Searcharxiv⌕ Search

arXiv subjects

Vitor Cardoso

Publications and source records attributed to Vitor Cardoso.

At least 19 recordsLinked to original sources
arXiv2026

How Fast do Black Holes Spin?

We present a systematic study of the spin evolution of accreting black holes. Using the BHAC code for general relativistic magnetohydrodynamic simulations, we conduct a total of 500 high resolution simulations of magnetized accretion disks around Kerr BHs with spin parameters ranging from $χ=0$ to $χ= 0.99$. The baseline configuration follows the default Fishbone--Moncrief torus setup in BHAC. We further explore the effects of: I) introducing multiple poloidal magnetic loops, II) the initial position of the torus, III) magnetic field strength and IV) radial loop modulation. From each simulation we extract the mass flux and angular momentum flux through the horizon to compute the time-averaged spin evolution rate after the system becomes quasi-stationary. The equilibrium spin ranges from $χ_{\rm eq}\simeq0.88$ in the fiducial model to $χ_{\rm eq}\simeq0.98$ for weak-flux model. Applied to Sgr A$^*$, even an optimistic estimate for rapid spin evolution gives a timescale longer than $10^{13}$ yr, indicating an earlier growth or merger history. These trends are consistent with the picture that larger net magnetic flux threading the horizon enhances jet-torques and reduces the equilibrium spins, whereas weak net flux yields near-maximal spins. Our results provide a pathway to interpret the high spins reported for X-ray binaries and active galactic nuclei and they sharpen the astrophysical viability of near-horizon high-energy processes that rely on near-extremal black holes.

astro-ph.HE↗
arXiv2026

The School on Gravity: from motion to commotion

Understanding the gravitational interaction is one of the great scientific endeavors of the 21st century. The Nobel Prize-winning discovery of gravitational waves in 2015, and subsequent detections by the LIGO-Virgo-KAGRA Collaboration, have marked a revolutionary step forward in physics and astrophysics by opening an entirely new window for exploring the Universe. Future upgrades to these detectors, along with upcoming facilities such as LISA, the Einstein Telescope, and radio and optical instruments, promise other breakthrough discoveries that the next generation of scientists will need to interpret. The School on Gravity (22--26 June 2026, Copenhagen) aimed to introduce junior scientists--graduate students and advanced undergraduates with a background in general relativity--to a wide range of foundational topics in the exciting new field of gravitational-wave physics. Topics spanned from the theory of motion, to new challenges introduced by quantum physics, to the astrophysics of black holes and source modeling for gravitational-wave detectors. The School on Gravity took place at the legendary Auditorium A of the Niels Bohr Institute, inviting students and lecturers to challenge current paradigms in gravitational physics in the spirit of the quantum revolution that unfolded in this very room. These lecture notes aim at opening up those new ideas presented in Auditorium A to the rest of the scientific community.

gr-qc↗
arXiv2026

Second-order scattering response of a Schwarzschild black hole

Gravitational waves couple in a nonlinear fashion in the vicinity of a black hole. Black hole perturbation theory can be readily applied to compute the magnitude of this coupling, its dependence on the parity content, frequencies, and angular structure of the incoming gravitational waves. In this work we carry out this calculation, showing how the nonlinear coupling of gravitational waves generically peaks when the driven frequency matches the oscillation frequency of the fundamental quasinormal mode of the black hole. We also demonstrate how this excitation is largest at the maximal harmonics allowed, and study its dependence on the parity content of the incoming modes. At low frequencies, the quadratic response computed here encodes the nonlinear, dynamical tidal deformability of the black hole spacetime itself. We demonstrate numerically that the quadratic black hole coupling coefficient scales quadratically with the driving frequency, and recover this scaling from the 5-point Compton graviton scattering amplitude.

gr-qc↗
arXiv2026

Cherenkov Bursts are Memory Steps

We show that the familiar Cherenkov burst from a finite superluminal charge track can be understood as a pair of interfering electromagnetic memory transitions. Endpoint accelerations of the track produce opposite memory steps in the transverse vector potential, such that the corresponding finite-frequency radiation is universally fixed, below the endpoint-acceleration-scale, by the memory amplitude and the time for which the temporary memory offset persists. As the Cherenkov direction is approached, the memory amplitudes grow while their separation in observer time shrinks, until the two vector-potential steps overlap and their divergent contributions cancel into a finite Cherenkov burst. This perspective opens the door to using the Askaryan effect as a setting in which individual macroscopic memory transitions may be resolved, providing a new experimental arena for memory-linked infrared physics.

