Impact of Multiple Phase Transitions in Dense QCD on Compact Stars

TL;DR

This review explores how multiple phase transitions in dense QCD influence the structure, stability, and thermal evolution of compact stars, highlighting new equations of state and phenomena like twin and triplet star configurations.

Contribution

It introduces an extended equation of state incorporating conformal fluid at high densities and analyzes the resulting complex mass-radius relationships of hybrid stars.

Findings

Identification of stable and unstable star configurations due to phase transitions.

Discovery of spiral features in mass-radius diagrams caused by conformal fluid transition.

Impact of phase transition timescales on star stability and thermal evolution.

Abstract

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. 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. We 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 $c_{\rm conf.}=1/\sqrt{3}$ at densities $\ge 10~n_{\rm sat}$, where $n_{\rm sat}$ 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.