  Extensions of the Standard Model are being considered as viable settings for
a first-order electroweak phase transition which satisfy Sakharov's three
conditions for the generation of the baryon asymmetry of the Universe. These
extensions provide a sufficiently strong phase transition and remove the main
obstacles which appear in the context of the Standard Model: A far-too-high
lower bound on the Higgs mass, immediate wipeout of the newly-created baryon
asymmetry, and insufficient CP violation. We describe the Universe
hydrodynamically as a fluid coupled to the Higgs field via a phenomenological
friction term, and study the time evolution of bubbles nucleated during the
phase transition. We express the friction term in the hydrodynamic equations in
terms of the particle content of the model, calibrate the friction on the basis
of existing calculations for the Standard Model, and produce predictions for
the velocity of the expanding bubble wall in the stationary regime. This way we
develop a very efficient approach to compute bubble velocities. As an example,
we apply our formalism to the first-order phase transition of a dimension-6
extension of the Standard Model which, within the present bounds on the Higgs
mass, can reproduce the observed baryon asymmetry of the Universe. Depending on
the strength of the phase transition, the wall velocity varies from about 0.3
to approaching the speed of light. Our method can easily be adapted to compute
wall velocities in other interesting extensions of the Standard Model.
