arXiv subjects

Paolo Pani

Publications and source records attributed to Paolo Pani.

At least 37 records · Page 2Linked to original sources

arXiv2026

Challenging the Weak Cosmic Censorship with Phantom Fields

Penrose's weak cosmic censorship conjecture asserts that spacetime singularities produced by gravitational collapse are generically hidden behind event horizons, thus preventing them from causally influencing distant observers and preserving the predictability of the exterior region. In this work, we probe this conjecture in a setup that deliberately violates one of its central assumptions - the dominant energy condition - by considering the spherical collapse of a phantom scalar field with negative energy density. In principle, such a field could produce a Schwarzschild geometry with negative mass and therefore no event horizon. Our aim is to assess whether, once the dominant energy condition is abandoned, the fully coupled evolution of matter and geometry can dynamically generate or expose naked singularities, thereby probing the robustness of cosmic censorship. To this end, we perform high-accuracy numerical relativity simulations based on fourth-order finite-difference schemes. Starting from smooth, asymptotically flat initial data representing regular phantom scalar wave packets, we follow their fully nonlinear evolution through collapse or dispersion. While an ordinary (positive-energy) scalar field exhibits the standard Choptuik critical behavior at the threshold of black-hole formation, the phantom field displays qualitatively different dynamics. For all amplitudes considered, we find no evidence for trapped surfaces, naked singularities, or alternative stationary end states. Instead, the phantom scalar field always disperses, suggesting that cosmic censorship remains dynamically preserved even in the presence of negative-energy matter.

gr-qc↗

arXiv2026

Tidal deformability of black holes surrounded by thin accretion disks

The tidal Love numbers of self-gravitating compact objects describe their response to external tidal perturbations, such as those from a companion in a binary system, offering valuable insights into their internal structure. For static tidal fields, asymptotically flat black holes in vacuum exhibit vanishing Love numbers in general relativity, even though this property is sensitive to the presence of an external environment. In this work we study the tidal deformability of black holes surrounded by thin accretion disks, showing that the Love numbers could be large enough to mask any effect of modified gravity and to intrinsically limit tidal tests of black-hole mimickers. Furthermore, we investigate the measurability of the tidal parameters with next-generation gravitational wave experiments, like LISA and Einstein Telescope. Our findings suggest that these parameters could be measured with high precision, providing a powerful tool to probe the environment around coalescing binary systems.

astro-ph.HE↗

arXiv2026

Black Hole Ringdown Amplitudescopy

Black hole ringdowns in extensions of General Relativity (GR) generically exhibit two distinct signatures: (1) theory-dependent shifts in the standard black-hole quasinormal modes, and (2) additional modes arising from extra fundamental fields -- such as scalar, vector, or tensor degrees of freedom -- that can also contribute to the gravitational-wave signal. As recently argued, in general both effects are present simultaneously, and accurately modeling them is essential for robust tests of GR in the ringdown regime. In this work, we investigate the impact of extra field-induced modes, which are often neglected in standard ringdown analyses, on the interpretation of gravitational-wave signals. To provide some concrete examples, we focus on dynamical Chern-Simons and Einstein-scalar-Gauss-Bonnet theories, well-motivated extensions of GR, characterized respectively by a parity-odd and a parity-even coupling between a dynamical scalar field and quadratic curvature invariants. We show that including extra field-induced modes improves the bounds on these theories compared to standard spectroscopy and also allows for equally constraining complementary tests not based on quasinormal mode shifts. Our analysis highlights the relevance of incorporating extra field-induced modes in ringdown templates and assesses their potential to either bias or enhance constraints on GR deviations.

