Impact of Multiple Phase Transitions in Dense QCD on Compact Stars

This review covers several recent developments in the physics of dense QCD with an emphasis on the impact of multiple phase transitions on astrophysical manifestations of compact stars. To motivate the multi-phase modeling of dense QCD and delineate the perspectives, we start with a discussion of the structure of its phase diagram and the arrangement of possible color-superconducting and other phases. It is conjectured that pair-correlated quark matter in beta-equilibrium is within the same universality class as spin-imbalanced cold atoms and the isospin asymmetrical nucleonic matter. This then implies the emergence of phases with broken space symmetries and tri-critical (Lifshitz) points. The beyond-mean-field structure of the quark propagator and its non-trivial implications are discussed in the cases of two- and three-flavor quark matter within the Eliashberg theory, which takes into account the frequency dependence (retardation) of the gap function. We then construct an equation of state (EoS) that extends the two-phase EoS of dense quark matter within the constant speed of sound parameterization by adding a conformal fluid with a speed of sound cconf.=1/3 at densities >= 10nsat, where nsat is the saturation density. With this input, we construct static, spherically symmetrical compact hybrid stars in the mass-radius diagram, recover such features as the twins and triplets, and show that the transition to conformal fluid leads to the spiraling-in of the tracks in this diagram. Stars on the spirals are classically unstable with respect to the radial oscillations but can be stabilized if the conversion timescale between quark and nucleonic phases at their interface is larger than the oscillation period. Finally, we review the impact of a transition from high-temperature gapped to low-temperature gapless two-flavor phase on the thermal evolution of hybrid stars.

Automated summary

This review article discusses recent developments in the field of dense QCD physics with a focus on the impact of multiple phase transitions on astrophysical manifestations of compact stars. The authors explore the structure of the phase diagram of dense QCD and the possible arrangement of various phases. They propose that pair-correlated quark matter in beta-equilibrium shares similarities with spin-imbalanced cold atoms and isospin asymmetrical nucleonic matter. The article also discusses the non-trivial implications of the beyond-mean-field structure of the quark propagator in two- and three-flavor quark matter. The authors construct an equation of state (EoS) that includes a conformal fluid and examine its effect on the mass-radius diagram of compact hybrid stars. Furthermore, they review the impact of a transition from gapped to gapless two-flavor phase on the thermal evolution of hybrid stars.