Neutron stars are the ultimate probe of dense matter physics under cold conditions not achievable in the laboratory. They represent extremes in terms of density, pressure, temperature, magnetic field and spin rate. The lecture will describe the interior structure of neutron stars and establish general limits to their properties from causality, general relativity and nuclear physics. It will...
We discuss some properties of superfluids that could be relevant for the evolution of compact stars.
Superfluids emerge in many different systems and can be produced and accurately studied in laboratory.
We will discuss how some of the results recently obtained in laboratory could be employed to better understand the interior of compact stars.
Of particular interest are...
A one-hour crash course on QFT at finite density and temperature with the aim to help the students understand basic methods of computing quantities such as the pressure in the finite-density (and also finite-temperature) background necessary for understanding neutron stars (and their collisions), as well as the complications that arise due to the presence of soft scales. Focus on computational...
The detection of gravitational waves has revolutionized astrophysics, offering a novel means to study phenomena that were previously beyond observational reach. This talk will provide a concise overview of current and future gravitational-wave sources, with a specific focus on individual compact remnants, in particular neutron stars formed in core-collapse supernovae and the remnants of binary...
Unknown neutron stars in binary systems might be one of the best sources of continuous gravitational waves (yet to be detected), due to their millisecond rotation rates, the accretion from their companion which can source the required asymmetry, and the vast unexplored parameter space. In this talk we will present the latest search results and will explain which neutron star parameters might...
Next-generation gravitational-wave detectors are expected to constrain the unknown neutron star equation-of-state from static and dynamical tides in binary neutron star inspirals. Although, the impact of dissipation on the tides are generally considered negligible while modeling the inspiral of binary neutron stars, recent studies shows that fluid dissipation from bulk viscosity of exotic...
I show that the delayed formation of a hybrid star can account for the unique properties of double-peaked superluminous supernovae (SLSNe). The results indicate that SLSNe can serve as probes of neutron star core physics, offering complementary constraints on the dense-matter equation of state and core magnetization through their light curves and spectra. This framework opens a new avenue for...
Gravitational wave astronomy provides a vital tool for probing extreme matter. This study investigates $f$-mode oscillations of cold, catalyzed Neutron stars as well as protoneutron stars with different evolutionary phases. We analyze the collective impact of nucleons, hyperons, phase transition to the quark matter, and dark matter admixtures on these oscillations employing full General...
The physical description of pulsars remains obscure. Although widely modelled as neutron stars, their correct description requires observations that probe their microphysics. Precise measurements of neutron star masses and radii by the NICER mission impose important constraints on the nuclear equation of state. We use state-of-the-art NICER measurements to date, including the most recent NICER...
In contrast to symmetric nuclear matter, which has been extensively studied in laboratory experiments, the matter inside neutron stars is highly isospin-asymmetric. We investigate the properties of strongly interacting matter under both symmetric and neutron star-like conditions to determine how electric charge neutrality and beta equilibrium influence the emergence of quark matter. In...
Some recent pulsar observations cannot naturally fit into the conventional picture of neutron stars: the compact objects associated with HESS J1731-347 and XTE J1814-338 have too small radii in the low-mass regime, while the secondary component of GW190814 is too massive for neutron stars to be compatible with constraints from the GW170817 event. In this study, we demonstrate that all these...
The post-merger phase of binary neutron star (BNS) mergers provides a unique opportunity to probe the equation of state (EOS) of dense matter at finite temperatures and out-of-equilibrium conditions. While current EOS constraints are dominated by cold inspiral signals, the hot and turbulent post-merger remnant encodes additional microphysics that remains unexplored. In this talk, I will...
Relativistic theories of dissipative fluids have long been known to suffer from pathologies. In recent years, a novel consistent formulation has been proposed: a well-posed and causal version of the relativistic Navier-Stokes equations. This theory provides a promising alternative to existing approaches and represents a key development in the foundations of relativistic hydrodynamics. We will...
It has long been hypothesized that above nucleaer density there might be a first-order transition from dense nuclear matter to quark matter, and hence that sufficiently heavy neutron stars might have quark matter cores. I will discuss some of the observable signatures of such a transition, including features of the mass radius relation and of the dynamics of neutron star mergers.
