Electroweak baryogenesis in a cold universe

We discuss the possibility of generating the baryon asymmetry of the Universe when the temperature of the Universe is much below the electroweak scale. As discussed by Nagatani recently the evaporation of primordial black holes may re-heat the surrounding plasma to temperatures above the electroweak transition temperature leading to the restoration of electroweak symmetry locally. The symmetry is broken again spontaneously as the plasma cools and a baryon asymmetry is generated during the phase transition. We re-analyse the transfer of heat in the plasma surrounding the black hole and point out that the diffusion approximation used earlier is not valid due to the breakdown of hydrostatic equilibrium. We then provide an estimate of the temperature profile of the plasma outside the black hole after including outward motion of the plasma due to the presence of a pressure gradient and an estimate of the baryon asymmetry that may be generated in the plasma. Simple estimates for a second order electroweak phase transition imply that this scenario may generate sufficient asymmetry. For a first order phase transition, sufficient asymmetry may be generated if viscous effects slow down the heated plasma as it moves away from the black hole. In this scenario there is no wash-out of the asymmetry after the phase transition as the plasma rapidly cools to lower temperatures thereby shutting off the sphaleron processes.