Gravitational waves from a very strong electroweak phase transition
  We investigate the production of a stochastic background of gravitational
waves in the electroweak phase transition. We consider extensions of the
Standard Model which can give very strongly first-order phase transitions, such
that the transition fronts either propagate as detonations or run away. To
compute the bubble wall velocity, we estimate the friction with the plasma and
take into account the hydrodynamics. We track the development of the phase
transition up to the percolation time, and we calculate the gravitational wave
spectrum generated by bubble collisions, magnetohydrodynamic turbulence, and
sound waves. For the kinds of models we consider, we find parameter regions for
which the gravitational waves are potentially observable at the planned
space-based interferometer eLISA. In such cases, the signal from sound waves is
generally dominant, while that from bubble collisions is the least significant
of them. Since the sound waves and turbulence mechanisms are diminished for
runaway walls, the models with the best prospects of detection at eLISA are
those which do not have such solutions. In particular, we find that heavy extra
bosons provide stronger gravitational wave signals than tree-level terms.
