Velocity of electroweak bubble walls

We study the velocity of bubble walls in the electroweak phase transition. For several extensions of the Standard Model, we estimate the friction and calculate the wall velocity, taking into account the hydrodynamics. We find that deflagrations are generally more likely than detonations. Nevertheless, for models with extra bosons, which give a strongly first-order phase transition, the deflagration velocity is in general quite high, 0.1 less than or similar to nu(omega) less than or similar to 0.6. Therefore, such phase transitions may produce an important signal of gravitational waves. On the other hand, models with extra fermions which are strongly coupled to the Higgs boson may provide a strongly first-order phase transition and small velocities, 10(-2) less than or similar to nu(omega) less than or similar to 10(-1), as required by electroweak baryogenesis. (C) 2009 Elsevier B.V. All rights reserved.