Cosmological consequences of a first-order phase transition in the SU5grand unified model

Abstract

The S${\mathrm{U}}_{5}$ grand unified model is considered in the context of big-bang cosmology. If the Higgs potential contains a cubic term there is a first-order transition to the S${\mathrm{U}}_{3}$\ifmmode\times\else\texttimes\fi{}S${\mathrm{U}}_{2}$\ifmmode\times\else\texttimes\fi{}${\mathrm{U}}_{1}$-symmetric phase. The bubble nucleation rate for this transition is calculated. For typical choices of parameters, the transition proceeds according to one of two scenarios depending on whether or not its rate is ever large relative to the expansion rate of the universe. Both possibilities lead to difficulties: In the former case the transition is rapidly completed, but leads to the production of too many superheavy magnetic monopoles. In the latter case monopole production is suppressed, but there is an extreme supercooling from which the universe never recovers.