The quest for the maximum mass possibly achieved by a neutron star (NS) is an old venerable subject. 50+ years ago very low values were considered, mainly put forward from a theoretical point of view. The discovery of the binary pulsar PSR 1913+16, and later on, similar systems, allowed the determination of the mass with two or more decimal places, leading to the (now proved wrong) idea of a...
To more precisely constrain the Equation of State (EOS) of supradense neutron-rich nuclear matter, future high-precision X-ray and gravitational wave observatories are proposed to measure the radii of neutron stars (NSs) with an accuracy better than about 0.1 km. However, it remains unclear which aspects of the EOS will be better constrained and by how much. In this talk, we address this issue...
In QCD at nonzero isospin chemical potential and zero baryon chemical potential, the Dirac determinant is manifestly real and the system does not suffer from the sign problem. Consequently, one can perform lattice simulations and study pion condensation non-perturbatively. For this reason, pion condensation is an ideal test bed for low-energy models and theories. In this talk, I will discuss...
In this work we present hybrid stars with a superconducting quark matter core covered by a hyperonic nuclear matter. The deconfined phase is modelled within a chirally symmetric density functional approach and follows a first order phase transition via a Maxwell construction from a hyperonic DD2 equation of state. While the hadronic phase is fixed, the range of properties of the quark matter...
We construct the equation of state of hypernuclear matter and study the structure of neutron stars employing
a chiral hyperon-nucleon interaction of the Julich–Bonn group tuned to femtoscopic ¨ Λp data of the ALICE Collaboration, and ΛΛ and ΞN interactions determined from lattice QCD calculations by the HAL QCD Collaboration that
reproduce the femtoscopic ΛΛ and Ξ
− p data. We employ the...
Usually an effective model can be used at low energy, as long as it incorporates the relevant global symmetries, and the appropriate spectrum. Recently, however, it has been understood that the anomaly structure at large distance has to match that at short distances, known as the 't Hooft anomaly. I outline how for two flavors, a standard linear sigma model satisfies the 't Hooft anomaly,...
The bulk viscosity of dense quark matter (QM) plays a central role in the dynamics of neutron star mergers by controlling the dissipation of density oscillations. In the neutrino-transparent regime at low temperatures, this transport coefficient is governed by the non-leptonic weak process u + d ⇆ u + s, whose microscopic rate enters directly into the QM bulk-viscosity coefficient. Despite its...
I will discuss two aspects of compact stars with strong phase transitions. The first topic covers the structure and transport in the quark phases of compact stars, while the second deals with oscillation modes, boundary conditions at the interface between quark and hadronic phases, and the resulting oscillation modes.
Phase-quenched lattice simulations combined with perturbation theory are an emerging precision approach to determining the thermodynamics of QCD across a wide arc of the phase diagram where the strong coupling constant $\alpha_s$ remains small. In this talk we will introduce this phase-quenched approach to determining QCD's Equation of State (EoS) and argue that at sufficiently high...
In this work, we investigate the combined effects of pressure anisotropy and the inclusion of $\Delta$ resonances on the structural properties of compact stars. The study is performed within a relativistic hadronic framework that incorporates $\Delta$ baryons, where the equation of state (EoS) is constructed under the conditions of charge neutrality and $\beta$-equilibrium.
Pressure...
We aim to construct a unified Equation of State (EOS) capable of describing strongly interacting matter over a wide range of densities and temperatures. As a first step, we validate a Bayesian framework to rigorously incorporate astrophysical constraints into the cold, dense matter sector by considering Relativistic Mean-Field (RMF) models based on the exchange of $\sigma, \omega$, and $\rho$...
First-order phase transitions (1st-order PT) have been described in the magnetized Nambu--Jona-Lasinio (NJL) model when the anomalous magnetic moment (AMM) of quarks is included at zero temperature. The AMM quark effect is a consequence of the dynamical chiral symmetry breaking for massive quark in the nonperturbative Quantum Chromodynamics (QCD) regime. The Lattice QCD has described the...
