Hydrodynamic obstruction to bubble expansion
  We discuss a hydrodynamic obstruction to bubble wall acceleration during a
cosmological first-order phase transition. The obstruction results from the
heating of the plasma in the compression wave in front of the phase transition
boundary. We provide a simple criterion for the occurrence of the obstruction
at subsonic bubble wall velocity in terms of the critical temperature, the
phase transition temperature, and the latent heat of the model under
consideration. The criterion serves as a sufficient condition of subsonic
bubble wall velocities as required by electroweak baryogenesis.
