Upper limit on first-order electroweak phase transition strength

For a cosmological first-order electroweak phase transition, requiring no sphaleron washout of baryon number violating processes leads to lower bound on the strength transition. The velocity boundary between phases, so-called bubble wall, can become ultra-relativistic if friction due plasma particles is not sufficient retard wall's acceleration. This ``runaway'' should occur successful asymmetry generation transition required. Using Boedeker-Moore criterion for wall runaway, within context an extension standard model particle physics with real gauge-single scalar field, we show that non runaway requirement puts upper