Big Bang: Phase Transitions

The seventies were an exciting time in particle physics. It has just become clear that non-abelian gauge theories could be normalized, and the experimental evidence increasingly pointed to a possible grand unification of the weak, electromagnetic, and strong interactions, all of which could now be described by gauge theories. For astrophysicists, too, the implications were profound: for the first time it became meaningful to ask why the universe is asymmetric, in that everything seems to be made out of matter, rather than antimatter. In the late seventies a possible answer arose: the fact that grand unified theories naturally violate baryon number conservation, together with the time asymmetry of the rapid expansion of the early universe, could lead to out-of-equilibrium processes that in turn produced a slight net baryon number. After the annihilation of the remaining baryons and antibaryons, this slight initial overproduction of baryons may have caused the current dominance of matter over antimatter.

Starting with some of the early calculations of net baryon number production, we included the extra complications of phase transitions in the vacuum, caused by changes in gauge symmetries necessary to break the original unification into the observed low-energy symmetries of SU(3)xSU(2)xU(1). Using a simple model, we made various quantitative estimates for the effect of such phase transitions in our paper

• Baryon Number Creation and Phase Transitions in the Early Universe, by Hut, P. & Klinkhamer, F.R., 1982, Astron. Astrophys. 106, 245-253.

In this paper we also discussed the generation of density perturbations necessary for the formation of galaxies.

Another consequence of some grand unified theories was the predicted presence of right-handed neutrinos in the early universe, presumably heavy enough so as not to violate cosmological bounds on the matter density in the universe. We discussed the effects of such neutrinos for baryon number creation, and the bounds we can place on their masses, in our paper

• Heavy Right-Handed Neutrinos and the Early Universe, by Klinkhamer, F.R., Branco, G., Derendinger, J.P., Hut, P. & Masiero, A., 1981, Astron. Astrophys. 94, L19-L22.

In another paper, we discussed the effect of grand unified phase transitions on the global space-time structure of the early universe. Following the supercooling that might take place near a first-order vacuum phase transition, we discussed the nature of barrier penetration in the nucleation of true vacuum states in a background of a false vacuum in curved spacetime in the paper:

• Global Space-Time Effects on First-Order Phase Transitions from Grand Unification, by Hut, P. & Klinkhamer, F.R., 1981, Phys. Lett. 104B, 439-443.