Since Witten's proposal that symmetric deconfined u, d, and s quark matter might be the true absolute ground state, properties of quark stars have been extensively studied. By choosing an equation of state to describe the matter inside these stars, it is possible to solve the Tolman-Oppenheimer-Volkoff equations to obtain the mass and radius of the star. However, it has become clear that...
Dileptons are an important electromagnetic probe of hot and dense QCD matter and are widely regarded as an effective thermometer of heavy-ion collisions. In this work, we investigate dilepton production from an isospin-asymmetric hot and dense quark medium in order to explore the role of isospin imbalance on electromagnetic radiation. We focus in particular on modifications of the dilepton...
We construct an equation of state describing cold and dense matter in the core of neutron stars which includes an admixture of fermionic dark matter and incorporates nucleon effective masses derived from the relativistic Brueckner-Hartree-Fock (BHF) many-body approach within a relativistic mean-field model. Such a BHF-informed mixed-model approach increases stellar compactness, with...
The nuclear equation of state (EoS) of hot and dense matter plays a crucial role in understanding extreme astrophysical phenomena such as proto-neutron stars and binary neutron star (BNS) mergers. In BNS mergers that do not undergo prompt collapse, the post-merger remnant emits gravitational waves (GWs) with characteristic frequencies that encode valuable information about the underlying...
NICER has enabled mass–radius inferences for pulsars using pulse profile modeling (PPM), providing constraints on the equation of state (EOS) of cold, dense matter. To date, PPM and EOS inference have been carried out as two separate steps, with the former using EOS-agnostic priors. This approach has several drawbacks. Ideally, one would perform a fully hierarchical Bayesian inference where...
We investigate the impact of a stiff dark matter equation of state (EoS) on the structure and stability of neutron stars. For dark matter, we use bosonic, self-interacting scalar fields that generate ultra-compact boson stars with compactness exceeding 1/3. Varying the dark matter particle mass and stiffness shifts stellar configurations across distinct regions of the mass–radius diagram,...
The investigation of the composition and evolution of rotating protoneutron stars (PNSs) encodes crucial information about their observable signatures while providing knowledge to advance observational investigations. We study the microphysical and macroscopic evolution of rotating PNSs using a relativistic mean-field model with density-dependent couplings that include finite temperature and...
We develop a family of thermodynamic models for fluid systems based on a virial expansion of the internal energy in terms of the volume density. We prove that the models, formulated for systems with finite number of degrees of freedom $N$, are exactly solvable to any expansion order, as expectation values of physical observables are determined from solutions to nonlinear C-integrable PDEs of...
Isospin-equilibrating weak processes, called ``Urca" processes, are of fundamental importance in astrophysical environments like proto-neutron stars, neutron star mergers, and supernovae. In these environments, matter can reach high temperatures of in the MeV range and be subject to large magnetic fields. Previous studies on the effect of magnetic fields on isospin-equilibration processes...
We revisit the procedure to construct hybrid star EoS introduced in [1] in order to predict sequences of third families of compact stars, based on a non-local chiral quark model equation of state.
The goal of our study is to find a hybrid EoS which maximises the mass defect occurring in the accretion-induced transition from a hadronic star to its hybrid star twin configuration at 1.4...
We explore the changes in the masses of $D$ ($\bar D$) and $B$ ($\bar B$) mesons within isospin asymmetric $\Delta$ resonance matter, utilizing the chiral SU(3) hadronic model extended to the SU(4) and SU(5) sectors, respectively.
In addition to nucleons, the dispersion relations explicitly incorporate the interactions of $D$ and $B$ mesons with decuplet baryons ($\Delta^{++,+,0,-}$).
The...
I will present results from a recent model-agnostic analysis of the neutron-star-matter equation of state (EoS), informed by both ab-initio theoretical limits and astrophysical observations. Allowing for explicit first-order phase transitions, we systematically search for twin-star solutions, i.e. stars of equal mass but differing radii. We find that current constraints exclude all but two...
We constrain the equation of state (EoS) of dense nuclear matter using a relativistic mean-field (RMF) model within a Bayesian inference framework. Constraints from chiral effective field theory ($\chi$-EFT), the observed maximum neutron star mass ($\sim 2 M_\odot$), gravitational-wave data from GW170817, and NICER X-ray mass–radius measurements are combined to obtain posterior distributions...
