Impurities in the early universe

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Abstract
We investigate the role of impurities in first-order phase transitions in the early universe. General arguments are given of how impurities serve as seeds for nucleation of bubbles of a new phase, when the temperature drops below the critical temperature. We clarify various conditions for this kind of bubble nucleation to take place to complete a phase transition. In the minimal SU(5) theory the fundamental Higgs scalar fields play the role of impurities in the grand-unified-theory phase transitions. It is shown, however, that impurity effects are very small in most cases where weak-coupling perturbation theory is reliable. Only in special cases for which fine tuning of parameters is necessary do impurities become quite relevant. It is also seen that the Coleman-Weinberg SU(5) theory adopted in the new inflationary-universe scenario contains no impurities.