gr-qc↗
arXiv2026

The quasinormal modes of Kerr-Newman black holes: a separated approach

We compute Kerr-Newman quasinormal modes using a recently found exact separation of the gravitoelectric perturbation equations. For each spin-label $ς=\pm1$, a matrix angular ordinary differential equation and a scalar second-order radial ordinary differential equation define a joint spectral problem for the complex frequency $ω$ and separation constant $λ$. We solve this problem by imposing regularity at the angular poles, ingoing behavior at the event horizon, and outgoing behavior at infinity. We benchmark the numerical code built to solve the separated equations, and the resulting frequencies, against recently published spectral calculations of the coupled system. Comparisons with those results yield absolute differences of order $10^{-10}$ to $10^{-5}$, depending on the BH parameters and overtone number. We also find numerical hints of spin-label isospectrality in both the electromagnetic-led and gravitational-led sectors of fundamental quasinormal modes, even at near-extremal Kerr-Newman configurations. These results provide numerical evidence that the separated equations reproduce Kerr-Newman quasinormal frequencies and offer a practical route to exploring the ringdown of rotating, charged black holes.

gr-qc↗
arXiv2026

Spectral instabilities in the time domain

We investigate the effect of a spectral instability on the time domain waveform using a one-dimensional double Pöschl-Teller model. By analytically following successive scatterings between the primary potential barrier and a weak, spatially separated perturbation, we identify a secular contribution that first appears after one causal round trip between the barriers. Before the first echo, all overtones remain at their unperturbed frequencies. After the first echo reaches the observer, there is a secular, linear-in-time correction. When this correction is perturbative, it can be interpreted as a shift of the quasinormal mode frequency. In that case, we show that its coefficient reproduces the frequency-domain result. More generally, it generates finite-time effective frequencies that need not coincide with either the unperturbed or fully perturbed quasi-normal mode spectrum. We confirm the results of our analytic calculation using numerical time-domain evolutions.

gr-qc↗
arXiv2026

Inspirals into bosonic dark matter stars and chirp mimickers

We investigate extreme--mass--ratio inspirals in which a stellar-mass compact object orbits a supermassive bosonic dark matter star, modeled as a boson star, using fully relativistic perturbative methods. Unlike inspirals around electro-vacuum black holes, these systems can shed scalar matter through dynamical friction which significantly alters the inspiral dynamics. We show that this additional dissipation can induce a chirp-like gravitational-wave signal closely resembling that of black hole binaries, allowing boson stars to act as gravitational-wave chirp mimickers even when they are not ultracompact. The inspiral evolution and resulting waveform depend sensitively on the compactness of the central boson star: highly compact configurations trigger dipolar scalar radiation, leading to a rapid plunge, whereas less compact stars yield smoother inspirals dominated by gravitational and quadrupolar scalar waves. To support waveform modeling, we derive semi-analytical prescriptions for the gravitational and scalar energy fluxes that remain accurate deep into the relativistic regime. Our findings indicate that future space-based detectors such as LISA could distinguish these mimicker signals from true black hole inspirals through measurable phase dephasings induced by scalar dissipation.

gr-qc↗
arXiv2026

Wave optical imaging of an oscillating electric dipole orbiting a black hole

We study the electromagnetic radiation and wave-optical imaging of an oscillating electric dipole orbiting a Kerr black hole. We derive the effective 4-current associated with a pointlike oscillating electric dipole in curved spacetime, and use black hole perturbation theory to compute the resulting radiation field at future null infinity, from first principles. We then develop a wave-optical imaging framework for a moving electromagnetic source in curved spacetime. We obtain images of an orbiting electric dipole, displaying relativistic beaming, gravitational lensing, and Einstein rings. We study polarization-dependent scattering by comparing the images produced by spinning dipoles with opposite helicities, finding a displacement that roughly decreases with the inverse of the radiation frequency, as expected for a beyond-geometric-optics effect. Our results provide a first-principles benchmark for beyond-geometric-optics descriptions of electromagnetic radiation in Kerr spacetime.

