Speaker
Description
First-order cosmological phase transitions proceed through the nucleation and growth of bubbles of the new phase, and can be a leading source of gravitational waves, baryogenesis, and dark matter in the early Universe. A central question is what happens to a bubble immediately after it nucleates: does the wall settle into a slow, subsonic regime, or does it accelerate to ultra-relativistic velocities? This outcome is controlled by hydrodynamic obstruction, whereby heating of the surrounding plasma counteracts the vacuum pressure that drives the wall.
In this talk I focus on the early evolution of the bubble right after nucleation, a stage usually left out of wall-velocity calculations. Standard analyses assume that the fluid around the wall has already relaxed to a steady-state profile, but this assumption can break down precisely while the wall is still accelerating. Working in local thermal equilibrium (LTE) and combining analytical approximations with numerical hydrodynamic simulations, we find that the heating wave often takes longer to form than the wall takes to accelerate, so steady-state predictions fail near the speed of sound. From this we derive a revised criterion for the maximal driving pressure, separating deflagration and hybrid solutions from detonations and runaway walls. Validated against simulations, the criterion shows that hydrodynamic obstruction is less
restrictive than steady-state LTE predictions suggest.