AXIONS AND THE QCD PHASE-TRANSITION

Axion dynamics during a first-order QCD phase transition are investigated. If the mass of the axion is different on either side of the phase boundary, low-energy axions are trapped in the shrinking regions of the high-temperature phase. A number of different possibilities present themselves: the axions may escape, resulting in the generation of isocurvature density perturbations; if a substantial population of quark nuggets is produced the axions remain trapped for the lifetime of the nugget; or the pressure of the trapped axions may itself prevent the collapse of the bubble of the high-temperature phase, leading to the formation of "axion nuggets."