Cosmological bubble friction in local equilibrium

Abstract In first-order cosmological phase transitions, the asymptotic velocity of expanding bubbles is crucial relevance for predicting observables like spectrum stochastic gravitational waves, or establishing viability mechanisms explaining fundamental properties universe such as observed baryon asymmetry. these dynamic it generally accepted that subluminal bubble expansion requires out-of-equilibrium interactions with plasma which are captured by friction terms in equations motion scalar field. This has been disputed works pointing out velocities local equilibrium arising either from hydrodynamic effects deflagrations entropy change across wall general situations. We argue both related and can be understood conservation degrees freedom equilibrium, leading to speeds detonations. The effect arises background field dependence density plasma, accounted simply imposing stress-energy including dependent thermal contributions effective potential. illustrate this explicit calculations static a electroweak transition Standard Model extension additional fields.