Speaker
Description
If dark matter is produced non-thermally via non-renormalisable interactions (so-called UV-dominated freeze-in), the resulting abundance depends sensitively on the details of reheating. This sensitivity can however be much reduced in the presence of a cosmological first-order phase transition, after which the decaying scalar field transfers its energy density to the SM radiation, thereby diluting pre-existing particle abundances. I will discuss how this phase-in mechanism introduces a dependence of the final DM abundance on the details of the phase transition, in particular the equation of state of the universe. Using results from lattice simulations, I will show that large gradients in the scalar field persist even after the bubble collisions, leading to an equation of state that differs significantly from the one of non-relativistic matter. Larger Lorentz factors of the bubble walls lead to higher-momentum modes in the power spectrum of the scalar field, corresponding to a more radiation-like equation of state. These results affect the dark matter abundance as well as other cosmological observables linked to the phase transition.