arXiv subjects

Luis Lehner

Publications and source records attributed to Luis Lehner.

At least 19 recordsLinked to original sources

arXiv2026

Love at the String Scale: Tidal Deformability Across the Black Hole-String Transition

Tidal deformability provides a sensitive probe of the structure of compact objects, and black holes are exceptional in having vanishing static Love numbers in four-dimensional Einstein gravity. We ask what happens to this fine-tuned rigidity across the black hole-string transition, where the same states are expected to admit a weakly coupled description as a self-gravitating highly excited string, or "string star". We compute the static tidal Love numbers of the Horowitz-Polchinski (HP) string star for multipoles $\ell=2,3,4$ in $D=4,5,6$. The response is non-zero in all cases, as in the available $\alpha'$-corrected black hole results, so the zero-Love structure of four-dimensional Einstein gravity does not survive on either side of the transition. On the string side, however, the response has a distinctive multipolar structure: the Love numbers grow rapidly with $\ell$, and we show analytically that this growth originates from the competition between the multipolar weight of the tidal field and the exponential tail of the winding condensate, with the result that the radial scale probed by the deformability grows linearly with $\ell$ at large multipole number. This provides a direct tidal signature of the extended, surface-less nature of the string star. We discuss the comparison with $\alpha'$-corrected black holes and the limitations of the currently available perturbative results at large $\ell$.

hep-th↗

arXiv2026

Gravitational Enstrophy: Local Geometric Origin and Inverse-Cascade Constraints

Two-dimensional fluids conserve energy and enstrophy, driving inverse energy cascades via Fj\o rtoft's argument. We show General Relativity admits an analogous structure: for linear radiative perturbations of Petrov type D backgrounds (Kerr, Kerr--AdS), the gravitational-wave energy $W = \sum_k W_k$ and magnetic Weyl enstrophy $\mathcal{Z} = \int B_{ab} B^{ab} \sqrt{\gamma} \, d^3x \approx \sum_k \omega_k^2 W_k$ are approximately conserved in the zero-angular momentum frame, where vorticity coupling vanishes identically and curl exchange cancels mode-by-mode. This yields a gravitational Fj\o rtoft constraint implying nonlinear energy transfer proceeds preferentially toward lower frequencies. The constraint is dynamically active in near-extremal Kerr ($\tau_{\text{damp}} \gg \tau_{\text{nl}}$) and confined geometries (AdS), but suppressed in generic ringdown. In AdS, $\mathcal{Z}$ maps holographically to the boundary fluid enstrophy.

gr-qc↗

arXiv2026

Agnostically decoding gravitational wave model deficiencies in GWTC-3

Gravitational Wave (GW) data bring an exceptional avenue to test the underlying models of coalescing compact objects. In the regime of strong gravity and high curvature, they allow the exploration of minute deviations from the best-fit models, which are difficult to uncover with other observational modalities. These deviations can stem from departures from General Relativity (GR) or unaccounted astrophysical effects. They may not be explainable within the current description of GW strain data, or may simply be difficult to model. However, they are expected to be correlated between detectors and across the population of observed events. The recently developed SCoRe analysis pipeline leverages these properties by focusing on the correlated power between detectors and combining results from multiple events. In this paper, we apply the framework on the Third Gravitational-Wave Transient Catalog to search for source-dependent deviations. In particular, we explore whether there is evidence for a mass-scale in the observed events, which can act like a line of demarcation in their physical properties by exhibiting a deviation that is different above and below this mass-scale. This mass scale dependency naturally arises in gravitational theories described through effective field theories, due to environmental effects or in scenarios involving exotic compact objects, where the GW signature can differ from the standard binary black holes in GR. Using the 30 highest Signal-to-Noise Ratio events in the catalog, we find Bayes factors ranging from 0.16--0.5 (depending on where the threshold mass is set), thus disfavoring the hypothesis of existence of any mass-scale between $\sim 2.5$ M$_\odot$ and $60$ M$_\odot$. We also compute the distribution of excess cross-correlated power across events and find a Bayes factor of $0.07$, which agrees with expected noise statistics.

