First-order phase transition of a vacuum and the expansion of the Universe

The progress of a first-order phase transition of a vacuum in the expanding Universe is investigated. The expansion of bubbles of a stable vacuum is calculated simultaneously with the cosmic expansion with the aid of the following two simplified nucleation rates of bubbles p: (i) p = pT Tc δ (T−Tc) in the hot Universe models, (ii) p = 0 for n > nc and p = pQ for n nc in the cold Universe models, where T is the cosmic temperature, Tc the critical temperature, n the cosmic number density of the fermions coupled to the order parameter of the vacuum, nc the critical density, and pT and pQ are parameters. The following results are obtained: (1) If the nucleation rates are small and the vacuum stays at the metastable state for a long time, the Universe begins to expand exponentially. As a result, the progress of the phase transition is delayed more and more by the rapid cosmic expansion. In particular, in model (i), if pT is less than a critical value, the phase transition never finishes. (2) The lower limits of the nucleation parameter pT and pQ are obtained from observation of the number ratio of photons to baryons in the present Universe. (3) If the phase transition of the vacuum in SU(5) GUT is of first order or if there exists a hypothetical first-order phase transition of the vacuum in the very early stage in which baryon number is not conserved, the density and the velocity fluctuations created by the phase transition may account for the origin of galaxies.