Cosmological magnetic fields from nonlinear effects

In the standard cosmological model, magnetic fields and vorticity are generated during the radiation era via second-order density perturbations. In order to clarify the complicated physics of this second-order magnetogenesis, we use a covariant approach and present the electromagnetodynamical equations in the fully nonlinear regime. We use the tight-coupling approximation to analyze Thomson and Coulomb scattering. At the zero-order limit of exact tight-coupling, we show that the vorticity is zero and no magnetogenesis takes place at any nonlinear order. We show that magnetogenesis also fails at all orders if either protons or electrons have the same velocity as the radiation, and momentum transfer is neglected. At first-order in the tight-coupling approximation, magnetic fields and vorticity still cannot be generated even via nonlinear effects. However, at second-order both of them are generated, and we derive a closed set of nonlinear evolution equations that governs this generation.