arXiv subjects

Paolo Pani

Publications and source records attributed to Paolo Pani.

At least 55 records · Page 3Linked to original sources

arXiv2025

Cosmology and nuclear-physics implications of a subsolar gravitational-wave event

Detecting a compact subsolar object would have profound implications in physics, the reach of which depends on the nature of the object. Here we explore such consequences for a putative subsolar-mass gravitational wave event detected by the LIGO-Virgo-KAGRA Collaboration. We forecast that the nature of a subsolar binary (made of light neutron stars, primordial black holes, or more exotic compact objects) can be inferred with a great statistical confidence level already during the ongoing fourth observing run, based on the large tidal deformability effects on the signal. The detection of a primordial black hole would have implications for cosmology and dark matter scenarios, while the measurement of the tidal deformability of a subsolar neutron star could rule out or confirm the existence of strange stars made of quarks.

astro-ph.HE↗

arXiv2025

Source multipoles and energy-momentum tensors for spinning black holes and other compact objects in arbitrary dimensions

Working in momentum space and at linear order in the gravitational coupling, we derive the most general class of energy-momentum tensors associated with a given multipolar structure of the spacetime in arbitrary dimensions, and built out of a mass and an angular momentum, at any order in the spin expansion. In this formalism, we are able to derive directly the full multipolar structure of any solution from the multipole expansion of the energy-momentum tensor, in complete analogy to Newtonian gravity. In particular, we identify the recurrence relations that allow obtaining the multipolar structure of the Kerr and the Myers-Perry black hole solutions, defining source multipoles in a General Relativity context for the first time. For these solutions, we are able to resum the energy-momentum tensor in momentum space at all orders in the angular momentum, and compute its real-space version. In the Kerr case we exactly obtain the matter source found by Israel, namely an equatorial, pressureless thin disk rotating at superluminal speed. For Myers-Perry in five dimensions, the matter distribution is a three-ellipsoid in four spatial dimensions with nontrivial stresses. Remarkably, for any dimensions, the matter configuration is a lower-dimensional distribution which has the same singularity structure as the fully non-linear black-hole solution. Our formalism underscores the advantage of working in momentum space to generate nontrivial matter sources for non-linear spacetimes, and could be used to construct regular non-exotic matter configurations that source spinning black hole solutions or horizonless compact objects with the same multipolar structure as black holes.

gr-qc↗

arXiv2025

Possible Causes of False General Relativity Violations in Gravitational Wave Observations

General relativity (GR) has proven to be a highly successful theory of gravity since its inception. The theory has thrivingly passed numerous experimental tests, predominantly in weak gravity, low relative speeds, and linear regimes, but also in the strong-field and very low-speed regimes with binary pulsars. Observable gravitational waves (GWs) originate from regions of spacetime where gravity is extremely strong, making them a unique tool for testing GR, in previously inaccessible regions of large curvature, relativistic speeds, and strong gravity. Since their first detection, GWs have been extensively used to test GR, but no deviations have been found so far. Given GR's tremendous success in explaining current astronomical observations and laboratory experiments, accepting any deviation from it requires a very high level of statistical confidence and consistency of the deviation across GW sources. In this paper, we compile a comprehensive list of potential causes that can lead to a false identification of a GR violation in standard tests of GR on data from current and future ground-based GW detectors. These causes include detector noise, signal overlaps, gaps in the data, detector calibration, source model inaccuracy, missing physics in the source and in the underlying environment model, source misidentification, and mismodeling of the astrophysical population. We also provide a rough estimate of when each of these causes will become important for tests of GR for different detector sensitivities. We argue that each of these causes should be thoroughly investigated, quantified, and ruled out before claiming a GR violation in GW observations.

gr-qc↗

arXiv2025

Nonradial stability of topological stars

Topological stars are regular, horizonless solitons arising from dimensional compactification of Einstein-Maxwell theory in five dimensions, which could describe qualitative properties of microstate geometries for astrophysical black holes. They also provide a compelling realization of ultracompact objects arising from a well-defined theory and display all the phenomenological features typically associated with black hole mimickers, including a (stable) photon sphere, long-lived quasinormal modes, and echoes in the ringdown. By completing a thorough linear stability analysis, we provide strong numerical evidence that these solutions are stable against nonradial perturbations with zero Kaluza-Klein momentum.

