Registration/Opening Room: Room Galileo Galilei (131), ground floor

Collisions of Einstein-Maxwell-Scalar Black Holes

• Robin Croft (Sapienza University of Rome)

Room: Room Galileo Galilei (131), ground floor Black Holes in EMS theory can support scalar hair. This feature gives rise to a double peak in the effective potential which can support gravitational echoes. In this topic I will discuss ongoing work in progress towards the simulation of headon collisions of EMS black holes with numerical relativity.

Echoes of the black hole microstructure

• Alexandru Dima (Università Sapienza)

Room: Room Galileo Galilei (131), ground floor The LIGO-VIRGO-KAGRA observations are so far compatible with the Kerr black hole paradigm, though they cannot rule out entirely the existence of black hole mimickers. These are ultracompact objects that reproduce some observable properties of black holes, while possibly predicting characteristic signatures such as non-trivial tidal deformability and/or repeated gravitational wave echoes in the ringdown. Interesting families of ultracompact objects called "topological solitons" consist in regular and horizonless solutions to five-dimensional Einstein-Maxwell theory, which resemble static black holes upon reduction to four dimensions. These mimickers can also serve as a classical toy model for the quantum black hole microstates constructed in the ``fuzzball” program in the context of string theory. In this talk we will present our latest results concerning the linear response of topological solitons, with a focus on their characteristic spectrum, gravitational wave echoes and (linear) stability.

Hyperbolicity and minimum black hole masses in scalar Gauss-Bonnet gravity

• Dario Rossi (Pisa University)

Room: Room Galileo Galilei (131), ground floor Scalar Gauss–Bonnet gravity is a promising class of extensions of General Relativity (GR), providing a simple test-bed for possible deviations in the gravitational wave signals from coalescing binary black holes (BHs). These theories are characterized by a scalar field non-minimally coupled to the Gauss–Bonnet invariant through a coupling function, and naturally encounter a breakdown of the effective description when the BH radius becomes comparable to, or smaller than, the characteristic length-scale of the GR correction. This breakdown manifests itself either as the existence of a minimum BH mass or as a loss of hyperbolicity of the perturbed field equations. We investigate how different choices of the coupling function affect the hyperbolicity properties of the theory and the resulting observable quantities, such as the scalar charge.

Asymmetric binaries and scalar fields on generic orbits

• Sara Gliorio (Gran Sasso Science Institute)

Room: Room Galileo Galilei (131), ground floor The LISA satellite, recently adopted by ESA, is set to open a new gravitational wave window, targeting sources that are inaccessible to ground-based detectors like LIGO and Virgo. Extreme mass-ratio inspirals (EMRIs), consisting of a massive black hole and a stellar-mass secondary, are among the most distinctive members of this new class of binaries. The inspiral phase of these systems falls within the mHz regime of the LISA band. Depending on their mass ratios, EMRIs will be continuously observed over long periods, ranging from months to years. This prolonged evolution is key to enabling exceptionally precise measurements of source parameters and performing stringent tests of gravity. In this talk, I consider a beyond-GR scenario in which the secondary compact object carries a scalar charge. I show how to compute the scalar energy fluxes emitted by such EMRIs when the secondary follows generic orbits—both eccentric and inclined. This computation is carried out using a new C++ code, MEW (Modified EMRI Waveforms), which calculates both scalar and gravitational energy fluxes. I present the first results obtained with this code. The total energy flux is decomposed into a sum over radial, polar, and azimuthal modes. I show the spectra of these scalar fluxes, a crucial first step toward efficient mode summation. Additionally, I present preliminary results on the detectability of the scalar charge in asymmetric binaries, and discuss how constraints on this scalar charge depend on the mass ratio.

Constraining Einstein-Aether gravity with binary pulsar observations

• Massimo Vaglio (SISSA)

Room: Room Galileo Galilei (131), ground floor We constrain Einstein-Aether gravity, a leading alternative to General Relativity that introduces a preferred-frame via a dynamical time-like vector field, using high-precision pulsar timing observations from EPTA. Our analysis incorporates both conservative and dissipative first post-Newtonian corrections and utilizes full times of arrival from multiple pulsars, processed with Vela.jl, a Bayesian tool based on PINT. The model spans many parameters, including binary component masses, Einstein-Aether coupling constants and center-of-mass velocity components. We extract posteriors on post-Keplerian parameters from timing data and employ normalizing flows combined with resampling techniques to map these onto constraints for the fundamental theory parameters. Our results demonstrate the power of pulsar timing to probe deviations from General Relativity and provide insights into the phenomenology of preferred-frame gravity.