gr-qc↗

arXiv2025

Theory-agnostic searches for non-gravitational modes in black hole ringdown

In any extension of General Relativity (GR), extra fundamental degrees of freedom couple to gravity. Besides deforming GR forecasts in a theory-dependent way, this coupling generically introduces extra modes in the gravitational-wave signal. We propose a novel theory-agnostic test of gravity to search for these nongravitational modes in black hole merger ringdown signals. To leading order in the GR deviations, their frequencies and damping times match those of a test scalar or vector field in a Kerr background, with only amplitudes and phases as free parameters. By applying this test to GW150914, GW190521, and GW200129, we find no strong evidence for an extra mode; however, its inclusion modifies the inferred distribution of the remnant spin. This test will be applicable for future detectors, which will achieve signal-to-noise ratios higher than 100 (and as high as 1000 for space-based detectors such as LISA). Such sensitivity will allow measurement of these modes with amplitude ratios as low as 0.02 for ground-based detectors (and as low as 0.003 for LISA), relative to the fundamental mode, enabling stringent agnostic constraints or detection of scalar/vector modes.

gr-qc↗

arXiv2025

Inferring black hole formation channels in GWTC-4.0 via parametric mass-spin correlations derived from first principles

We investigate the differences between several proposed formation scenarios for binary black holes (BBHs), including isolated stellar evolution, dynamical assembly in dense clusters and AGN disks, and primordial BHs. Our approach exploits the predicted spin features of each formation channel, and adopts parameterized models of the predicted correlations between the spin magnitudes (and orientations) and mass, inspired by first principles. 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, 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 is not informative enough about spin orientation: the cluster (random orientations) and AGN (aligned orientations) scenarios have comparable Bayesian evidence. Finally, the mass-spin correlation predicted by the primordial scenario gives a poor fit to the data, and this scenario can only account for a subset of the observed events.

gr-qc↗

arXiv2025

The Great Impersonation: $\mathcal{W}$-Solitons as Prototypical Black Hole Microstates

We analyze a new class of static, smooth geometries in five-dimensional supergravity, dubbed $\mathcal{W}$-solitons. They carry the same mass and charges as four-dimensional Reissner-Nordström-like black holes but replace the horizon with a Kaluza-Klein bubble supported by electromagnetic flux. These solutions provide analytically tractable prototypes of black hole microstates in supergravity, including a new, relevant neutral configuration involving a massless axion field. Focusing on photon scattering and scalar perturbations, we compute their key observables, aiming to identify mesoscopic observables. We find that $\mathcal{W}$-solitons feature a single photon sphere, qualitatively similar to that of the black hole but with quantitative differences. They have only short-lived quasinormal modes~(QNMs), as black holes, while long-lived echo modes seen in other ultracompact horizonless objects are absent. As a result, the ringdown closely resembles that of a black hole while still showing sizable deviations. The latter are at the ${\mathcal{O}}(10\%)$ level, compatible with the recent measurement of GW250114 and potentially falsifiable in the near future. Finally, we show that $\mathcal{W}$-solitons are stable under scalar perturbations. Our results underscore the qualitative similarities between $\mathcal{W}$-solitons and black holes, reinforcing their relevance as smooth black hole microstate prototypes.

gr-qc↗

arXiv2025

Accuracy of ringdown models calibrated to numerical relativity simulations

The ''ringdown'' stage of gravitational-wave signals from binary black hole mergers, mainly consisting of a superposition of quasinormal modes emitted by the merger remnant, is a key tool to test fundamental physics and to probe black hole dynamics. However, ringdown models are known to be accurate only in the late-time, stationary regime. A key open problem in the field is to understand if these models are robust when extrapolated to earlier times, and if they can faithfully recover a larger portion of the signal. We address this question through a systematic time-domain calculation of the mismatch between non-precessing, quasi-circular ringdown models parameterised by the progenitor binary's degrees of freedom and full numerical relativity inspiral-merger-ringdown waveforms from the Simulating eXtreme Spacetimes (SXS) simulation catalog. For the best-performing models, the mismatch is typically in the range $[10^{-6}, 10^{-4}]$ for the $(\ell,|m|)= (2,2)$ harmonic, and $[10^{-4}, 10^{-2}]$ for higher-order modes. Our findings inform ongoing observational searches for quasinormal modes, and underscore the need for improved modeling of higher-order modes to meet the sensitivity requirements of future gravitational-wave detectors.

