We analyze families of hybrid equations of state of cold QCD matter, which combine input from gauge-gravity duality and from various ab initio methods for nuclear matter at low density, and predict that all neutron stars are fully hadronic without quark matter cores. We focus on constraints from recent measurements by the NICER telescope on the radius and mass of the millisecond pulsar PSR <math><mrow><mi>J</mi><mn>0740</mn><mo>+</mo><mn>6620</mn></mrow></math>. These results are found to be consistent with our approach: they set only mild constraints on the hybrid equations of state and favor the most natural models which are relatively stiff at low density. Adding an upper bound on the maximal mass of neutron stars, as suggested by the analysis of the GW170817 neutron star merger event, tightens the constraints considerably. We discuss updated predictions on observables such as the transition density and latent heat of the nuclear to quark matter transition as well as the masses, radii, and tidal deformabilities of neutron stars.

Finland

Niko Jokela

South Korea

Matti Järvinen

Spain

Jere Remes

Holographic QCD in the NICER era

Ab initio methods using weakly interacting nucleons give a good description of condensed nuclear matter up to densities comparable to the nuclear saturation density. At higher densities strong interactions between overlapping nucleons become important; we propose that the interactions will continuously switch over to follow a holographic model in this region. In order to implement this, we construct hybrid equations of state (EOSs) where various models are used for low-density nuclear matter, and the holographic V-QCD model is used for nonperturbative high-density nuclear matter as well as for quark matter. We carefully examine all existing constraints from astrophysics of compact stars and discuss their implications for the hybrid EOSs. Thanks to the stiffness of the V-QCD EOS for nuclear matter, we obtain a large family of viable hybrid EOSs passing the constraints. We find that quark matter cores in neutron stars are unstable due to the strongly first-order deconfinement transition and predict bounds on the tidal deformability as well as on the radius of neutron stars. By relying on universal relations, we also constrain characteristic peak frequencies of gravitational waves produced in neutron star mergers.

Israel

Netherlands

Govert Nijs

Unified weak and strong coupling framework for nuclear matter and neutron stars

Following a series of similar calculations in simpler nonconformal holographic setups, we determine the quasinormal mode spectrum for an operator dual to a gauge-invariant scalar field within the improved holographic QCD framework. At temperatures somewhat above the critical temperature of the deconfinement transition, we find a small number of clearly separated modes followed by a branch-cut-like structure parallel to the real axis, the presence of which is linked to the form of the IHQCD potential employed. The temperature dependence of the lowest nonzero mode is furthermore used to study the thermalization time of the corresponding correlator, which is found to be of the order of the inverse critical temperature near the phase transition and decrease slightly faster than <math><mn>1</mn><mo>/</mo><mi>T</mi></math> at higher temperatures.

Timo Alho

Kimmo Tuominen

Aleksi Vuorinen

Quasinormal modes and thermalization in improved holographic QCD

We use the holographic V-QCD models to analyse the physics of dense QCD and neutron stars. Accommodating lattice results for thermodynamics of QCD enables us to make generic predictions for the Equation of State (EoS) of the quark matter phase in the cold and dense regime. We demonstrate that the resulting pressure in V-QCD matches well with a family of neutron-star-matter EoSs that interpolate between state-of-the-art theoretical results for low and high density QCD. After implementing the astrophysical constraints, i.e., the largest known neutron star mass and the recent LIGO/Virgo results for the tidal deformability, we analyse the phase transition between the baryonic and quark matter phases. We find that the baryon density n  B  at the transition is at least 2.9 times the nuclear saturation density n  s . The transition is of strongly first order at low and intermediate densities, i.e., for n  B  /n  s  ≲ 7.5.

Holographic QCD in the Veneziano limit and neutron stars