Characterisation of circular light rays in a plasma in Scalar-Tensor-Vector Gravity

• Sabatino Fatigati (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor In this talk, I will present a study of photonic circular orbits in plasma-rich environments around black holes in Scalar-Tensor-Vector Gravity, based on a potential-based approach originally developed in General Relativity. This work extends the analysis of light propagation to the strong-field regime of Scalar-Tensor-Vector Gravity and provides a consistent theoretical framework within this context. It also allows one to assess possible deviations from General Relativity or, at least, to place constraints on the parameter that characterises this alternative theory of gravity.

There and back again: outspiralling motion in non-Kerr compact objects

• Manuel Goncalo Oliveira Mariano

Room: Room Galileo Galilei (131), ground floor In Keplerian dynamics, a test body orbiting a point particle in circular motion has a monotonically increasing frequency, with decreasing radius. If a dissipative channel is introduced, such as gravitational wave (GW) emission, (say) under the quadrupole approximation, the corresponding GW strain has an ever increasing frequency with time. A similar statement holds for equatorial motion of a test particle on the Kerr manifold, except such inspiral is cut off at the ISCO, wherein stable circular orbits cease to exist and a plunge is expected. We analyse circular timelike orbits in generic spinning spacetimes and study the conditions in which exotic motion can occur, arising from non-Kerr features. In particular, we derive conditions under which an inspiral towards a compact object is naturally followed by an outspiral motion, and give concrete examples, as well as the corresponding GW phenomenology. This analysis serves both as a theoretical exploration of non-Kerrness and as an example of a concrete smoking gun of exotic spacetimes.

Inspiral tests of General Relativity with the Einstein Telescope

• Joachim Pomper (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor In this talk, we present forecasts for the accuracy with which GR-deviations of the Post-Newtonian inspiral coefficients can be constrained using third-generation ground-based interferometers, focusing on Einstein Telescope (ET) and binary black hole mergers. Given the expected high detection rate and the resulting computational infeasibility of a full Bayesian analysis, we adopt a Fisher matrix approach, simulating parameter estimation in an idealized observation scenario. This allows us to study populations of tens of thousands of compact binary coalescences, as expected for ET, with feasible computational efforts.

Doughnuts in the sky?

• Gerardo García Moreno (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor From a purely geometric (kinematic) perspective, black holes in four dimensional spacetimes can have event horizons with arbitrary topologies. It is only when energy conditions are imposed that the horizon’s topology is constrained to be that of a sphere. Despite this, exploring exotic horizon topologies remains theoretically intriguing since it allows to unveil structural aspects of General Relativity and gain intuition on energy condition violations. In the axisymmetric case, besides the well-known spherical topology, only a toroidal topology is consistent with the symmetry. Complete solutions, describing the entire exterior region of such toroidal black holes without singularities, have not been reported yet. To the best of our knowledge, the construction we present here is the first explicit example of a toroidal black hole solution in four spacetime dimensions that is free of singularities in the external region.

pTEOBResumS: a parametrized eccentric, precessing GW model for theory-agnostic tests of GR

• Danilo Chiaramello (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor Gravitational waves (GWs) from binary black hole (BBH) mergers allow us to test General Relativity (GR) in the strong-field, high-curvature regime. In this work we present pTEOBResumS, a new parametrized, spin-precessing inspiral-merger-ringdown model for null tests of GR that also incorporates orbital eccentricity, which had thus far been neglected by existing GW-based frameworks. Building on the effective-one-body model TEOBResumS-Dalí, pTEOBResumS introduces parametrized deviations from GR both in the inspiral and the merger-ringdown regimes. The model is validated via parameter estimation (PE) of synthetic signals, including from numerical simulations of BBHs and exotic systems, demonstrating its self-consistency and ability to recognize beyond-GR effects, as well as showcasing the impact of eccentricity on tests of GR. The model is then used to re-analyze of a set of BBH events observed by the LIGO-Virgo-KAGRA collaboration, assuming either a quasi-spherical, spin-precessing or a non-precessing, eccentric hypothesis, while also searching for deviations from the GR expectation of the remnant BH's properties. We discuss the results obtained from the analyzed sample, which shows no statistically significant evidence for violations of GR, and delve into particularly interesting single events, such as GW200129, for which we infer support for orbital eccentricity.

