Second Holography and Dense Matter Workshop (featuring Meeting of Compact Objects working group of the GW GdR)

17 sept. 2025, 09:30 → 19 sept. 2025, 18:00 Europe/Paris

APC Laboratory (Paris)

The workshop will focus on applications of the gauge/gravity duality to compact objects and gravitational waves. The purpose is to bring together  researchers  from AdS/CFT, QCD and GR to have an open an informal discussion, point out the interesting open problems and discuss where holography can be useful and how it can interface with other techniques.

The first day of the the workshop will be dedicated to the 2025 Meeting of the Working Group "Neutron Stars, Supernovas and Heavy Element Production" , part of the French Gravitational Waves Network (GdR-GW). Information about this specific event can be found here. 

The workshop will be folllowed, on Saturday September 20, by a program to commemorate our colleague and friend Umut Gûrsoy, who was an integral part of the collaboration that initiated this workshop. The event will take place at APC. More information will be avaliable soon on this page

Workshop Organizers: Elias Kiritsis, Francesco Nitti, 

GDR Meeting Organizers: Anthea Fantina, Jerome Novak

SPONSORS

 

mer. 17 septembre

09:30 → 10:00 Welcome/Registration - Coffee

10:00 → 12:30 Compact Objects WG (GdR on Gravitational Waves)

10:00 Pairing beyond BCS

Neutron pairing and superfluidity have important consequences for pulsar glitches and neutron star cooling and have been studied for decades. Nevertheless, the density dependence of the pairing gap and of the critical temperature are still a matter of debate, mostly because of medium polarization (screening) effects. After a brief review of recent quantum Monte-Carlo results and screening calculations, I will discuss a new method similar to Bogoliubov many-body perturbation theory to include consistently different types of in-medium effects, including not only screening but also the effective mass, vertex corrections, and wave-function renormalization.

Orateur: Michael Urban

10:40 Superfluid fraction in the inner crust of neutron stars

The question of whether there are enough superfluid neutrons in the inner crust of neutron stars to explain pulsar glitches remains a topic of debate. Previous band structure calculations suggest that the entrainment effect significantly reduces the superfluid density. In this talk, HFB calculations are presented and a new derivation of the BCS expression for the superfluid density is given. These results suggest that about 90% of the neutrons are effectively superfluid, making possible to explain glitches with models that involve the crust only.

Orateur: Giorgio Almirante

11:30 Bayesian Inferences of Neutron Stars' Properties: When Astro and Nuclear physics go hand in hand

Understanding the equation of state (EoS) of dense matter is crucial in simulations of neutron star structure and dynamics, as well as for the interpretation of astrophysical data. In particular, the EoS of cold neutron stars defines their stationary structure, from their masses and radii to their tidal deformabilities and the oscillation spectra of their normal modes. However, a first-principles derivation remains out of reach across the density range relevant to neutron stars. To address this, Bayesian methods have been developed to explore nucleonic, hadronic, or agnostic EoS models constrained by theoretical, nuclear, and astrophysical inputs.
Gravitational wave signals from neutron stars encode information about their structure and, indirectly, their internal composition, offering a unique observational window for next-generation detectors like the Einstein Telescope. For this reason, Bayesian frameworks will be essential for extracting information from these observations and assessing the residual uncertainties in the EoS. In this seminar, I will present a causal "meta-model" for the EoS that systematically explores the nucleonic hypothesis and discusses its implications for neutron star structure and oscillation modes.

Orateur: Gabriele Montefusco

12:30 → 14:00 Lunch

14:00 → 15:20 Compact Objects WG (GdR on Gravitational Waves)

14:00 Simulations of rotating neutron star oscillations with Roxas

Relativistic hydrodynamics has undergone major development over the past three decades, notably through high-resolution shock-capturing schemes enabled by conservative formulations of the relativistic fluid equations.
Simultaneously, advances in Numerical Relativity have allowed fully relativistic simulations of compact binary systems, particularly binary neutron stars, up to merger.
The GW170817 event marked a turning point, demonstrating the scientific potential of multi-messenger observations.
Post-merger remnants are expected to emit gravitational waves in the kHz band before collapsing into black holes, offering an additional probe of the dense-matter equation of state.

