Cosmology and broken discrete symmetry

Abstract

It is argued that under various circumstances spontaneously broken discrete symmetry, which is commonly thought to lead to grave cosmological problems, need not do so. A particularly interesting example is the two-doublet version of the standard model. This extended model avoids unacceptable flavor-changing neutral currents in a natural way only if an appropriate discrete symmetry is postulated. Here it is shown that this discrete symmetry is anomalous. As a result, when it is spontaneously broken it does not produce stable domain walls (which would be problematic for cosmology). Instead an interesting cosmological scenario results, which incorporates substantial deviations from thermal equilibrium at the QCD scale. If axions are used to address the strong CP problem, then the analysis is altered. Depending on the Peccei-Quinn charge assignments, domain walls may be either stable or nonexistent.