Quasinormal ringing of Kerr black holes: excitation coefficients for equatorial inspirals from the ISCO

• Matteo Della Rocca (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor The remnant of a black hole binary merger settles into a stationary configuration by “ringing down” through the emission of gravitational waves that consist of a superposition of damped exponentials with discrete complex frequencies – the remnant black hole’s quasinormal modes. While the frequencies themselves depend solely on the mass and spin of the remnant, the mode amplitudes depend on the merger dynamics. We investigate quasinormal mode excitation by a point particle plunging from the innermost stable circular orbit of a Kerr black hole. Our formalism is general, but we focus on computing the quasinormal mode excitation coefficients in the frequency domain for equatorial orbits, and we analyze their dependence on the remnant black hole spin. We find that higher overtones and subdominant multipoles of the radiation become increasingly significant for rapidly rotating black holes. This suggests that the prospects for detecting overtones and higher-order modes are considerably enhanced for highly spinning merger remnants.

Modelling ring-down with greybody factors

• Romeo Felice Rosato (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor Post-merger ringdown signals in compact-binary coalescences encode characteristic imprints of the underlying spacetime, traditionally described in terms of quasinormal modes. Motivated by a recently uncovered connection between post-merger black-hole signals and greybody factors, we investigate greybody factors as gravitational observables, providing a complementary approach to QNMs. We show that greybody factors remain stable under small perturbations and are not affected by several ambiguities that limit the robustness of a purely QNM-based description. We then test this greybody factor–based framework against a broad set of numerical-relativity waveforms from the SXS catalog, focusing on comparable-mass binaries with generic spins. By extracting and fitting the frequency-domain ringdown signal across the catalog, we study how the model parameters vary throughout the binary parameter space. This allows us to assess the domain of validity of the approach, identify systematic trends linked to binary properties, and explore its potential as an alternative parametrization of ringdown signals for gravitational-wave data analysis.

Gravity with higher-curvature terms and second-order field equations: f(R) meets Gauss-Bonnet

• Andrea Pierfrancesco Sanna (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor General Relativity is expected to break down in the high-curvature regime. Beyond effective field theories with higher-order operators, it is crucial to identify consistent nonperturbative theories including higher-curvature terms. Two well-studied cases are f(R) gravity and Einstein–dilaton–Gauss–Bonnet (EdGB) gravity. The former shares GR’s vacuum solutions, while the latter faces well-posedness issues in the strong-coupling regime. We show that combining them yields genuinely new phenomena beyond simple superposition. This framework naturally extends EdGB gravity to include arbitrary higher-curvature terms. Focusing on quadratic and quartic corrections, we find: (i) black holes are modified by f(R) terms, unlike in pure EdGB; (ii) the solutions preserve key nonperturbative EdGB features, such as minimum mass and multiple branches; (iii) a mechanism suppresses Ricci-scalar divergence in the interior; yet (iv) the singularity and elliptic regions remain similar to EdGB. Thus, adding higher-order terms does not resolve the theory’s ill-posedness at the nonperturbative level (based on: arXiv:2510.17965).

Excitation of f-modes in dynamical capture neutron star binaries

• Nicholas Loutrel (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor When in dynamical capture binaries, neutron stars will necessarily undergo dynamical tidal interactions, due to the tidal field becoming sharply peaked around pericenter passage. This results in two effects: 1) a slow evolution of the neutron star's tides, proportional to the perturbing tidal field, and 2) the excitation of fluid oscillation modes inside of the star. Using analytic techniques, I will discuss the impact of these two effects on the gravitational wave emitted by dynamical capture binaries within the effective fly-by (EFB) framework, specifically focusing on the change in burst arrival times.

Binary Neutron Star Merger Simulations with Microphysical Equation of State using Spritz

• Fatemeh Hossein Nouri (University of Milano-Bicocca)

Room: Room Galileo Galilei (131), ground floor We present results from new simulations performed with Spritz, a general-relativistic magnetohydrodynamics (GRMHD) code built on the Einstein Toolkit infrastructure and developed for high-precision studies of binary neutron star (BNS) mergers with realistic microphysical equations of state (EOS). These simulations build upon recent developments in the code, which enable the evolution of magnetized systems with tabulated EOS. We model both magnetized and non-magnetized equal-mass binaries employing two EOS inspired by the GW170817 event. Our analysis focuses on the gravitational-wave (GW) emission, as well as on the potential formation of relativistic jets and neutron-rich outflows.