Anisotropic phases are hypothesised to play a role in the small temperature and large chemical potential regime of the QCD phase diagram, making their existence in the core of neutron stars a concrete possibility. I will present the study of such a phase, the chiral density wave (CDW), defined as an anistropic chiral condensate that breaks spatial invariance. Within a mean-field nucleon-meson...
The microphysical composition of neutron star cores remains an unresolved problem, with current multimessenger data being insufficient to identify the correct description of dense nuclear matter. Quarkyonic matter, where baryons coexist with quarks deep in the Fermi sea, provides a framework which naturally reconciles the issue of massive neutron stars with relatively small radii, made...
Recent astronomical observations now tightly constrain the neutron-star EoS at intermediate densities, where matter may be neither purely hadronic nor weak-coupling quark matter. The favored “stiff” EoS can feature c_s^2 > 1/3 and even a negative (normalized) trace anomaly, challenging normal-phase NNLO pQCD predictions. Based on arXiv:2411.03781, I examine whether this tension can be...
The dynamics of binary neutron star (BNS) mergers are a unique environment to study the state of the matted at extreme conditions. In this context standard matter may undergo a phase transition to the state of deconfined quarks. It has been suggested that such a transition could leave observable imprints in the gravitational waves.
Nevertheless a similar behaviour is also reproduced by...
Neutron stars provide a high-density laboratory to test dark matter (DM) through its gravitational imprint on stellar structure and early evolution. In this talk, I present a unified set of results based on two-fluid modeling of cold neutron stars and evolving proto-neutron stars (PNSs), together with Bayesian model selection, multi-messenger constraints, and inverse parameter inference. For...
We present a unified overview of our recent studies on hybrid neutron stars constrained by NICER observations of PSR J0740+6620 and PSR J0030+0451, and by the low-mass compact object in HESS J1731–347. The hadronic phase is modeled using relativistic mean-field (RMF) equations of state: multiple RMF parameterizations with different interaction schemes are employed for the NICER analysis to...
In this work, we study the effects of ΛΛ-hyperons on neutron star properties employing a metamodel framework for the equation of state (EoS). Different choices for defining the hyperonic couplings with different levels of parametric freedom are discussed. In all models, the predicted NS maximum masses are reduced compared with the purely nucleonic composition as expected. In the case of...
We explore the role of color superconductivity in quarkyonic matter under the conditions of color and electric neutrality at β- and strong equilibrium, as relevant for neutron stars. By explicitly incorporating the color-superconducting pairing gap into the phenomenological model of a smooth transition from hadron to quark matter, we extend the known quarkyonic framework to include this...
Spin polarization in nuclear matter has been recognized as a key ingredient in the description of highly vortical systems formed in heavy-ion collisions, motivating detailed studies of the associated phase structure under extreme conditions. Recently, a spin potential, $\mu_{\Sigma}$, has been proposed in the context of lattice quantum chromodynamics (LQCD) as a quantity that measures the...
The behavior of strongly interacting matter at supranuclear densities and the nature of the associated deconfinement phase transition remain central open problems in nuclear physics and astrophysics. Such extreme conditions are realized in compact stars and related phenomena.
A key uncertainty concerns the energy per baryon of strange quark matter (SQM) relative to that of iron at zero...
The Bodmer-Witten hypothesis proposes that the presence of strange quarks decreases the binding energy of deconfined quark matter, allowing it to stabilize at high densities and favoring its appearance in the cores of compact objects. From this perspective, we investigate the conditions for stellar matter to be composed of strange quark matter. To do so, we employ the equiparticle model [1,...
In the magnetospheres of magnetars, strongly magnetized neutron stars, the magnetic field can be tens of times the critical Schwinger field $B_Q = m^2/e \approx 4.41\cdot 10^{13}$ G. In this strong field regime quantum electrodynamics (QED) becomes nonlinear, which has profound effects on the plasma dynamics of the magnetosphere. Most notably the energies of electrons and positrons become...
We derive a novel BPS bound from chiral perturbation theory minimally coupled to electrodynamics at finite isospin chemical potential. The BPS configurations represent magnetic multi-vortices with quantized flux supported by a superconducting current. The corresponding topological charge density is related to the magnetic flux density, but is screened by the hadronic profile. Such a screening...
