Speaker
Description
When reheating is delayed, supercooled first-order phase transitions are often assumed to hand the Universe over to an early era of matter domination. In this talk I ask whether this assumption actually holds, and argue that it generally does not — at least not immediately. Bubble collisions leave behind a highly inhomogeneous scalar field with persistent relativistic gradients, producing an equation of state between those of matter and radiation. Using lattice simulations, we find that the equation of state is controlled by the wall Lorentz factor at collision, $\gamma_\star$: walls with larger $\gamma_\star$ populate higher-momentum modes and drive the fluid closer to radiation. Matter domination begins only after these modes redshift, at $a/a_\star \simeq \gamma_\star$, or after the field thermalises through self-scattering and number-changing processes. I close by discussing what this delay means for gravitational waves and primordial black holes.