gr-qc↗

arXiv2024

Ringdown stability: greybody factors as stable gravitational-wave observables

The quasinormal mode spectrum of black holes plays a crucial role in the modelling of post-merger ringdown signals. However, the spectrum is extremely sensitive to small deformations of the system and describes the linear response only in a certain (not precisely defined) timeframe after the merger. We argue here that the greybody factors, recently shown to describe the ringdown spectral amplitude at relatively high frequencies, are instead stable under small perturbations of the system and free of certain ambiguities that plague the quasinormal mode spectrum. Our analysis also unveils a nontrivial interplay: while certain ringdown quantities are dominated by the contribution of spectrally unstable quasinormal modes, these modes conspire to produce stable observables. Thus, we propose a complementary approach to ringdown studies, which circumvents some limitations of the standard quasinormal mode description.

gr-qc↗

arXiv2024

Beyond The Standard Model electrodynamics in the time domain

Many motivated extensions of the standard model include new light bosons, such as axions and dark photons, which can mix with the ordinary photon. This latter, when in a dilute plasma, can be dressed by an effective plasma mass. If this is equal to the mass of the new degree of freedom, then a resonance takes place and the probability of transition between states is enhanced. This phenomenon of resonance conversion is at the very basis of multiple probes of dark matter, and it is typically studied within the so-called Landau-Zener approximation for level crossings. This latter is known to break down in a variety of scenarios, such as multiple level crossings or when the de Broglie wave length of the new boson is comparable to the scale over which the background plasma varies. We develop a flexible code adopting a 3+1 formalism in flat spacetime to perform non-linear simulations of systems with photons and new ultralight bosons in the presence of plasma. Our code currently allows to evolve one-dimensional systems, which are the ones of interest for this first study, but can be easily extended to treat three-dimensional spaces, and can be adapted to describe a plethora of realistic astrophysical and cosmological situations. Here we use it to study the breakdown of the Landau-Zener approximation in the case of multiple level crossings and when the slowly-varying plasma approximation ceases to be valid. In this first paper we detail our code and use it to study non-turbulent plasma, where small scale fluctuations can be neglected; their treatment will be considered in an upcoming publication.

hep-ph↗

arXiv2024

Black Holes Inside and Out 2024: visions for the future of black hole physics

The gravitational physics landscape is evolving rapidly, driven by our ability to study strong-field regions, in particular black holes. Black Holes Inside and Out gathered world experts to discuss the status of the field and prospects ahead. We hope that the ideas and perspectives are a source of inspiration. Structure: Black Hole Evaporation - 50 Years by William Unruh The Stability Problem for Extremal Black Holes by Mihalis Dafermos The Entropy of Black Holes by Robert M. Wald The Non-linear Regime of Gravity by Luis Lehner Black Holes Galore in D > 4 by Roberto Emparan Same as Ever: Looking for (In)variants in the Black Holes Landscape by Carlos A. R. Herdeiro Black Holes, Cauchy Horizons, and Mass Inflation by Matt Visser The Backreaction Problem for Black Holes in Semiclassical Gravity by Adrian del Rio Black Holes Beyond General Relativity by Enrico Barausse and Jutta Kunz Black Holes as Laboratories: Searching for Ultralight Fields by Richard Brito Primordial Black Holes from Inflation by Misao Sasaki Tests of General Relativity with Future Detectors by Emanuele Berti Black Holes as Laboratories: Tests of General Relativity by Ruth Gregory and Samaya Nissanke Simulating Black Hole Imposters by Frans Pretorius Black Hole Spectroscopy: Status Report by Gregorio Carullo VLBI as a Precision Strong Gravity Instrument by Paul Tiede Testing the nature of compact objects and the black hole paradigm by Mariafelicia De Laurentis and Paolo Pani Some Thoughts about Black Holes in Asymptotic Safety by Alessia Platania Black Hole Evaporation in Loop Quantum Gravity by Abhay Ashtekar How the Black Hole Puzzles are Resolved in String Theory by Samir D. Mathur Quantum Black Holes: From Regularization to Information Paradoxes by Niayesh Afshordi and Stefano Liberati

