Can electroweak bubble walls run away?

Bödeker D, Moore GD. Can electroweak bubble walls run away? JOURNAL OF COSMOLOGY AND ASTROPARTICLE PHYSICS. 2009;2009(05): 009.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