gr-qc↗

arXiv2025

Dynamical Love numbers of black holes: Theory and gravitational waveforms

In General Relativity, the static tidal Love numbers of black holes vanish identically. Whether this remains true for time-dependent tidal fields -- i.e., in the case of dynamical tidal Love numbers -- is an open question, complicated by subtle issues in the definition and computation of the tidal response at finite frequency. In this work, we analyze the dynamical tidal perturbations of a Schwarzschild black hole to quadratic order in the tidal frequency. By employing the Teukolsky formalism in advanced null coordinates, which are regular at the horizon, we obtain a particularly clean perturbative scheme. Furthermore, we introduce a response function based on the full solution of the perturbation equation which does not depend on any arbitrary constant. Our analysis recovers known results for the dissipative response at linear order and the logarithmic running at quadratic order, associated with scale dependence in the effective theory. In addition, we find a finite, nonvanishing conservative correction at second order in frequency, thereby possibly demonstrating a genuine dynamical deformation of the black hole geometry. Although removing any ambiguity in the dynamical tidal response would require a matching with some gauge-invariant coefficient, we assess the impact of these effects on the gravitational-wave phase. These contributions enter at eighth post-Newtonian order, and can be expressed in terms of generic $\mathcal{O}(1)$ coefficients, which have to be matched to the perturbative result. Regardless of the matching ambiguities, we argue that such corrections are too small to be observable even with future-generation gravitational wave detectors. Moreover, the corresponding phase shifts are degenerate with unknown point-particle contributions entering at the same post-Newtonian order.

gr-qc↗

arXiv2025

Gravity with higher-curvature terms and second-order field equations: $f(\mathcal{R})$ meets Gauss-Bonnet

General Relativity is expected to break down in the high-curvature regime. Beyond an effective field theory treatment with higher-order operators, it is important to identify consistent theories with higher-curvature terms at the nonperturbative level. Two well-studied examples are $f(\mathcal{R})$ gravity and Einstein-dilaton-Gauss-Bonnet (EdGB) gravity. The former shares the same vacuum solutions as General Relativity, including black holes, while the latter suffers from well-posedness issues due to quadratic curvature terms in the strong-coupling regime. We show that combining these two theories leads to genuinely new phenomena beyond their simple superposition. The resulting framework falls outside Horndeski's class, as it can be recast as a gravitational theory involving two nonminimally coupled scalar fields with nontrivial mutual interactions. This construction naturally extends EdGB gravity to include arbitrary higher-curvature terms, providing a versatile setting to address fundamental questions. Focusing on quadratic and quartic corrections, we find that: (i) black holes are modified by $f(\mathcal{R})$ terms, unlike the case without Gauss-Bonnet interactions; (ii) the resulting solutions retain the qualitative nonperturbative features of EdGB black holes with certain couplings, such as a minimum mass and multiple branches; (iii) a nontrivial mechanism suppresses the divergence of the Ricci scalar in the black-hole interior; (iv) still, even with quartic corrections, the singularity structure and elliptic regions inside the horizon remain similar to those of pure EdGB gravity. This suggests that, at the nonperturbative level, the theory's ill-posedness cannot be resolved by adding individual higher-order terms. This conjecture could be tested by studying the nonlinear dynamics, which remains governed by second-order field equations.

gr-qc↗

arXiv2025

Asymptotically Flat Rotating Topological Stars

We construct a new class of smooth, horizonless, non-supersymmetric solutions in five-dimensional minimal supergravity, which we call rotating topological stars. Built from a Kerr-Taub-bolt geometry embedded in five dimensions, they constitute the first rotating generalization of the topological star compatible with both smoothness in the interior and standard Kaluza-Klein asymptotics, S$^1\times\mathbb{R}^{1,3}$. The solutions carry angular momentum, magnetic and electric charges, and form a discrete tower of states labeled by a primary quantum number controlling the spin. Remarkably, despite lying outside the black-hole extremality bound, they can approach arbitrarily closely (in conserved charges) the Kerr black string with a large boost along the fifth dimension, making them relevant prototypes for rotating and astrophysical black-hole microstates. We analyze their geometry in detail, including their gravitational multipoles that can significantly deviate from those of black holes and the presence of an ergoregion, and show that both geodesics and scalar perturbations separate, paving the way for analyzing their dynamics in future work.