gr-qc↗

arXiv2024

Dynamical tidal Love numbers of Kerr-like compact objects

We develop a framework to compute the tidal response of a Kerr-like compact object in terms of its reflectivity, compactness, and spin, both in the static and the frequency-dependent case. Here we focus on the low-frequency regime, which can be solved fully analytically. We highlight some remarkable novel features, in particular: i) Even in the zero-frequency limit, the tidal Love numbers (TLNs) depend on the linear-in-frequency dependence of the object's reflectivity in a nontrivial way. ii) Intriguingly, the static limit of the frequency-dependent TLNs is discontinuous, therefore the static TLNs differ from the static limit of the (phenomenologically more interesting) frequency-dependent TLNs. This shows that earlier findings regarding the static TLNs of ultracompact objects correspond to a measure-zero region in the parameter space, though the logarithmic behavior of the TLNs in the black hole limit is retained. iii) In the non-rotating case, the TLNs generically vanish in the zero-frequency limit (just like for a black hole), except when the reflectivity is ${\cal R}=1+{\cal O}(Mω)$, in which case they vanish with a model-dependent scaling, which is generically logarithmic, in the black-hole limit. The TLNs initially grow with frequency, for any nonzero reflectivity, and then display oscillations and resonances tied up with the quasi-normal modes of the object. iv) For rotating compact objects, the TLNs decrease when the reflectivity decreases or the rotation parameter increases. Our results lay the theoretical groundwork to develop model-independent tests of the nature of compact objects using tidal effects in gravitational-wave signals.

gr-qc↗

arXiv2024

Spectroscopy of magnetized black holes and topological stars

We study the linear response of four dimensional magnetized black holes and regular topological stars arising from dimensional compactification of Einstein-Maxwell theory in five dimensions. We consider both radial and nonradial perturbations and study the stability of these solutions, both in the frequency and in the time domain. Due to the presence of magnetic fluxes in the background, axial (i.e., odd-parity) gravitational perturbations are coupled to polar (i.e., even-parity) electromagnetic perturbations (Type-I sector) whereas polar gravitational and scalar perturbations are coupled to axial electromagnetic ones (Type-II sector). We provide a comprehensive analytical and numerical study of the radial perturbations and of the Type I sector, finding no evidence of linear instabilities (besides the already known Gregory-Laflamme instability of black strings occurring only in a certain range of the parameters), even despite the fact that the effective potential for radial perturbations of topological stars is negative and divergent near the inner boundary. Ultracompact topological stars exhibit long-lived trapped modes that give rise to echoes in the time-domain response. Their prompt response is very similar to that of the corresponding black hole with comparable charge-to-mass ratio. This provides a concrete realization of ultracompact objects arising from a well-defined theory. The numerical analysis of the Type-II sector will appear in a companion paper.

gr-qc↗

arXiv2024

Analytical model of precessing binaries using post-Newtonian theory in the extreme mass-ratio limit I: General Formalism

We develop a fully analytical waveform model for precessing binaries with arbitrary spin vectors using post-Newtonian~(PN) theory in the extreme mass-ratio limit and a hierarchical multi-scale analysis. The analytical model incorporates leading PN order spin precession dynamics from spin-orbit, spin-spin, and quadrupole-monopole couplings, and 2PN order dissipative dynamics truncated to first order in the mass ratio $q \ll 1$. Due to the pure analytic nature of the model, the framework developed herein can readily be extended to both higher PN and higher-$q$ order. Although the PN series is asymptotic to this limit, our results can be used to estimate how precession affects the measurability of certain binary parameters, and to inform and compare with other waveform approximants, such as effective-one-body models, hybrid waveforms, and self-force calculations.

gr-qc↗

arXiv2024

Relevance of Precession for Tests of the Black Hole No Hair Theorems

The multipole moments of black holes in general relativity obey certain consistency relations known as the no-hair theorems. The details of this multipolar structure are imprinted into the gravitational waves emitted by binary black holes, particularly if the binary is precessing. If black holes do not obey the vacuum field equations of general relativity, then the no-hair theorems may be broken, and the observed gravitational waves will be modified, thus providing an important test of the no-hair theorems. Recently, analytic solutions to the precession dynamics and inspiral waveforms were computed within the context of binaries possessing non-axisymmetric mass quadrupole moments, which are parametrized by a modulus $q_{m}$ and phase $a_{m}$ with $m = 1,2$ the azimuthal spherical harmonic number. Here, we use a Fisher analysis to study plausible constraints one may obtain on generic, non-axisymmetry quadrupole configurations using current and future ground-based detectors. For non-precessing binaries, we generically find that no meaningful constraints can be placed with current detectors on the non-axisymmetry parameters $(q_{m}, a_{m})$ due to the presence of strong degeneracies with other waveform parameters, while with next generation detectors, only weak constraints are possible. For precessing configurations, the exact value of the uncertainty is strongly dependent on the sky location, system orientation relative to the line of sight, and initial inclination angle of the orbital angular momentum. After averaging over these parameters, we find that with GWTC-3-like events, one should be able to plausibly constraint non-axisymmetric mass quadrupole deviations to $Δq_{m} \sim 10^{-2}$ for LIGO at design sensitivity, and $Δq_{m} \sim 10^{-4}$ for the same sources with Einstein Telescope and Cosmic Explorer.

