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It is believed that first-order phase transitions occurred in the very early universe when the temperature dropped below the grand-unification and Weinberg-Salam energies. Bubbles of the new, broken-symmetry phase would have formed surrounded by the symmetric phase. The energy released in the phase transition would have caused the walls of the bubbles to accelerate outwards. We study what happens when the walls collide with each other. We find that the energy in the walls would not be thermalized for a considerable time. In the inflationary-universe scenario, in which the bubble nucleation rate is low, thermalization could not occur until long after the baryon and nucleosynthesis eras and would not be complete. We also investigate the formation of primordial black holes in bubble collisions. The Weinberg-Salam phase transition is not likely to produce black holes but, under certain circumstances, the grand-unified phase transition might give rise to black holes of 10^3 g.
Hawking et al. (Mon,) studied this question.