the strength of the magnetic field and the ultraviolet cutoff of its spectrum, stru cture can be generated on small or intermediate scales early in the hi story of the universe. For a present rms magnetic held of 10(-10) G on intergalactic scales, we find that perturbations on galactic scales c ould have gone nonlinear at z similar or equal to 6. Finally, we discu ss how primordial magnetic fields affect scenarios of structure format ion with nonbaryonic dark matter.