Can electroweak bubble walls run away?
  In extensions of the Standard Model with SU(2) singlet scalar fields, there
can be regions of parameter space for which the electroweak phase transition is
first order already at the mean-field level of analysis. We show that in this
case the phase interface (bubble wall) can become ultra-relativistic, with the
relativistic gamma factor gamma = (1-v_{wall}^2)^{-1/2} growing linearly with
the wall's propagation distance. We provide a simple criterion for determining
whether the bubble wall "runs away" in this way or if gamma approaches a
terminal value.