Muonic Interactions in Binary Neutron Star Mergers

• Raffaele Ferrari (Trento University)

Room: Room Galileo Galilei (131), ground floor An accurate inclusion of weak processes and neutrino radiation in the modeling of BNS mergers is essential to reach a correct description of these systems and their observable signatures. Current simulations usually include only a minimal set of neutrino interactions, often neglecting the presence of heavy leptons, as well as their antiparticles. Due to the increase in complexity and computational costs related to their consistent inclusion inside simulations, muons have been neglected so far in BNS merger models. However, recent works have confirmed their potential relevance in this context. In this talk, the most important muon-neutrino interactions are presented and characterized, with the purpose of addressing the question of whether the inclusion of such interactions can be relevant during a BNS merger.

Fundamental physics in simulations of extreme astrophysical explosions

• Andrea Celati (Florence University)

Room: Room Galileo Galilei (131), ground floor The gravitational collapse of massive stars at the end of their life leads to powerful supernova explosions that produce new stellar-sized compact objects, regulate the dynamics of their host galaxies, and produce new heavy elements that contribute to the cosmic chemical evolution. In presence of fast rotation and strong magnetic fields, such explosions reach extremely high energies that can explain sources such as hypernovae and long gamma-ray bursts, which are the most violent transients observable in the Universe. The goal of our work is to produce state-of-the-art axisymmetric models of magnetorotational explosions rely on nuclear equations of state (EoS) whose microphysical uncertainties remain one of the dominant sources of variability in the predicted dynamics. In particular, differences in the stiffness, composition, and finite-temperature behavior of the EoS can significantly affect the collapse, bounce, and jet-launching phases, even when employing advanced relativistic magnetohydrodynamic (RMHD) treatments, neutrino-matter interactions, and general-relativistic corrections. In particular, we aim to quantify the impact of the chosen EoS for dense matter on the explosion dynamics and the resulting multi-messenger emission, including neutrinos and gravitational waves. We use the Aenus-Alcar RMHD code to perform simulations for different EoS. Each simulation starts from the same initial condition, using a standard pre-supernova model with solar metallicity and a zero-age main sequence mass of 20 solar masses, endowed with a dipolar magnetic field configuration. Variations in the stiffness, nuclear interactions, and treatment of nuclei among the different EoS lead to significant differences in the explosion dynamics, particularly in the bounce time, moment of inertia, explosion energy, and mass of the ejected material. In addition, we test different numbers of energy bins in the spectral neutrino-transport scheme to assess the sensitivity of neutrino heating and emission to the energy resolution. We also investigate the impact of different pseudo-Newtonian gravity treatments, evaluating how alternative approximations to the gravitational potential influence collapse dynamics, shock propagation, and jet formation. These additional tests provide a more complete picture of the numerical and physical uncertainties affecting magnetorotational supernova models.

Gravitational signatures beyond Newton: Exploring hierarchical three-body dynamics

• Pietro Farina (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor Hierarchical three-body systems offer a compelling framework to explore the subtle interplay between Newtonian and relativistic gravitational effects in astrophysical environments. In this work, we investigate post-Newtonian corrections to the periastron shift within such systems, focusing on the impact of orbital eccentricity. Modeling the secondary body’s influence as a quadrupolar perturbation, we compare Newtonian, Schwarzschild, and post-Newtonian quadrupolar contributions to orbital precession. Our analysis demonstrates that Newtonian quadrupolar effects could be observable, for a long monitoring time, in the orbit of the S87 star around Sagittarius A* if an intermediate-mass black hole is present, under the assumptions of our model. Additionally, post-Newtonian quadrupolar corrections may influence the dynamics of small Solar System bodies in the presence of massive companions. Although the predicted effects are minute and require long monitoring periods to be measurable, our analysis clarifies how relativistic corrections enter the dynamics of the third body and outlines the conditions under which future observations could reveal them.

