Above a critical temperature in the Universe, there was probably a phase of unbroken approximate QCD chiral symmetry. Mathematically, the symmetry breaking is triggered when the operators $\bar{\psi}{\psi}$ for the quark fields $\psi$ acquired a nonzero vacuum expectation value (vev). This vev is called a quark condensate. For me this is a very mathematical description and I failed to translate it into a physical picture. Can someone explain what is really going on above and below this transition temperature in physical terms? My feeling is that above the transition temperature quarks where free and below it, the quarks and antiquarks became trapped to form mesons.
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