gr-qc↗

arXiv2024

Rotating metrics and new multipole moments from scattering amplitudes in arbitrary dimensions

We compute the vacuum metric generated by a generic rotating object in arbitrary dimensions up to third post-Minkowskian order by computing the classical contribution of scattering amplitudes describing the graviton emission by massive spin-1 particles up to two loops. The solution depends on the mass, angular momenta, and on up to two parameters related to generic quadrupole moments. In $D=4$ spacetime dimensions, we recover the vacuum Hartle-Thorne solution describing a generic spinning object to second order in the angular momentum, of which the Kerr metric is a particular case obtained for a specific mass quadrupole moment dictated by the uniqueness theorem. At the level of the effective action, the case of minimal couplings corresponds to the Kerr black hole, while any other mass quadrupole moment requires non-minimal couplings. In $D>4$, the absence of black-hole uniqueness theorems implies that there are multiple spinning black hole solutions with different topology. Using scattering amplitudes, we find a generic solution depending on the mass, angular momenta, the mass quadrupole moment, and a new stress quadrupole moment which does not exist in $D=4$. As special cases, we recover the Myers-Perry and the single-angular-momentum black ring solutions, to third and first post-Minkowksian order, respectively. Interestingly, at variance with the four dimensional case, none of these solutions corresponds to the minimal coupling in the effective action. This shows that, from the point of view of scattering amplitudes, black holes are the "simplest" General Relativity vacuum solutions only in $D=4$.

hep-th↗

arXiv2024

Tidal heating as a discriminator for horizons in equatorial eccentric extreme mass ratio inspirals

Tidal heating in a binary black hole system is driven by the absorption of energy and angular momentum by the black hole's horizon. Previous works have shown that this phenomenon becomes particularly significant during the late stages of an extreme mass ratio inspiral (EMRI) into a rapidly spinning massive black hole, a key focus for future low-frequency gravitational-wave observations by (for instance) the LISA mission. Past analyses have largely focused on quasi-circular inspiral geometry, with some of the most detailed studies looking at equatorial cases. Though useful for illustrating the physical principles, this limit is not very realistic astrophysically, since the population of EMRI events is expected to arise from compact objects scattered onto relativistic orbits in galactic centers through many-body events. In this work, we extend those results by studying the importance of tidal heating in equatorial EMRIs with generic eccentricities. Our results suggest that accurate modeling of tidal heating is crucial to prevent significant dephasing and systematic errors in EMRI parameter estimation. We examine a phenomenological model for EMRIs around exotic compact objects by parameterizing deviations from the black hole picture in terms of the fraction of radiation absorbed compared to the BH case. Based on a mismatch calculation we find that reflectivities as small as $|\mathcal{R}|^2 \sim \mathcal{O}(10^{-5})$ are distinguishable from the BH case, irrespective of the value of the eccentricity. We stress, however, that this finding should be corroborated by future parameter estimation studies.

gr-qc↗

arXiv2024

Tidal Love numbers and approximate universal relations for fermion soliton stars

Fermion soliton stars are a consistent model of exotic compact objects which involve a nonlinear interaction between a real scalar field and fermions through a Yukawa term. This interaction results in an effective fermion mass that depends upon the vacuum structure in the scalar potential. In this work we investigate the tidal deformations of fermion soliton stars and compute the corresponding tidal Love numbers for different model parameters. Furthermore, we discuss the existence of approximate universal relations for the electric and magnetic tidal deformabilities of these stars, and compare them with other solutions of general relativity, such as neutron stars or boson stars. These relations for fermion soliton stars are less universal than for neutron stars, but they are sufficiently different from the ordinary neutron star case that a measurement of the electric and magnetic tidal Love numbers (as potentially achievable by next-generation gravitational wave detectors) can be used to disentangle these families of compact objects. Finally, we discuss the conditions for tidal disruption of fermion soliton stars in a binary system and estimate the detectability of the electromagnetic signal associated with such tidal disruption events.