Eccentricity distribution of extreme mass ratio inspirals

• Davide Mancieri (University of Trento / Milano-Bicocca)

Room: Room Galileo Galilei (131), ground floor We present realistic eccentricity distributions for extreme mass ratio inspirals (EMRIs) forming via the two-body relaxation channel in nuclear star clusters, tracking their evolution up to the final plunge onto the central Schwarzschild massive black hole (MBH). We find that EMRIs can retain significant eccentricities at plunge, with a distribution peaking at $e_\mathrm{pl} \approx0.2$, and a considerable fraction reaching much higher values. In particular, up to $20\%$ of the forming EMRIs feature $e_\mathrm{pl} > 0.5$ for central MBH masses $M_\bullet$ in the range $10^5 \, \mathrm{M_\odot} \leq M_\bullet \leq 10^6 \, \mathrm{M_\odot}$, partially due to EMRIs forming at large semi-major axes and ``cliffhanger EMRI'', usually neglected in literature. This highlights the importance of accounting for eccentricity in waveform modeling and detection strategies for future space-based gravitational wave observatories such as the upcoming Laser Interferometer Space Antenna (LISA). Furthermore, we find that the numerical fluxes in energy and angular momentum currently implemented in the FastEMRIWaveforms (FEW) package may not adequately sample the full parameter space relevant to low-mass MBHs ($M_\bullet < 10^6 \, \mathrm{M_\odot}$), potentially limiting its predictive power in that regime. Specifically, for $M_\bullet=10^5 \, \mathrm{M_\odot}$ we find that about $75\%$ ($50 \%$) of EMRIs at 2 years (6 months) from plunge fall outside the currently available flux parameter space. Our findings motivate the development of extended flux grids and improved interpolation schemes to enable accurate modeling of EMRIs across a broader range of system parameters.

Interpreting the spin properties of GWTC-4.0 events from first principles

• Francesco Crescimbeni (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor We investigate the differences between several proposed formation scenarios for binary black holes (BBHs), including isolated stellar evolution, dynamical assembly in dense clusters and active galactic nuclei (AGN) disks, and primordial BHs. Our approach exploits the distinguishing spin features of each formation channel, and focuses on the models' predicted correlations between the spin magnitudes (and orientations) with their mass. Using hierarchical Bayesian inference on the recent GWTC-4.0 dataset, we compare these features across all models and assess how well each scenario explains the data. We find that the data strongly favor the presence of a positive correlation between mass and spin magnitude, in agreement with previous studies. Furthermore, we find that the hierarchical scenario provides a better fit to the observations, due to the inclusion of second-generation mergers leading to higher spins at larger masses. The current dataset does not allow us to distinguish orientation properties: cluster (random orientations) and AGN (aligned orientations) scenarios show similar Bayesian evidence. Finally, we find that the mass–spin correlation predicted by the primordial scenario gives a poor fit to the data and can only account for a subset of the observed events.

A multi-parameter expansion for Extreme Mass Ratio Inspirals in astrophysical environments

• Sayak Datta (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor Asymmetric binaries have attracted increasing attention as golden sources for probing the astrophysical environment in which they evolve. Black holes do not exist in isolation; they inhabit diverse environments where particles and fields, possible of unknown nature, interact with each others and with compact objects. For example, massive BHs are surrounded by accreting matter, dark matter halos, which may consist of exotic fields or beyond-standard-model candidates. Moving beyond vacuum general relativity presents significant challenges due to the lack of relativistic solutions describing BHs embedded in matter and the complexities introduced by metric-matter couplings. As a result, modeling environmental effects on extreme mass ratio inspirals often relies on post-Newtonian approaches, though fully relativistic descriptions remain key to confidently extract small deviation from vacuum predictions. I along with Prof. Andrea Maselli, develope a multi-parameter framework to describe environmental effects on BHs, where the surrounding matter is modeled as a fluid stress-energy tensor. We adopt a general anisotropic prescription, incorporating both radial and tangential pressure components. We compute axial and polar perturbations at first order in the mass ratio, induced by the secondary on the geometry of a non-rotating BH embedded in an environment. We discuss practical, ready-to-use expressions for computing gravitational and fluid perturbations, as well as the resulting GW emission, as functions of environmental parameters and the secondary’s orbital motion.

