Bubble wall velocity from hydrodynamics

Terminal velocity reached by bubble walls in cosmological first-order
phase transitions is an important parameter determining both primordial
gravitational wave spectrum and the production of baryon asymmetry in
models of electroweak baryogenesis. In this talk I discuss the recent
results for local thermal equilibrium approximation for which, using
hydrodynamic simulations, we have confirmed that pure hydrodynamic
backreaction can lead to steady-state expansion. However, this is not
the generic outcome. Instead, it is much more common to observe
runaways, as the early-stage dynamics right after the nucleation allow
the bubble walls to achieve supersonic velocities before the heated
fluid shell in front of the bubble is formed. In order to capture this
effect, we generalized the analytical methods beyond the local thermal
equilibrium and find a qualitative way to predict whether the runaway is
physical, which has a crucial impact on cosmological observables.