First-Order Phase Transitions in an Early-Universe Environment
  In first-order phase transitions in the early universe, the bubble wall is expected to be significantly slowed-down by its interaction with the surrounding plasma. We examine the behaviour of the phase of the Higgs field after two-bubble collisions, and find that phase differences equilibrate much more quickly in slow-moving bubbles than in those which expand at the speed of light. This could lead to a significant reduction in the initial density of topological defects formed at a first-order phase transition. 
Matthew Lilley