When Vacuum Breaks

• Lorenzo Copparoni (SISSA)

Room: Room Galileo Galilei (131), ground floor Gravitational-wave signals are typically interpreted under the vacuum hypothesis, i.e. assuming negligible influence from the astrophysical environment. This assumption is expected to break down for low-frequency sources such as extreme mass ratio inspirals (EMRIs), which are prime targets for the Laser Interferometer Space Antenna (LISA) and are expected to form, at least in part, in dense environments such as Active Galactic Nuclei or dark-matter spikes/cores. Modeling environmental effects parametrically is challenging due to the large uncertainties in their underlying physics. We propose a non-parametric test for environmental effects in EMRIs, based on assessing the self-consistency of vacuum parameter posteriors inferred from different portions of the signal. Our results demonstrate that this approach can reveal the presence of an unmodeled features without introducing additional parameters or assumptions about the underlying physics.

TEONGRAV Assembly Room: Room Galileo Galilei (131), ground floor

Systematic bias in LISA ringdown analysis due to waveform inaccuracy

• Lodovico Capuano

Room: Room Galileo Galilei (131), ground floor

Functional inference of deviations from General Relativity

• Costantino Pacilio (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor Gravitational waves provide a powerful means to perform tests of strong-gravity physics. Statistical methods based on hierarchical inference, adapted from population studies, have been developed to confidently identify potential signatures of new physics. While these methods are well-suited for detection, they provide limited insight into how exotic physics depends on standard degrees of freedom, such as the mass and spin of an observed black hole. In this talk, we present an extension of hierarchical tests that enables the modeling of such dependencies in a flexible and theory-agnostic manner. The method adopts an optimization strategy based on (a queer use of) Gaussian Process Regression.

Chasing evidence of spin precession in the ringdown: a fresh look at GW190521

• Chiara Anselmo (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor The ringdown phase of a binary black-hole merger provides a uniquely clean probe of strong-field gravity, as it can be modelled with minimal assumptions. The quasi-normal-mode spectrum encodes the mass and spin of the Kerr remnant, while the excitation of these modes depends on the properties of the progenitor binary. In this work, we present the first implementation of a recently developed amplitude model that incorporates spin precession into a simulation-based inference framework designed specifically for ringdown signals. We apply this to GW190521- a short, merger-dominated event with competing interpretations- performing both spin-aligned and precessing analyses. Allowing for precession produces modest but systematic shifts in the inferred remnant parameters and in the amplitudes of subdominant modes, although the ringdown alone does not provide compelling evidence for precession. These results demonstrate the feasibility of physics-informed, precessing ringdown modelling and pave the way toward identifying spin precession directly from the ringdown stage, where waveform systematics are expected to be substantially reduced.

Population-level effects due to mismodeling and degeneracy at the single-event level

• Caroline Owen (University of Milano-Bicocca)

Room: Room Galileo Galilei (131), ground floor Identifying population-level correlations among black hole parameters is an important goal of gravitational wave astronomy. Such correlations will likely be critical in understanding the formation channels of compact bodies in our Universe. However, from signal detection to single-event parameter estimation to population-level hierarchical inference, there are many factors that could lead to the incorrect identification of correlations that do not exist in the data. We are particularly interested in understanding the circumstances under which mismodeling and degeneracy at the level of single-event parameter estimation can produce false-positive identification of correlation between uncorrelated parameters at the population level. We explore these effects in the context of a toy model.

Gravitational wave astronomy without waveform approximants

• Matteo Boschini (University of Milano-Bicocca)

Room: Room Galileo Galilei (131), ground floor Gravitational wave astronomy relies on waveform approximants for both detection and parameter estimation. However, these models inherit uncertainties that grow increasingly relevant as detectors sensitivity improves. Notably, GW231123 revealed discrepancies between different waveform models. In this work we explore the possibility to perform parameter estimation directly from numerical relativity waveform using simulation-based inference. This approach avoids approximant calibration errors, even with the limited coverage of existing NR catalogs. I will present ongoing studies, highlighting both the advantages and current challenges of this "approximant-free" pathway.

