Bubble Expansion and the Viability of Singlet-Driven Electroweak
  Baryogenesis
  The standard picture of electroweak baryogenesis requires slowly expanding
bubbles. This can be difficult to achieve if the vacuum expectation value of a
gauge singlet scalar field changes appreciably during the electroweak phase
transition. It is important to determine the bubble wall velocity in this case,
since the predicted baryon asymmetry can depend sensitively on its value. Here,
this calculation is discussed and illustrated in the real singlet extension of
the Standard Model. The friction on the bubble wall is computed using a kinetic
theory approach and including hydrodynamic effects. Wall velocities are found
to be rather large ($v_w \gtrsim 0.2$) but compatible with electroweak
baryogenesis in some portions of the parameter space. If the phase transition
is strong enough, however, a subsonic solution may not exist, precluding
non-local electroweak baryogenesis altogether. The results presented here can
be used in calculating the baryon asymmetry in various singlet-driven
scenarios, as well as other features related to cosmological phase transitions
in the early Universe, such as the resulting spectrum of gravitational
radiation.