hep-th↗

arXiv2025

Taming systematics in distance and inclination measurements with gravitational waves: role of the detector network and higher-order modes

Gravitational-wave (GW) observations of compact binaries have the potential to unlock several remarkable applications in astrophysics, cosmology, and nuclear physics through accurate measurements of the source luminosity distance and inclination. However, these parameters are strongly correlated when performing parameter estimation, which may hamper the enormous potential of GW astronomy. We comprehensively explore this problem by performing Bayesian inference on synthetic data for a network of current and planned second-generation GW detectors, and for the third-generation interferometer Einstein Telescope~(ET). We quantify the role of the network alignment factor, detector sensitivity, and waveform higher-order modes in breaking this degeneracy. We discuss the crucial role of the binary mass ratio: in particular, we find that ET can efficiently remove the error in the distance as long as the compact binary is asymmetric in mass.

gr-qc↗

arXiv2025

Black hole spectroscopy and nonlinear echoes in Einstein-Maxwell-scalar theory

In the context of Einstein-Maxwell-scalar theory with a nonminimal coupling between the electromagnetic and scalar field, we study linear (non)radial perturbations and nonlinear radial dynamics of spherically symmetric black holes. In a certain region of the parameter space, this theory admits hairy black holes with a stable photon sphere. This has a counterpart in the effective potential of linear perturbations, featuring multiple maxima and minima. The corresponding quasinormal mode spectrum contains long-lived modes trapped in the potential cavity and the time-domain linear response displays echoes, as previously observed for horizonless compact objects. Interestingly, the black-hole dynamics in this theory can be studied at the nonlinear level. By performing fully-fledged 1+1 simulations, we show that echoes are present even when the nonlinearities are significant. To our knowledge, this is the first example of echoes appearing in a consistent theory beyond a linearized analysis. In a follow-up work we will study whether this feature is also present in the post-merger signal from black hole collisions in this theory.

gr-qc↗

arXiv2025

Extreme mass ratio inspirals around topological stars

We study a point scalar charge in circular orbit around a topological star, a regular, horizonless soliton emerging from dimensional compactification of Einstein-Maxwell theory in five dimensions, which could describe qualitative properties of microstate geometries for astrophysical black holes. This is the first step towards studying extreme mass-ratio inspirals around these objects. We show that when the particle probes the spacetime close to the object, the scalar-wave flux deviates significantly from the corresponding black hole case. Furthermore, as the topological star approaches the black-hole limit, the inspiral can resonantly excite its long-lived modes, resulting in sharp features in the emitted flux. Although such resonances are too narrow to produce detectable dephasing, we estimate that a year-long inspiral down to the innermost stable circular orbit could accumulate a significant dephasing for most configurations relative to the black hole case. While a full parameter-estimation analysis is needed, the generically large deviations are likely to be within the sensitivity reach of future space-based gravitational-wave detectors.

gr-qc↗

arXiv2025

One Membrane to Love them all: Tidal deformations of compact objects from the membrane paradigm

The tidal deformability is a key observable to test the nature of compact objects in a binary coalescence. Within vacuum General Relativity, the tidal Love numbers of a four-dimensional black hole are strictly zero, while they are non-zero and model-dependent for material objects, matter distributions around black holes, or in alternative theories of gravity. Here, we develop a model-agnostic framework based on the membrane paradigm, where the tidal properties of a spherically symmetric object are encoded in the response of a fictitious membrane in terms of viscosity coefficients. We show that both neutron stars and exotic compact objects (in particular, we provide an explicit example for thin-shell gravastars) are included in this framework, provided that the bulk and shear viscosity coefficients of the membrane have a nontrivial frequency dependence. We derive a general result for the electric and magnetic tidal Love numbers for non-rotating and spherically symmetric compact objects in terms of the viscosity coefficients, discussing their behavior with the compactness of the object, and identifying the conditions for the logarithmic behavior found in previous literature for various ultracompact objects. Finally, we show that the membrane shear viscosity coefficients associated with neutron star models feature novel quasi-universal relations, which depend only mildly on the neutron-star equation of state.

