Cosmological Phase Transitions

It is generally believed that several phase transitions have taken place in the early Universe. The effects of cosmological phase transitions may well have been crucial for the evolution of the Universe, and thus for the existence of life as we know it. The cosmological phase transitions investigated here are related to strong and electroweak interactions. When the Universe was about 10 seconds old and the horizon radius equaled one centimeter, symmetry between weak and electromagnetic interactions was broken. It is quite possible that the baryon asymmetry, one of the most important properties of our Universe, was generated in this transition. Later happened the phase transition from the quark–gluon plasma to the hadron matter. At the quark–hadron transition the size of a causally connected region of space, the horizon radius, was ten kilometers and the age of the Universe 10 seconds. The new Z(3) phase transition suggested in this work is situated to a temperature or energy two orders of magnitude above the electroweak scale. At that time the Universe was roughly 10 seconds old and the horizon radius was of the order of one micrometer. This hypothetical phase transition is caused purely by the hightemperature properties of strong interactions. The discussion of the different phase transitions is based on the assumption that they are first order. It is pointed out that especially the onset of a cosmological phase transition shows a universal behavior. General methods are presented, applicable to an analysis of the sequence of events taking place in cosmological first-order phase transitions. An equation of state is derived for the electroweak matter near the phase transition point. The thermodynamically allowed region for the velocities of the phase transition front is determined. The nucleation rate of bubbles of the broken-symmetry phase is computed in generic first-order cosmological phase transitions. The radial dependence of the Helmholtz free energy of the bubbles is also discussed.