Electroweak monopoles and the electroweak phase transition

We consider an isolated electroweak monopole solution within the Standard Model with a nonlinear Born-Infeld extension of the hypercharge gauge field. Monopole ( and dyon) solutions in such an extension are regular and their masses are predicted to be proportional to the Born-Infeld mass parameter. We argue that cosmological production of electroweak monopoles may delay the electroweak phase transition and make it more strongly first order for monopole masses M greater than or similar to 9.3 center dot 10(3) TeV, while the nucleosynthesis constraints on the abundance of relic monopoles impose the bound M less than or similar to 2.3 center dot 10(4) TeV. The monopoles with a mass in this shallow range may be responsible for the dynamical generation of the matter-antimatter asymmetry during the electroweak phase transition.