Inverse phase transitions via dark sector chemical equilibration

Abstract

We study the cosmological evolution of the effective potential in a dark sector with a spontaneously broken Abelian gauge symmetry, populated out of equilibrium through the freeze-in mechanism. Initially dilute and in the broken phase, the dark sector is described by a temperature as well as a chemical potential that reflects the low occupancy of dark sector states. We show that the thermal corrections to the effective potential depend on both of these quantities, and that they are suppressed while the sector remains dilute, but grow as internal number-changing reactions increase the occupancy and drive the system toward chemical equilibrium. We solve the coupled evolution of the temperature and chemical potential and the resulting impact on the effective potential to show that, for sufficiently large portal couplings, chemical equilibration can evolve the effective potential across a first-order barrier and drive the dark sector toward its symmetric phase through an inverse phase transition.