Dark matter filtering-out effect during a first-order phase transition

Abstract

If the mass of dark matter is generated from a cosmological phase transition involving the nucleation of bubbles, the corresponding bubble walls can filter out dark matter particles during the phase transition. Only particles with sufficient momentum to overcome their mass inside the bubbles can pass through the walls. As a result, the dark matter number density after the phase transition has a suppression factor $\exp(-M_χ/2\tilde γT)$, where $M_χ$ is the dark matter mass, and $\tilde γ$ and $T$ are the Lorentz factor and temperature of the incoming fluid in the bubble wall rest frame, respectively. Under certain assumptions, we show that the filtering-out process can naturally provide a large suppression consistent with the observed dark matter density for a wide range of dark matter masses up to the Planck scale. Since the first-order phase transition is the decisive ingredient in our mechanism, a new connection is made between heavy dark matter scenarios and gravitational wave observations.