Quantum chromodynamics (QCD) at finite baryon chemical potential remains hard to access by first-principles methods, making effective models an essential tool for exploring this region of the phase diagram. The quark–meson (QM) model provides a viable alternative to more established approaches such as the Nambu–Jona-Lasinio model, with the advantage that it can be matched to physical...
We present a comprehensive study of the thermal evolution of isolated neutron stars (NSs) based on a statistical analysis of cooling curves for five purely nucleonic equations of state (EoS). Cooling curves are computed using the publicly available NScool code and statistically compared with X-ray measurements of NS surface luminosities. The predicted luminosity for each source depends on its...
There exist multiple possible phases and microscopic structures of strongly interacting matter at extreme densities. Due to the the non-perturbative nature of QCD and lack of experimental data, there is significant uncertainty in our understanding of dense matter. By combining ground experiments and astronomical observations, we aim to construct a unified theoretical description starting from...
Astrophysical observations, such as neutron star mass–radius measurements inferred from Shapiro delay and X-ray observation of NICER, provide stringent constraints on the electrically neutral equation of state of strongly interacting matter relevant for compact stars. Incorporating heavy-ion collision data, in particular collective flow measurements from the STAR experiment, into the Bayesian...
Dark matter can strongly influence the internal structure of compact stars, reshaping the conditions for quark matter formation. In this work, we investigate its role in hybrid stars - objects that contain both hadronic and quark matter phases. Using a two-fluid approach, where normal matter and dark matter interact only through gravity, we demonstrate that dark matter raises the central...
Neutron stars provide unique laboratories for probing the physics of dark matter. I begin by reviewing the scenario proposed by Goldman and Nussinov, in which dark matter accumulates inside neutron stars and can trigger their collapse into solar-mass black holes. In this picture, dark matter cores form seed black holes that consume their host stars, producing solar-mass black holes beyond the...
The physics and astrophysics of compact stars experiences major advancements in quite diverse research areas opening up new directions for future investigations. Here, we report on recent extensions of compact star physics as the inclusion of color superconducting phases in neutron stars, the implications for proto-neutron star evolution, and the properties of compact stars with...
I will discuss recent developments in perturbative QCD at high baryon density, focusing on work towards completing the next-to-next-to-next-to-leading order pressure of cold quark matter. This result is expected to substantially improve theoretical control over the high-density regime of the neutron-star-matter equation of state (EoS), with direct implications for model-agnostic EoS inference....
Traditional Bayesian methods for inferring neutron star properties from observations begin by parameterizing equation of state (pressure-energy density) space in order to generate mass-radius information. These methods contain uncertainties from their arbitrary choices of equation of state models and their parameters. Those uncertainties are at least as large as the observational...
Astronomical observations of neutron stars provide data on the kilometer scale, while the nuclear interaction, fundamental for neutron stars, operates on the femtometer scale. To describe physical processes across so many orders of magnitude, one needs effective models. The inner crust of a neutron star is a complex system, where a lattice of nuclei strongly interacts with superfluid neutrons....
In this talk, I will discuss the predicition of finite temperature effects in dense matter from first principles. I will first discuss a simple, data-driven approach, the virial expansion, which allows for predictions of thermal properties across a wide range of densities and temperatures [1]. I will then discuss extensions to higher density with the Self-Consistent Green's Functions approach...
This study investigates how hyperons influence neutron star mergers using a large sample of equations of state. Our systematic analysis reveals that the presence of thermal hyperons induces a characteristic increase (several percent) in the dominant post-merger gravitational wave frequency.
Additionally, the presence of hypernuclear matter leads to lower average temperatures and higher...
The equation of state of deconfined strongly interacting matter at high densities remains an open question, with effects from quark pairing in the preferred color-flavor-locked (CFL) ground state possibly playing an important role. Recent studies suggest that at least large pairing gaps in the CFL phase are incompatible with current astrophysical observations of neutron stars. At the same...
The violation of the conformal limit for the speed of sound, $c_s^2=1/3$, has emerged as a critical feature of dense strongly interacting matter. Astrophysical observations — including gravitational-wave data from LIGO/Virgo and precise neutron-star radius measurements from NICER — indicate that the equation of state must undergo a rapid stiffening at intermediate baryon densities. This...
