arXiv subjects

David Pereñiguez

Publications and source records attributed to David Pereñiguez.

At least 19 recordsLinked to original sources

arXiv2026

Accurate Black Hole Quasinormal Modes, Regge Poles, and Greybody Factors from Light Ring Ladder Symmetry

The characteristic oscillations of a black hole can be understood in terms of null rays trapped near its unstable light ring, with this correspondence becoming exact in the high-frequency limit. In this regime, we uncover a hidden ladder symmetry in the perturbation equations, which maps the calculation of quasinormal modes, Regge poles, and greybody factors for a spherically symmetric black hole onto the quantum-mechanical problem of an anharmonic oscillator. This provides a systematic, efficient framework for computing the black hole response, with improved convergence properties compared with existing approximation schemes. As an explicit application, we analytically compute all three quantities for a Schwarzschild black hole through eighteenth order in inverse angular momentum, obtaining highly accurate results. We explore different resummation schemes and find that they can further improve the accuracy of the Regge poles and, particularly, the greybody factors. Our framework can be readily extended to rotating black holes and to modified theories of gravity.

gr-qc↗

arXiv2026

Tidal perturbations of an extreme mass ratio inspiral around a Kerr black hole

We determine the metric of a Kerr black hole subject to external tidal fields using metric reconstruction techniques. Working within the Newman-Penrose formalism, we solve the Teukolsky master equation for static, quadrupolar modes associated with a slowly varying tidal environment, and reconstruct the corresponding metric perturbation in the outgoing radiation gauge. As an application, we derive the secular Hamiltonian governing the motion of a test particle in the tidally deformed Kerr spacetime and investigate long-term tidal effects relevant to extreme-mass-ratio inspirals. In particular, we compute tidal-induced shifts of the innermost stable circular orbit and the light ring. We find that these tidal corrections are strongly spin dependent, with significantly larger effects for retrograde orbits around rapidly rotating black holes. Our results provide a fully analytic framework for studying tidal interactions and secular dynamics in rotating black-hole spacetimes, with direct applications to gravitational-wave modeling and tests of gravity in the strong-field regime.

gr-qc↗

arXiv2026

Black Hole Ringdown Nonlinearities in the Large-D Limit

We initiate the study of nonlinear effects in the ringdown phase of black hole mergers using the effective theory of black hole dynamics in the large-D limit. This framework offers several advantages: the quasinormal mode spectrum, including nonlinear corrections, is analytically tractable; numerical simulations of collisions are computationally inexpensive; and the extraction and analysis of the ringdown signal are clean and controlled. As a proof of concept, we derive analytic expressions for the third-order response of a static black hole driven by a single quasinormal mode, and apply them to study the ringdown following head-on collisions of non-spinning black holes across a range of velocities and mass ratios. We find that including nonlinear effects, up to quadratic and cubic order, improves the accuracy of quasinormal-mode modelling of black hole relaxation by several orders of magnitude. The results also show a clear growth in the strength of nonlinear effects as the collision velocity increases.

gr-qc↗

arXiv2026

Unifying the Regge-Wheeler-Zerilli and Bardeen-Press-Teukolsky formalisms on spherical backgrounds

We develop a formulation of perturbation theory on spherically symmetric backgrounds based on self-dual curvature equations combined with spherical harmonic expansions. The resulting framework unifies the Regge-Wheeler-Zerilli (RWZ) and Bardeen-Press-Teukolsky (BPT) formalisms and is designed to combine key advantages of both. The use of self-dual variables is crucial, and makes quasinormal mode isospectrality manifest, when present. We present the formalism first for a general energy-momentum tensor, and then specialize to vacuum General Relativity with matter sources to illustrate its practical advantages. A central result is that the RWZ and BPT equations arise directly as different components of a single tensorial curvature equation. We also show that, in the frequency domain, the metric can be reconstructed algebraically from any of the proposed master functions and their derivatives, and we comment on possible obstructions to such a reconstruction in the time domain. A Mathematica notebook, based on xAct, that implements the formalism and was used in our computations is released alongside this work.