gr-qc↗

arXiv2026

Matching Tidal Deformability (Wilson) Coefficients to Black Hole Love Numbers in Higher-Curvature Gravity

We present a consistent mapping between tidal deformability coefficients (tidal Love numbers) and Wilson coefficients in effective field theory (EFT) descriptions of higher-curvature theories of gravity. In this work, we focus on the connection between the static response of a non-spinning black hole and the corresponding Wilson coefficient governing tidal imprints in gravitational-wave signals. We analyze a set of control cases to identify the key ingredients required for a systematic computation and matching procedure. In doing so, we highlight shortcomings in existing results that rely on the standard matching approach used in General Relativity when applied to higher-curvature gravity theories. As an explicit demonstration, we compute the relevant coefficients for cubic gravity theories. Our findings bridge an important gap in the correspondence between tidal Love numbers and Wilson coefficients in EFT extensions of General Relativity, which had not been thoroughly explored previously.

gr-qc↗

arXiv2026

Modified Teukolsky formalism: Null testing and numerical benchmarking

Next-generation gravitational-wave detectors will make black-hole ringdown an increasingly sensitive probe of small departures from General Relativity in the strong-field regime. This motivates obtaining high-precision predictions of gravitational effective field theory, as spectral shifts can be quite small. Here we perform a focused stress test of the modified-Teukolsky framework by designing two null diagnostics. First, we consider an action with redundant operators that must produce zero first-order vacuum QNM shifts. Second, we exploit a Ricci-flat identity relating two physical cubic Riemann to test such a relation is satisfied by the ringdown spectra obtained. We compute the shifts using two independent numerical approaches: the eigenvalue-perturbation and generalized continued-fraction (Leaver-type) methods. Both null tests are passed across multiple multipoles and overtones, and the control-operator results agree in magnitude with the benchmark values reported in Ref. [1]. These validations support using the framework for obtaining accurate predictions for robust strong-field tests, with straightforward extensions to rotating backgrounds and coupling with matter fields.

gr-qc↗

arXiv2025

Sourced Carrollian Fluids Dual to Black Hole Horizons

The (degenerate) geometry of event horizons is linked to Carrollian fluids. We investigate the behavior of event horizons via a perturbative coupling to a massless scalar field, making connections to Carrollian hydrodynamics with a driving source, and discuss the fluid equilibration in tandem with the horizon's relaxation to equilibrium. We observe that after the perturbation dies off, the Carrollian fluid energy and momentum densities approach equilibrium as the horizon asymptotically becomes non-expanding. We connect the equilibration of the Carrollian fluid dual to the black hole horizon through the expansion of its background geometry.

gr-qc↗

arXiv2025

Emergent Turbulence in Nonlinear Gravity

Gravity in nonlinear and dynamical regimes underpins spectacular astrophysical phenomena and observable consequences, from the early universe to black hole collisions. In these extreme environments, inverse energy cascades - mediated by nonlinear interactions - may help explain the near scale-invariance of cosmic structure and the simplicity of gravitational waves from binary black hole mergers. Yet the presence, characteristics, and generality of such interactions in full General Relativity remain largely unexplored. Here we show that two types of nonlinear interactions - a four-mode and a three-mode interaction - emerge in the fully nonlinear regime, and can indeed channel inverse energy cascades by inducing resonant and anti-damping instabilities. This establishes what was previously only hinted at in highly specialized perturbative contexts. We further demonstrate a ``laminar'' to ``turbulent'' transition for the largest-possible angular structure in General Relativity, whereas finer structures remain persistently turbulent. Our results reveal the impact and generality of these nonlinear interactions (instabilities), which can be key to understanding observations ranging from cosmological to kilometer scales. We anticipate that our work will shed new light on nonlinear gravitational phenomena and their consequences, such as constructing gravitational wave templates and testing General Relativity in the most extreme regime. Moreover, our work is a starting point for addressing nonlinear gravitational interactions using ideas and methods inspired by fluid dynamics.

