Electroweak Baryogenesis and the Expansion Rate of the Universe

Abstract

The standard requirement for the production of baryons at the electroweak phase transition, that the phase transition be first order and the sphaleron bound be satisfied, is predicated on the assumption of a radiation dominated universe at that epoch. One simple alternative - domination by the energy in a kinetic mode of a scalar field which scales as $1/a^6$ - gives a significantly weakened sphaleron bound for the preservation of a baryon asymmetry produced at a first-order phase transition, and allows the possibility that the observed baryon asymmetry be produced when the phase transition is second-order or cross-over. Such a phase of `kination' at the electroweak scale can occur in various ways as a scalar field evolves in an exponential potential after inflation.