gr-qc↗

arXiv2025

Towards a Non-singular Paradigm of Black Hole Physics

The study of regular black holes and black hole mimickers as alternatives to standard black holes has recently gained significant attention, driven both by the need to extend general relativity to describe black hole interiors, and by recent advances in observational technologies. Despite considerable progress in this field, significant challenges remain in identifying and characterizing physically well-motivated classes of regular black holes and black hole mimickers. This report provides an overview of these challenges, and outlines some of the promising research directions -- as discussed during a week-long focus programme held at the Institute for Fundamental Physics of the Universe (IFPU) in Trieste from November 11th to 15th, 2024.

gr-qc↗

arXiv2025

Black hole mimickers: from theory to observation

The black hole paradigm, while remarkably successful, raises fundamental questions-both classical and quantum-about the nature of spacetime, horizons, and singularities. Black hole mimickers, horizonless ultra-compact objects, have emerged as potential alternatives that seek to resolve some of these puzzles while remaining consistent with current observational constraints. Recent breakthroughs in gravitational-wave astronomy and horizon-scale electromagnetic imaging have opened new avenues to test this paradigm-making this an opportune moment to systematically investigate such alternatives. This vision document presents a snapshot of the field as discussed at the Black Hole Mimickers: From Theory to Observation workshop, where experts from gravitational wave astronomy, very long baseline interferometry, numerical and mathematical relativity, and high-energy physics converged to assess the current frontiers. By highlighting key open questions and proposing concrete pathways forward, this document aims to guide future efforts to probe the nature of compact objects. As the field stands at the crossroads of theoretical innovation and observational breakthroughs, we outline strategies to harness upcoming observational capabilities to fundamentally test the black hole paradigm.

gr-qc↗

arXiv2025

Gravitational quasinormal modes of black holes in quadratic gravity

We study the gravitational perturbations of black holes in quadratic gravity, in which the Einstein-Hilbert term is supplemented by quadratic terms in the curvature tensor. In this class of theories, the Schwarzschild solution can coexist with modified black hole solutions, and both families are radially stable in a wide region of the parameter space. Here we study non-radial perturbations of both families of static, spherically symmetric black holes, computing the quasi-normal modes with axial parity and finding strong numerical evidence for the stability of these solutions under axial perturbations. The perturbation equations describe the propagation of a massless and a massive spin-two fields. We show that the Schwarzschild solution admits the same quasi-normal modes as in general relativity, together with new classes of modes corresponding to the massive spin-two degrees of freedom. The spectrum of the modified black hole solution has the same structure, but all modes are different from those of general relativity. We argue that both classes of modes can be excited in physical processes, suggesting that a characteristic signature of this theory is the presence of massive spin-two modes in the gravitational ringdown, even when the stationary solution is the same as in general relativity.

gr-qc↗

arXiv2025

Cumulative effect of orbital resonances in extreme-mass-ratio inspirals

Orbital resonances in extreme-mass-ratio inspirals (EMRIs) have been proven to be a key feature for accurate gravitational-wave template modeling. Decades of research have led to schemes that can not only model the adiabatic inspiral of such a binary system, but also account for the effects of resonances on their evolution. In this work, we use an effective resonance model that includes analytically derived corrections to the radiation reaction fluxes, to study the combined effects of both dominant (low-order) and subdominant (high-order) orbital resonances in EMRIs. We show that using single, universal shifts for all fluxes overestimates the resonance impact, and therefore individualized shifts for each resonance crossing are needed for accurate modeling. Our analysis reveals that the cumulative effects from multiple resonance crossings can significantly impact the orbital evolution of EMRIs, especially for highly eccentric orbits. Our results provide further evidence that resonance effects have to be included in template production to extract detailed astrophysical parameters from EMRI signals.

gr-qc↗