Chiral symmetry breaking and pion condensation in the early universe

TL;DR

This paper explores how the early universe's QCD phase transitions, including chiral symmetry breaking and pion condensation, could have generated primordial gravitational waves, using the quark-meson model to analyze phase trajectories.

Contribution

It introduces the possibility of a first-order pion condensation phase transition in the early universe due to high lepton asymmetry, which was not previously considered.

Findings

Universe may have undergone pion condensation via a first-order transition.

First-order transition could produce primordial gravitational waves.

Analysis uses the quark-meson model at finite temperature and chemical potentials.

Abstract

We determine the possible trajectories the universe may have followed in the QCD phase diagram during the QCD epoch. We focus on the roles of chiral symmetry breaking and pion condensation under high imbalances in lepton asymmetry. Adopting the quark-meson model as an effective description of QCD at finite temperature, charge and baryon chemical potentials we show that, for sufficiently large but physically motivated asymmetries, the universe may have entered the pion condensation phase through a first-order phase transition, followed by a second-order phase transition when exiting it. Such a first-order phase transition represents a new possible source of primordial gravitational waves during the QCD epoch.