Metastability of dense neutron matter

Abstract

We formulate a theory of metastability of dense neutron matter with respect to the first-order phase transition to a pion-condensed state. The theory is based on the idea of nucleation of the pion-condensed phase in the metastable normal matter through the appearance of spontaneously growing drops of the new phase. Different paths leading from the false to the true ground state of dense neutron matter are considered. Also, both quantum fluctuations — that dominate at the low temperatures typical of neutron stars-and thermal fluctuations — that dominate during the gravitational collapse of massive stars — are considered. The calculations performed for realistic models of cold neutron matter yield an interval of metastability (on the time scale of the age of the universe) which is as large as half of that between the baryon density ρ N where the metastability starts and ρ c where the potential barrier (without surface effects) between the true and the false ground state vanishes. Possible astrophysical implications of this finding for neutron stars are discussed. In the case of the pion condensation in hot neutron matter ( T ~ 5 MeV) the region of metastability (on the time scale of the gravitational collapse of a massive star) is much narrower than in cold neutron matter. Consequently additional energy release and entropy generation from the first-order phase transition in hot supercompressed matter are found to be negligible. The theory outlined here can be used for the description of other phase transitions in dense nuclear matter, such as the transition to the quark phase for instance.