gr-qc↗

arXiv2024

Novel tests of gravity using nano-Hertz stochastic gravitational-wave background signals

Gravity theories that modify General Relativity in the slow-motion regime can introduce nonperturbative corrections to the stochastic gravitational-wave background~(SGWB) from supermassive black-hole binaries in the nano-Hertz band, while remaining perturbative in the highly-relativistic regime and satisfying current post-Newtonian~(PN) constraints. We present a model-agnostic formalism to map such theories into a modified tilt for the SGWB spectrum, showing that negative PN corrections (in particular -2PN) can alleviate the tension in the recent pulsar-timing-array data if the detected SGWB is interpreted as arising from supermassive binaries. Despite being preliminary, current data have already strong constraining power, for example they set a novel (conservative) upper bound on theories with time-varying Newton's constant at least at the level of $\dot{G}/G \lesssim 10^{-5} \text{yr}^{-1}$ for redshift $z=[0.1\div1]$. We also show that NANOGrav data are best fitted by a broken power-law interpolating between a dominant -2PN or -3PN modification at low frequency, and the standard general-relativity scaling at high frequency. Nonetheless, a modified gravity explanation should be confronted with binary eccentricity, environmental effects, nonastrophysical origins of the signal, and scrutinized against statistical uncertainties. These novel tests of gravity will soon become more stringent when combining all pulsar-timing-array facilities and when collecting more data.

gr-qc↗

arXiv2024

Compact objects in and beyond the Standard Model from non-perturbative vacuum scalarization

We consider a theory in which a real scalar field is Yukawa-coupled to a fermion and has a potential with two non-degenerate vacua. If the coupling is sufficiently strong, a collection of N fermions deforms the true vacuum state, creating energetically-favored false-vacuum pockets in which fermions are trapped. We embed this model within General Relativity and prove that it admits self-gravitating compact objects where the scalar field acquires a non-trivial profile due to non-perturbative effects. We discuss some applications of this general mechanism: i) neutron soliton stars in low-energy effective QCD, which naturally happen to have masses around 2 solar masses and radii around 10 km even without neutron interactions; ii) Higgs false-vacuum pockets in and beyond the Standard Model; iii) dark soliton stars in models with a dark sector. In the latter two examples, we find compelling solutions naturally describing centimeter-size compact objects with masses around 10^-6 solar masses, intriguingly in a range compatible with the OGLE+HSC microlensing anomaly. Besides these interesting examples, the mechanism of non-perturbative vacuum scalarization may play a role in various contexts in and beyond the Standard Model, providing a support mechanism for new compact objects that can form in the early universe, can collapse into primordial black holes through accretion past their maximum mass, and serve as dark matter candidates.

hep-ph↗

arXiv2024

Spherically symmetric elastic bodies in general relativity

The purpose of this review it to present a renewed perspective of the problem of self-gravitating elastic bodies under spherical symmetry. It is also a companion to the papers [Phys. Rev. D105, 044025 (2022)], [Phys. Rev. D106, L041502 (2022)], and [arXiv:2306.16584 [gr-qc]], where we introduced a new definition of spherically symmetric elastic bodies in general relativity, and applied it to investigate the existence and physical viability, including radial stability, of static self-gravitating elastic balls. We focus on elastic materials that generalize fluids with polytropic, linear, and affine equations of state, and discuss the symmetries of the energy density function, including homogeneity and the resulting scale invariance of the TOV equations. By introducing invariant characterizations of physical admissible initial data, we numerically construct mass-radius-compactness diagrams, and conjecture about the maximum compactness of stable physically admissible elastic balls.

gr-qc↗

arXiv2024

Scale invariant elastic stars in General Relativity

We present a model of relativistic elastic stars featuring scale invariance. This implies a linear mass-radius relation and the absence of a maximum mass. The most compact spherically symmetric configuration that is radially stable and satisfies all energy and causality conditions has a slightly smaller radius than the Schwarzschild light ring radius. To the best of our knowledge, this is the first material compact object with such remarkable properties in General Relativity, which makes it a unique candidate for a black-hole mimicker.

gr-qc↗