gr-qc↗

arXiv2025

Generic EFT-motivated beyond General Relativity gravitational wave tests and their curvature dependence: from observation to interpretation

We present a "dictionary" to expedite the identification of potential deviations in gravitational waveforms from those predicted by General Relativity (GR) during the inspiral phase of black hole binaries. Assuming deviations from GR can be described by a local Effective Field Theory (EFT) formulated in terms of curvature operators (and possibly additional scalar fields), this dictionary characterizes how deviations scale with the masses of the binary components and identifies the leading order Post-Newtonian corrections in generic theories constructed within the EFT framework. By establishing a direct connection between observations and candidate theories beyond GR, this dictionary also aids in distinguishing genuine physical effects from systematic errors. These results can be readily incorporated into essentially all existing tests for the inspiral regime and, in particular, facilitate a more efficient combination of data from multiple events.

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

Tidal response of regular black holes

In this work, we investigate the tidal deformability of regular black holes (RBHs). Employing different phenomenological models, we analyze their response to both test fields and gravitational perturbations, interpreting the latter within the framework of Einstein's field equations in the presence of an appropriate exotic matter distribution. Numerical and analytical methods reveal that RBHs exhibit non-trivial tidal responses, influenced by their regularization parameters and exotic matter distributions. The results obtained for test fields and gravitational perturbations are in qualitative agreement. This hints at the possibility that similar conclusions could hold if these spacetimes were interpreted as solutions of a modified gravitational action. Our findings suggest that RBHs possess distinct, though subtle, tidal signatures, which may serve as observational probes of their internal structure in gravitational wave detections.

gr-qc↗

arXiv2024

Detecting Unmodeled, Source-dependent Signals in Gravitational Waves with SCoRe

New physics and systematic errors can lead to deviations between the models used to analyze gravitational wave data and the actual signal. Such deviations will generally be correlated between detectors and manifest differently across the gravitational wave source parameter space. The previously introduced \ttt{SCoRe} framework uses these features to distinguish these deviations from noise and extract physical information from their source-dependent variation. In this work, we further analyze the hierarchical component of the method -- we include the expected dependence of the deviations on the source parameters into the inference process, obtaining more physically informative results. As a specific example, we study a deviation that scales as a power law of the mass scale of black hole binaries -- as, for example, in Effective Field Theory of gravity. We show how the signal-to-noise ratio of the cross-correlated residual power can be used to recover the power-law index. We demonstrate how both the distribution in source and deviation strength determine which region of source parameter space influences the inference most. Finally, we forecast the constraint on the power law index for a network of two Cosmic Explorer-like detectors with a year of observation period.

gr-qc↗

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

The curvature dependence of gravitational-wave tests of General Relativity

High-energy extensions to General Relativity modify the Einstein-Hilbert action with higher-order curvature corrections and theory-specific coupling constants. The order of these corrections imprints a universal curvature dependence on observations while the coupling constant controls the deviation strength. In this Letter, we leverage the theory-independent expectation that modifications to the action of a given order in spacetime curvature (Riemann tensor and contractions) lead to observational deviations that scale with the system length-scale to a corresponding power. Focusing on gravitational wave observations, the relevant scale is the binary total mass, and deviations scale as a power of mass $p$ related to the action order. For example, $p=4,6$ arise in effective field theory for cubic and quartic theories respectively. We incorporate this universal scaling into theory-agnostic tests of General Relativity with current gravitational-wave observations, thus enabling constraints on the curvature scaling without compromising the agnostic nature of these tests. This introduces a flexible yet highly interpretable new paradigm for tests of General Relativity with gravitational-wave catalogs.

gr-qc↗

arXiv2024

Nonlinear studies of modifications to general relativity: Comparing different approaches