gr-qc↗

arXiv2026

Gravitational electric-magnetic duality at the light ring and quasinormal mode isospectrality in effective field theories

Black hole perturbations are characterized by a superposition of damped exponentials known as quasinormal modes. In general relativity, the spectra of parity-even and parity-odd quasinormal modes coincide -- a property known as isospectrality, which is typically broken by corrections beyond general relativity. Recently, certain higher-derivative operators were shown to preserve isospectrality in the high-frequency (eikonal) regime. Motivated by the relation between the light ring Penrose limit and the eikonal limit, we study isospectrality in a class of plane-wave spacetimes. In general relativity, we show that dynamical metric fluctuations on these backgrounds admit a gravitational analog of electric-magnetic duality, which enforces isospectrality. Requiring this duality to persist in the presence of higher-derivative corrections constrains the couplings so that isospectrality is preserved. We conclude that gravitational electric-magnetic duality at the light ring is the organizing principle behind isospectrality in the eikonal limit, and we conjecture that this remains true for other duality-invariant corrections to general relativity.

gr-qc↗

arXiv2026

On the non-zero Love numbers of magnetic black holes

Black holes are believed to possess vanishing Love numbers, which implies that they do not deform in the presence of external tides. This fact has been verified in a number of scenarios, that involve tides of bosonic fields of various natures (e.g. scalar, electromagnetic and gravitational), and has triggered active research in trying to identify the underlying reason. Surprisingly, two counterexamples have been found recently. The first, concerns charged-field tides on electrically-charged black holes. In that case, however, the response cannot be disentangled from dissipative effects, and might thus be argued to not consist of a truly conservative deformation. The second concerns fermionic tides on neutral holes. While these yield a purely conservative response, they lack a classical interpretation, which is the physical regime where black hole deformability is understood. Here, we consider magnetic Reissner-Nordstrom black holes and show that electrically-charged, scalar-field tides induce non-vanishing tidal Love numbers. We prove that this is a purely non-dissipative effect, in contrast to the cases of rotating or electrically-charged black holes, and hence consists of a genuine deformation. In addition, the magnetic charge resolves common ambiguities in defining Love numbers, so our result does not rely on any regularisation scheme. This constitutes a clear realisation of how new physics can influence black hole tidal deformability, and offers new perspectives on the study of black hole Love numbers.

gr-qc↗

arXiv2026

Gravitational Atoms from Topological Stars

We study the bound states of a massive scalar field around a topological star, and show that these are strictly normal modes. This yields a genuine gravitational atom, sharply distinguishing horizonless objects from black holes. We show that the modes are controlled by the field's Compton wavelength compared to the size of the star. When the Compton wavelength is large, the field forms a cloud with a hydrogen-like spectrum, while in the opposite regime it is localized along timelike trajectories. When the two scales are comparable the spectrum becomes richer, and we characterize it in detail allowing the field to carry electric charge and Kaluza--Klein momentum.

gr-qc↗

arXiv2025

Perturbations of Plane Waves and Quadratic Quasinormal Modes on the Lightring

We study second order gravitational perturbations on plane wave spacetimes from both the metric and curvature perturbation points of view. For the former, we explicitly use the isometries of the background to introduce tensor oscillator harmonics, which render Einstein's equations algebraic around symmetric plane waves. For the latter, we formulate the first and second order Teukolsky equations in a Geroch-Held-Penrose covariant way. Both approaches are useful in their own right, and together with our discussion on gauge freedom, they provide a foundation for the study of higher-order gravitational dynamics around plane wave spacetimes. Taking the perspective that these plane wave spacetimes arise from Penrose limits, we subsequently use these results to explore the nonlinear gravitational dynamics close to black hole lightrings. Specifically, we define and discuss quadratic quasinormal mode ratios, observe that they satisfy emergent selection rules, and make publicly available a code to compute them.

