Electroweak phase transition and bubble wall velocity in local thermal equilibrium

The dynamics of the electroweak phase transition in the early Universe has profound implications for cosmology and particle physics. We systematically study the steady-state dynamics of bubble walls in scenarios where the transition is first order within three representative beyond the Standard Model frameworks, characterized by the presence of an additional scalar in different electroweak representations. Focusing on the local thermal equilibrium regime, we numerically solve the coupled scalar and hydrodynamic equations to extract key properties of the phase transition front: the wall velocity, width, plasma, and field profiles. We find a near-universal behavior across models when expressed in terms of thermodynamic quantities, which can be captured by simple fitting functions, useful for phenomenological applications. These results also provide an upper bound on the bubble velocity and represent the first necessary step for the full inclusion of out-of-equilibrium effects. As an application, we determine the gravitational wave signal and the amount of baryon asymmetry generated by the transition in local thermal equilibrium.