Studying the dynamical, nonlinear regime of modified theories of gravity remains a theoretical challenge that limits our ability to test general relativity. Here we consider two generally applicable, but approximate methods for treating modifications to full general relativity that have been used to study binary black hole mergers and other phenomena in this regime, and compare solutions obtained by them to those from solving the full equations of motion. The first method evolves corrections to general relativity order by order in a perturbative expansion, while the second method introduces extra dynamical fields in such a way that strong hyperbolicity is recovered. We use shift-symmetric Einstein-scalar-Gauss-Bonnet gravity as a benchmark theory to illustrate the differences between these methods for several spacetimes of physical interest. We study the formation of scalar hair about initially non-spinning black holes, the collision of black holes with scalar charge, and the inspiral and merger of binary black holes. By directly comparing predictions, we assess the extent to which those from the approximate treatments can be meaningfully confronted with gravitational wave observations. We find that the order-by-order approach cannot faithfully track the solutions when the corrections to general relativity are non-negligible. The second approach, however, can provide consistent solutions, provided the ad-hoc timescale over which the dynamical fields are driven to their target values is made short compared to the physical timescales.

gr-qc↗

arXiv2024

Exploring Black Hole Mimickers: Electromagnetic and Gravitational Signatures of AdS Black Shells

We study electromagnetic and gravitational properties of AdS black shells (also referred to as AdS black bubbles) -- a class of quantum gravity motivated black hole mimickers, that in the classical limit are described as ultra compact shells of matter. We find that their electromagnetic properties are remarkably similar to black holes. We then discuss the extent to which these objects are distinguishable from black holes, both for intrinsic interest within the black shell model, and as a guide for similar efforts in other sub-classes of exotic compact objects (ECOs). We study photon rings and lensing band characteristics, relevant for very large baseline inteferometry (VLBI) observations, as well as gravitational wave observables -- quasinormal modes in the eikonal limit and the static tidal Love number for non-spinning shells -- relevant for ongoing and upcoming gravitational wave observations.

gr-qc↗

arXiv2023

Fluid-gravity correspondence and causal first-order relativistic viscous hydrodynamics

The fluid-gravity correspondence is a duality between anti-de Sitter Einstein gravity and a relativistic fluid living at the conformal boundary. We show that one can accommodate the causal first-order viscous hydrodynamics recently developed by Bemfica, Disconzi, Noronha, and Kovtun in this framework, by requiring a set of natural conditions for the geometric data at the horizon. The latter hosts an induced Carrollian fluid, whose equations of motion are shown to be tightly tied to the ones describing the fluid at the boundary. Functional expressions for the transport coefficients are found --with those associated to viscosity and heat flux uniquely determined--, satisfying a set of known causality requirements for the underlying equations of motion.

hep-th↗

arXiv2023

Modelling self-consistently beyond General Relativity

The majority of extensions to General Relativity display mathematical pathologies (higher derivatives, character change in equations that can be classified within PDE theory, and even unclassifiable ones) that cause severe difficulties to study them, especially in dynamical regimes. We present here an approach that enables their consistent treatment and extraction of physical consequences. We illustrate this method in the context of single and merging black holes in a highly challenging beyond GR theory.

gr-qc↗

arXiv2022

Non-linear black hole dynamics and Carrollian fluids

The dynamics of black hole horizons has recently been linked to that of Carrollian fluids. This results in a dictionary between geometrical quantities and those of a fluid with unusual properties due its underlying Carrollian symmetries. In this work we explore this relation in dynamical settings with the interest of shedding light on either side by relevant observations. In particular: we discuss how the null surface where the Carrollian fluid evolves is affected by its behavior; that the fluid's equilibration properties are tied to teleological considerations; the connection of higher derivative contributions as both source of energy and dissipation for the fluid and the non-linear behavior of black holes. This latter point, connects with discussions of non-linear modes in the relaxation to equilibrium of perturbed black holes.

gr-qc↗