gr-qc↗

arXiv2025

Ringdown nonlinearities in the eikonal regime

The eikonal limit of black hole quasinormal modes (the large multipole limit $\ell \gg 1$) can be realized geometrically as a next-to-leading order solution to the geometric optics approximation, and also as linear fluctuations about the Penrose limit plane wave adapted to the lightring. Extending this interpretation beyond the linear order in perturbation theory requires a robust understanding of quadratic quasinormal modes for large values of $\ell$. We analyze numerically the relative excitation of quadratic to linear quasinormal modes of Schwarzschild black holes, with two independent methods. Our results suggest that the ratio of quadratic to linear amplitudes for the $\ell \times \ell \to 2\ell$ channel converges towards a finite value for large $\ell$, in sharp contrast with a recent proposal inspired by the Penrose limit perspective. On the other hand, the $2 \times \ell \to \ell + 2$ channel seems to have a linearly growing ratio. Nevertheless, we show that there is no breakdown of black hole perturbation theory for physically realistic initial data.

gr-qc↗

arXiv2025

Decoupled Gravitational Wave Equations in Spherical Symmetry from Curvature Wave Equations

Black hole perturbation theory on spherically symmetric backgrounds has been instrumental in establishing various aspects about the gravitational dynamics close to black holes, and continues to be an interesting avenue to confront current challenges in gravitational physics. In this paper, we present an approach to perturbation theory in spherical symmetry that addresses simultaneously some conceivably inconvenient aspects of the traditional methods. In particular, focusing on Schwarzschild's background we are able to derive a decoupled wave equation, for a single complex variable, by simply computing one component of the curvature wave equation satisfied by a complex self-dual version of the Riemann tensor. The real and imaginary parts of the variable consist only of even and odd pieces of the metric fluctuation, respectively, and both satisfy the Regge-Wheeler equation. Besides providing a systematic derivation of decoupled equations, an immediate corollary of our results is the isospectrality between even and odd sectors. We conclude by discussing potential extensions of our formalism to include matter and higher orders in perturbation theory.

gr-qc↗

arXiv2024

Charged Binaries in Gravitational Tides

Next-generation low-frequency interferometers are expected to detect binary systems near supermassive black holes, where tidal effects can alter significantly the motion of the binary. This motivates a broader investigation of how external gravitational fields influence the dynamics of physical systems. In this work, we consider a charged black hole binary system subject to a gravitational tide. We first construct a stationary gravitational tide acting on a dyonic Reissner-Nordström black hole and, focusing on the extreme mass-ratio limit, we analyze the motion of a test particle. By calculating the secular Hamiltonian of the test particle, we obtain the ISCO and light ring tidal shifts in terms of explicit functions of the parameters of the binary. Our results show that tidal corrections are suppressed as the charge of the black hole increases, but they persist in the extremal limit yielding a finite contribution. This work paves the way towards studying tidal effects on other charged systems, such as topological stars.

gr-qc↗

arXiv2024

Superradiant Instability of Magnetic Black Holes

Black hole superradiance has proven being very valuable in several realms of gravitational physics, and holds a promising discovery potential. In this paper, we consider the superradiant instability of magnetically-charged, rotating black holes and find a number of important differences with respect to neutral ones. Considering massive charged bosonic fields, we find that the instability timescale is much shorter, and this is true even if the black hole contains an order-one number of magnetic monopoles, or merely a single one, and possesses either low, moderate or large values of angular momentum. In particular, the instability is drastically faster than the radiative decay time of charged pions, potentially making it physically relevant. Furthermore, our analysis identifies the most unstable modes as a class of monopole spheroidal harmonics, that we dub north and south monopole modes, whose morphology is markedly different from the ones in standard superradiance since they extend along the rotational axis. For completeness, we also study the quasinormal mode spectrum and amplification factors of charged massless fields, finding no evidence of instabilities in that case.

gr-qc↗

arXiv2023

Ringdown of a dynamical spacetime

The gravitational waves emitted (some time) after two black holes merge are well described by the theory of linear perturbations on a spacetime characterized by the mass and spin of the remnant. However, in the very early stages right after merger, both the mass and spin are changing. In this work we explore, in a set up based on Vaidya's spacetime, the dynamical consequences of a change of mass in the spacetime due to the accretion of null matter (for example, gravitational waves). We show that accretion imprints time-dependent frequencies and amplitude to a ringdown waveform, and we show how to model accurately this effect in certain regimes. We also comment on the direct emission of gravitational waves due to perturbations in the in--falling matter, which is of relevance for black holes embedded in astrophysical environments.