Impact of facility timing and coordination for next-generation gravitational-wave detectors

• Ssohrab Borhanian (Penn State)

Room: Room Galileo Galilei (131), ground floor While the Einstein Telescope and Cosmic Explorer proposals for next-generation, ground-based detectors promise vastly improved sensitivities to gravitational-wave signals, only joint observations are expected to enable the full scientific potential of these facilities, making timing and coordination between the efforts crucial to avoid missed opportunities. This study investigates the impact of long-term delays on the scientific capabilities of next-generation detector networks. We use the Fisher information formalism to simulate the performance of a set of detector networks for large, fiducial populations of binary black holes, binary neutron stars, and primordial black-hole binaries. Bootstrapping the simulated populations, we map the expected observation times required to reach a number of observations fulfilling scientific targets for key sensitivity and localization metrics across various network configurations. We also investigate the sensitivity to stochastic backgrounds. We find that purely sensitivity-driven metrics such as the signal-to-noise ratio are not strongly affected by delays between facilities. This is contrasted by the localization metrics, which are very sensitive to the number of detectors in the network and, by extension, to delayed observation campaigns for a detector. Effectively, delays in one detector behave like network-wide interruptions for the localization metrics for networks consisting of two next-generation facilities. We examine the impact of a supporting, current-generation detector such as LIGO India operating concurrently with next-generation facilities and find such an addition will greatly mitigate the negative effects of delays for localization metrics, with important consequences on multi-messenger science and stochastic searches.

Inferring population properties of Galactic compact binaries in LISA with Simulation Based Inference

• Federico De Santi (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor The LISA space mission, set to launch in the mid 30s, is expected to open a new window on the “gravitational wave universe”. Thanks to its exceptional sensitivity in the low frequency spectrum ~10^-4-10^-1 Hz, it will observe a variety of different sources all at the same time: from massive black hole binaries to extreme mass ratio inspirals or Galactic compact binaries. Among these, Galactic compact binaries of double white dwarves, emitting nearly monochromatic waves, are expected to be the dominant source in the mHz band. Of ~10^7 binaries in the Milky Way, a small fraction of tens to thousands will be actually resolvable by LISA. The unresolved ones are indeed expected to produce a foreground (“confusion”) stochastic noise which must be accounted for in the recovery of the other sources. Recent works have shown that different astrophysical mechanism driving the binary evolution can affect the overall astrophysical population. In particular, different interacting binary populations can produce sensibly different foreground noises. However, the extraction of the astrophysics from confusion noise still remains a non-trivial challenge in the context of the Global Fit. In this talk, I will describe an alternative approach to overcome this challenge based on Simulation Based Inference. By simulating several catalogue realizations from different astrophysical populations we train a Neural Posterior Estimator to map the simulated confusion noises to the parameters of the populations. Our model represents a first step towards the linking of astrophysical populations to the output of a Global Fit analysis.

Gravitational-wave Populations Using Delaunay Triangulation with Optimized Complexity

• Rodrigo Tenorio (Istituto Nazionale di Fisica Nucleare)

Room: Room Galileo Galilei (131), ground floor We investigate the joint mass-redshift evolution of the binary black-hole merger rate in the latest Gravitational-Wave Transient Catalog, GWTC-4.0. We present and apply a novel non-parametric framework for modeling multi-dimensional, correlated distributions based on Delaunay triangulation. Crucially, the complexity of the model -- namely, the number, positions, and weights of triangulation nodes -- is inferred directly from the data, resulting in a highly efficient approach that requires about one to two orders of magnitude fewer parameters and significantly less calibration than current state-of-the-art methods. We find no evidence for a peak at Mtot∼70M⊙ at low redshifts (z∼0.2), where it would correspond to the m1∼35M⊙ feature reported in redshift-independent mass spectrum analyses, and we infer an increased merger rate at high redshifts (z∼1) around those masses, compatible with such a peak. When related to the time-delay distribution from progenitor formation to binary black-hole merger, our results suggest that sources contributing to the m1∼35M⊙ feature follow a steeper (shallower) time-delay distribution at high (low) redshifts. This hints at contributions from different formation channels -- for example dense environments and isolated binary evolution, respectively -- although firm identification of specific formation pathways will require further observations and analyses.

Cosmologically coupled black holes in population synthesis

• Ilaria Usai (Cagliari University)

Room: Room Galileo Galilei (131), ground floor In recent years, several studies have explored the possibility that black-hole masses may be cosmologically coupled, allowing them to grow over cosmic time purely as a consequence of the expansion of the Universe. According to recent theoretical developments, the nature and strength of this coupling could depend on whether a singularity is present inside the event horizon. In this talk, I will present the first steps of a project aimed at incorporating such cosmological coupling into a population-synthesis framework. I will provide preliminary results illustrating how black-hole populations with and without central singularities would evolve differently. Finally, I will discuss how these distinct populations could be distinguished by future gravitational-wave interferometers.