We developed ROXAS, a numerical code to evolve fast rotating isolated neutron stars and extract their oscillation modes.
A new hydrodynamic formalism was introduced, based on primitive rather than conserved variables, avoiding costly recovery procedures.
We employed multi-domain pseudospectral methods along with filters to mitigate aliasing.
The framework was validated on the scalar wave equation, then extended to self-gravitating configurations in both Newtonian and relativistic regimes.
A key feature is accurate stellar surface tracking. We extracted radial and non-axisymmetric modes of polytropic rapidly rotating stars.
More recently, the code was extended to differentially rotating configurations, a crucial step towards a realistic description of post-merger remnants.

Orateur: Gaël Servignat

14:40 Muonic opacities in (hot) dense matter

Neutrinos play an important role in compact star
astrophysics: neutrino-heating is one of the main ingredients in
core-collapse supernovae, neutrino-matter interactions determine the
composition of matter in binary neutron star mergers and have among
others a strong impact on conditions for heavy element nucleosynthesis
and neutron star cooling is dominated by neutrino emission except for
very old stars. Many works in the last decades have shown that in
dense matter medium effects considerably change the neutrino-matter
interaction rates, whereas many astrophysical simulations use analytic
approximations which are often far from reproducing more complete
calculations. In this talk I will present a scheme which allows to
incorporate improved rates into simulations and show in particular results for charged current rates with muon (anti-) neutrinos.

Orateur: Micaela Oertel

15:20 → 15:50 Coffee break

15:50 → 16:50 Holography & Dense Matter

15:50 Neutron Star Mergers: Multimessenger Probes of Extreme Matter

Binary neutron star mergers are critical for understanding the dynamics of dense matter, the origin of gravitational waves, and the formation channels of the heaviest elements through the r-process. I will review how long-lived remnants can act as central engines for multimessenger observations. I will then discuss how we can identify phase transitions within neutron stars or their remnants using such observations. Phase transitions alter the system’s dynamics and can produce distinct observable signatures, potentially detectable with next-generation facilities and observatories. These signatures can be used to probe matter at supranuclear densities and to test fundamental physics.

Orateur: Pablo Bosch

16:50 → 17:30 Discussion

17:30 → 19:30 Wine & Cheese

jeu. 18 septembre

09:00 → 10:00 Holography & Dense Matter

09:00 QCD in the cores of neutron stars

Neutron stars are the densest astrophysical objects in our universe, reaching densities as high as those realized in ultrarelativistic heavy-ion collisions at the LHC. In these collisions ordinary nuclear matter melts into a new phase of elementary particle matter, quark matter. This naturally raises the question: does quark matter also exist inside neutron stars? The rapid advancement in neutron-star observations in combination with state-of-the-art QCD calculations is providing us with an unprecedented view of the extreme matter deep in the cores of the stars. In my talk, I describe how recent advancements in theory of superdense matter inform us about what lies in the centers of neutron stars and how different constraints point to the existence of quark matter cores in large neutron stars.

Orateur: Aleksi Kurkela

10:00 → 10:30 Coffee break

10:30 → 12:30 Holography & Dense Matter

10:30 A new perturbative approach to dense matter: thermal Loop Tree Duality

It is well established that the high-density constraint from perturbative QCD, though directly applicable only at densities exceeding those inside neutron stars, plays a central role in the model-independent inference of the neutron-star equation of state (EoS). The completion of the next-to-next-to-next-to-leading order (N3LO) pressure of dense quark matter is expected to significantly sharpen this constraint. In this talk, I will discuss a recent methodological breakthrough: the extension of the Loop Tree Duality framework of vacuum QFT to nonzero temperature and density. This advance removes the last major conceptual obstacles to the N3LO calculation and opens the door to a wide range of high-order perturbative computations in thermal field theory.

Orateur: Aleksi Vuorinen

11:30 Microscopic description of critical bubbles

First-order phase transitions in the early Universe and inside neutron stars could leave observable imprints, including a stochastic gravitational wave background or characteristic waveforms in specific frequency ranges. Predicting these signals requires precise understanding of critical bubbles, often addressed using effective field theory or perturbation theory. In this talk, I will present the first explicit computation of fully-backreacted critical bubbles in a strongly coupled system, using gauge/gravity duality.