gr-qc↗

arXiv2023

Constraints on massive gravity from dipolar mode excitations

We study extreme-mass-ratio systems in theories admitting the Schwarzschild solution and propagating a massive graviton. We show that, in addition to small corrections to the quadrupolar and higher-order modes, a dipolar mode is excited in these theories and we quantify its excitation. While LIGO-Virgo-KAGRA observations are not expected to impose meaningful constraints in the dipolar sector, future observations by the Einstein Telescope or by LISA, together with bounds from dispersion relations, can rule out theories of massive gravity admitting vacuum General Relativistic backgrounds. For the bound to be circumvented, one needs to move away from Ricci-flat solutions, and enter a territory where constraints based on wave propagation and dispersion relations are not reliable.

gr-qc↗

arXiv2023

Black Hole Perturbations and Electric-Magnetic Duality

Black holes can be electromagnetically charged, or carry vector charge from new fundamental fields. Their response to small fluctuations is of paramount importance to study gravitational wave generation. However, the usual even and odd sectors of gravitoelectromagnetic waves couple if the black hole is magnetically charged, a fact that complicates significantly the perturbative approach. In this paper, perturbation theory based on harmonic expansion is extended to have manifest invariance under electric-magnetic duality. As a result, the equations decouple into two generalised even and odd sectors, each governed by master wave equations that include the most general coupling to a dyonic source. These can be used to compute, in a simple manner, the gravitational and electromagnetic radiation emitted in the interaction of the most general spherically symmetric black holes of the Einstein--Maxwell theory with electromagnetically charged matter.

gr-qc↗

arXiv2023

Magnetic Black Holes: from Thomson Dipoles to the Penrose Process and Cosmic Censorship

We consider accretion of charged test matter by rotating, magnetic black holes and discuss a number of aspects in which the interaction of the angular momentum contained in the electromagnetic field and the spin of the hole plays a fundamental role. First, we argue that such a black hole tends to lose its angular momentum by accreting charges while remaining globally neutral. Then, we show that accretion can happen in a superradiant manner via an enhanced Penrose process. In particular, we find that the regions from which energy and angular momentum can be extracted contain the axis of rotation and, in some cases, consist of floating bubbles disconnected from the black hole itself. Finally, we address the question of whether extremal dyonic rotating black holes can be overcharged or overspun via accretion of arbitrary matter, and prove that this can not happen if the null-energy condition holds. We conclude by discussing some future research directions.

gr-qc↗

arXiv2022

Magnetic charges and Wald entropy

Using Wald's formalism, we study the thermodynamics (first laws and Smarr formulae) of asymptotically-flat black holes, rings etc. in a higher-dimensional higher-rank generalization of the Einstein-Maxwell theory. We show how to deal with the electric and magnetic charges of the objects and how the electric-magnetic duality properties of the theory are realized in the first laws and Smarr formulae.

hep-th↗

arXiv2022

Love numbers and magnetic susceptibility of charged black holes

The response of black holes to companions is of fundamental importance in the context of their dynamics and of gravitational-wave emission. Here, we explore the effect of charge on the static response of black holes. With a view to constraining broader setups, we consider charged geometries in an arbitrary number of spacetime dimensions $D\geq4$. Tensor tidal Love numbers are shown to follow a power law in the black hole temperature $\sim T_{H}^{2l+1}$, and thus vanish at extremality. In contrast, the black hole charge $Q$ excites new modes of polarisation in the vector sector that are otherwise not responsive in the neutral limit. In four dimensions, Love numbers and magnetic susceptibilities vanish for all values of the charge that respect the extremality bound. Using the theory of Fuchsian equations we are able to obtain analytical results in most cases, even beyond the hypergeometric instances.

gr-qc↗