Topical Seminar: Scalar Field Dynamics and Dark Matter Production in Supercooled Phase Transitions

by Henda Mansour (KIT, Karlsruhe, TTP)

Thursday 4 Dec 2025, 12:00 → 13:30 Europe/Madrid

1001-Primera-1-1-1 - Paterna. Seminario (Universe)

Supercooled first-order phase transitions can reshape the thermal history of the universe. If the scalar field driving the transition is weakly coupled, an era of scalar field domination may follow. Using lattice simulations, I will show that the post-transition equation of state can deviate significantly from the commonly assumed matter domination. Our results indicate that the equation of state depends on how relativistic the bubble collisions are, with bigger bubble separations leading to more relativistic walls and to a more radiation-like expansion. 

These dynamics have direct implications on particle production and gravitational waves. In the second part of this talk, I will introduce the phase-in scenario, as a concrete application. In this setup, we study the impact of a supercooled phase transition on dark matter (DM) production via UV-freeze-in. In the phase-in regime, the DM abundance becomes insensitive to the reheating temperature, and instead depends on the details of the transition.