Relics of cosmic quark condensation

Abstract

The influence of the QCD phase transition on the standard cosmological model is examined. Physical mechanisms are analyzed which transport energy and baryon number during the cosmological transition from free-quark (unconfined) to hadron (confined) phases of matter, with particular attention to an effect described by Witten—the concentration of baryons in low-entropy bubbles. Two limiting regimes of transport, hydrodynamic flow and neutrino conduction, are discussed and their relative importance under various circumstances is clarified. The spatial distribution of specific entropy at the end of the transition, and its subsequent evolution, is described using a spherical shell model. Inhomogeneities which persist until the epoch of nucleosynthesis can lead to nuclear products with very different chemical composition from the standard hot big bang, both because of contamination from regions with entropy orders of magnitude less than the cosmic average and because of variation in neutron-to-proton ratio caused by differential diffusion of these reactants into voids of higher-than-average entropy. These events may produce observable distortions in cosmic light-element abundances; in particular, the deuterium abundance may be increased by orders of magnitude. Thus, it is perhaps more appropriate to regard nucleosynthesis as a constraint on the parameters of the phase transition than as a precise probe of the cosmic baryon density or the number of neutrino species.