Orateur: Javier Subils

12:30 → 14:30 Lunch

14:30 → 16:30 Holography & Dense Matter

14:30 Forming and Evaporating Black Holes from Strongly Coupled Matter

Holography has provided valuable insights into the time evolution of strongly coupled gauge theories in a fixed spacetime. However, this framework is insufficient if this spacetime is dynamical. We present a scheme to evolve a four-dimensional, strongly interacting gauge theory coupled to four-dimensional dynamical gravity in the semiclassical regime. We apply this framework to the description of the gravitational collapse and the subsequent formation of a black hole at the boundary. In the bulk, this corresponds to the formation of a black funnel. We will discuss how black hole evaporation may also be seen in this framework. If time permits, we will peek behind the horizon and study the so-called BKL dynamics near the singularity.

Orateur: David Mateos

15:30 From Ringdowns to Critical Points: Constraining Dense Matter with Neutron Stars

Astrophysical measurements, especially gravitational waves (GW) from binary neutron star (BNS) mergers, are promising tools to infer the largely unknown properties of dense QCD matter at densities that are presently inaccessible to first-principles theory and laboratory experiments.
In the first part of my talk, I will show how the correlation between the ratio of energy and angular-momentum losses in the late-time ‘long-ringdown’ of the BNS post-merger GW signal and the equation-of-state (EOS) properties of neutron-star cores reduces EOS uncertainties at densities several times the nuclear saturation density.
In the second part, I will present how nuclear-theory and astrophysical constraints, when applied to a large family of holographic V-QCD models, enable Bayesian inference of the conjectured QCD critical endpoint.

Orateur: Christian Ecker

16:30 → 17:00 Coffee break

17:00 → 18:00 Discussion

20:00 → 23:00 Dinner

ven. 19 septembre

09:00 → 10:00 Holography & Dense Matter

09:00 On effective action from holography

First-order phase transitions in strongly coupled systems are of central interest, both for fundamental physics and for their potential cosmological signatures such as gravitational waves. In this talk, I will explain how holography provides a framework to study critical bubble nucleation. I will describe two complementary approaches: constructing a two-derivative effective action and solving directly for bubble solutions in the bulk. I will conclude with an outlook on open directions and future applications.

Orateur: Niko Jokela

10:00 → 10:30 Coffee break

10:30 → 12:30 Holography & Dense Matter

10:30 Phases of dense QCD and neutron stars from holography

I will review recent progress in the Witten-Sakai-Sugimoto model to understand strongly coupled phases of matter and their application to neutron stars. I will start from pion condensation at finite isospin chemical potential, where lattice QCD results are available to test the holographic results. Baryonic matter can be added to predict the phase structure at finite baryon and isospin chemical potentials, which is poorly understood from first principles. This setup can also be used to construct neutron stars entirely from the holographic model, and I will discuss the comparison with astrophysical data. Finally, I will briefly present a very recent study on improving the holographic description of nuclear matter.

Orateur: Andreas Schmitt

11:30 Towards an improved holographic equation of state for hot and dense QCD

I discuss recent studies which aim at improving the description of the equation of state and the phase diagram of QCD by using the holographic V-QCD framework. Such improvements include turning on a finite strange quark mass, comparison to lattice QCD data at finite mass, developing a simple model for a color superconducting phase, and analyzing isospin asymmetry in holographic nuclear matter.

Orateur: Matti Järvinen

12:30 → 14:30 Lunch

14:30 → 15:30 Holography & Dense Matter

14:30 Hall droplet baryons in holographic QCD

I will present a first-principle construction of baryons as quantum Hall droplets in single-flavor holographic QCD, focusing on the top-down Witten-Sakai-Sugimoto model. The baryons are described as charged D6-branes with a circular boundary on a flavor D8-brane in the Type IIA background sourced by N D4-branes wrapped on a circle. The holographic description allows to calculate their properties, such as spin, mass and size. Other related objects with baryonic charge, such as vortons, domain walls with holes, and “sandwich vortons”, will be briefly discussed.

Orateur: Francesco Bigazzi

15:30 → 16:00 Coffee break

16:00 → 17:00 Holography & Dense Matter: Holography and Dense Matter

16:00 From top-down to bottom-up: a new construction for holographic color-superconductivity

Taking inspiration from top-down constructions, I revisit the problem of constructing a color superconducting phase in bottom-up holography. The model introduced describes the five-dimensional dynamics of a scalar field dual to a chiral symmetry breaking condensate, which is coupled to the RR 4-form. The coupling is given by a topological WZ term, which is elegantly responsible both for scalar condensation at high density, and color Higgsing in the condensed phase. This construction realizes both properties at finite density and Higgsing fraction.

Orateur: Edwan Préau

17:00 → 18:00 Discussion