The composition of the core of neutron stars is still under debate. Agnostic descriptions of the equation of state are a powerful tool to determine the allowed region in the pressure-energy density or mass-radius space defined by observations and theoretical ab-initio calculations. These methods, however, cannot really give information on the neutron star composition. Understanding the...
As our understanding of cold, extremely dense matter grows, a multidisciplinary approach that combines recent progress in multimessenger neutron-star observations with theoretical knowledge of the equation of state (EoS) becomes increasingly essential. In this talk, I present a new physically motivated framework for encoding prior knowledge about dense matter arising from chiral effective...
In this contribution, I will start with an overview of different types of equation of state modelling in the Bayesian formalism, to demonstrate the িmpact of different experimental and observational constraints. Further, I will present equations of state at finite temperature obtained with Brussels-Skyrme-on-a-Grid (BSkG) energy density functionals developed at Brussels, which are unified...
Neutron star mergers create environments of hot, ultra-dense matter where the strong interaction governs the behavior but cannot be solved exactly or perturbatively using current methods. These collisions throw matter out of equilibrium and provide a unique laboratory to explore the phases and properties of dense matter. Simulations of neutron star mergers let us follow this matter in detail...
The constituent of the compact star matter is a one of the most fundamental and long-standing problems in nuclear- and astro-physics. The known properties of nuclear matter, together with astronomical observations, impose stringent and interconnected constraints on theoretical descrip-
tions. In this work, by using the most general quantum hadrodynamics model including σ, ω, ρ and a0, and...
We consider the renormalization group optimized perturbation theory (RGOPT) at next-to-next-to-leading order (NNLO) to evaluate the equation of state (EoS) for cold quark matter, incorporating full strange quark mass dependence. RGOPT entails an all-order RG resummation that generically reduces the renormalization scale uncertainties as compared to perturbative QCD (pQCD). We obtain...
The possibility of quark deconfinement in the interiors of neutron stars is investigated within the physics-informed Bayesian analysis of the observational data on neutron stars, which allows for distinguishing between the scenarios with quark cores and without them [1]. For this the recently proposed three-flavor nonlocal NJL model of quark matter with the scalar attractive, vector repulsive...
We present a field-theoretical description of quarkyonic matter in which quark–nucleon duality is implemented through ghost fields that compensate the extra nucleonic degrees of freedom. The framework reduces to a nucleon effective field theory at low density and describes the dynamical formation of a nucleon shell near the Fermi surface as baryon density increases. A phenomenological equation...
I will report on a novel, somewhat analytical way to produce equations of state (EOSs) that generate particular values of neutron star mass, radius, and tidal deformability. This is possible because our description for the EoS of dense matter can produce recurring regions, small areas where several EoSs cross in the mass-radius and mass-tidal deformability diagrams. We can place recurring...
Compact stars provide unique astrophysical laboratories for exploring the properties of dense nuclear matter. Observations of pulsars, together with recent gravitational-wave detections, have placed stringent constraints on the nuclear equation of state (EOS). Among various thermodynamic quantities, the speed of sound plays a central role in understanding the structure of neutron star and...
An accurate theoretical description of pressure of cold and dense quark matter is a key ingredient for constraining models of the equation of state of neutron stars at large baryon density. While soft-gluon logarithmic corrections at third order in the strong coupling constant have been obtained recently, the corresponding hard contribution has so far only been evaluated partially.
In this...
We present an analysis based on Finite Energy Sum Rules (FESR) formulated in a general in-medium framework. This approach allows us to investigate the in-medium evolution of parameters in both the nonperturbative sector of QCD and the hadronic sector. We discuss the methodology, emphasizing its advantages as well as its main challenges and limitations. As specific high-density applications, we...
This work presents the rotational properties of self-bound quark stars within general relativity using two representative quark matter equations of state: the vector MIT bag model and the density-dependent quark mass model. Uniformly rotating equilibrium sequences are constructed to explore their mass--radius relations, moments of inertia, quadrupole moments, surface redshifts, Keplerian...