gr-qc↗
arXiv2026

Ringing of rapidly rotating black holes in effective field theory

Within the effective field theory approach to gravity, deviations from general relativity can be systematically described by higher-curvature operators. However, computing the resulting corrections to black hole quasinormal mode spectra remains challenging in the rapidly rotating regime, where perturbative expansions in the spin break down. We use recently constructed numerical rotating black hole solutions to compute quasinormal mode frequency corrections at leading order in the effective field theory. Focusing on scalar perturbations, we evaluate cubic-curvature corrections, which constitute the leading modifications. We employ a pseudo-spectral collocation method to solve the resulting perturbation equations on these backgrounds, enabling accurate computation across a broad parameter range. We obtain frequency corrections for fundamental modes with $l\le5$ for all $m$, and the first overtone of $2 \le l \le 5$ modes for all $m$ for spins up to $a=0.99M$, with relative errors below $10^{-4}$. We observe that corrections to certain modes grow significantly as the spin approaches the near-extremal regime.

gr-qc↗
arXiv2026

Dynamics and Frequency Conversion of Accreting Axion Clouds

Axion fields can form exponentially growing gravitational clouds around compact objects through self-interaction-driven relaxation of ambient axion waves. As the field amplitude approaches the axion decay constant, nonlinear effects become important. We identify two distinct regimes of late-time evolution, determined by the gravitational fine-structure constant and the cloud growth rate: a Bosenova regime, characterized by collapse accompanied by explosive axion bursts, and a saturation regime, in which self-interaction-induced axion emission balances accretion. In the latter regime, the emitted axion radiation exhibits stable discrete spectral lines at odd multiples of the bound-state energy, directly probing the global structure of the axion potential beyond its quadratic minimum. We show that single-cosine potentials and QCD axion-like potentials predict distinct emission spectra, enabling probes of the underlying axion self-interaction structure and its ultraviolet completion through terrestrial detection of relativistic axion fluxes from compact objects.

hep-ph↗
arXiv2026

Ultralight Boson Ionization from Comparable-Mass Binaries

Detection of gravitational waves enables probes of environmental effects around compact binaries. Ultralight bosons, well motivated in particle physics and capable of forming core-like dark matter structures, induce environmental dynamics that differ qualitatively from those produced by stars or particle dark matter. For comparable-mass binaries, such bosons can form gravitationally bound states analogous to molecules once the binary separation falls below the characteristic wavelength of the bound states, with an inner region co-moving with the binary. We combine numerical simulations and a semi-analytic framework to characterize the structure and ionization of these gravitational molecules. We determine the extent of the co-moving region and compute the ionization flux driven by orbital motion over a range of eccentricities. Using these results, we estimate the backreaction on the binary orbital evolution and identify a new environmental effect: eccentricity-induced ionization of the co-moving component leads to efficient circularization. We further show that this molecular phase can be astrophysically viable and significantly modify the stochastic gravitational wave background from supermassive black hole binaries.

gr-qc↗
arXiv2026

Black hole spectroscopy: from theory to experiment

The "ringdown" radiation emitted by oscillating black holes has great scientific potential. By carefully predicting the frequencies and amplitudes of black hole quasinormal modes and comparing them with gravitational-wave data from compact binary mergers we can advance our understanding of the two-body problem in general relativity, verify the predictions of the theory in the regime of strong and dynamical gravitational fields, and search for physics beyond the Standard Model or new gravitational degrees of freedom. We summarize the state of the art in our understanding of black hole quasinormal modes in general relativity and modified gravity, their excitation, and the modeling of ringdown waveforms. We also review the status of LIGO-Virgo-KAGRA ringdown observations, data analysis techniques, and the bright prospects of the field in the era of LISA and next-generation ground-based gravitational-wave detectors.

gr-qc↗
arXiv2026

Probing Fundamental Constant Oscillation in the Galactic Center with S-Star Spectroscopy

Astrophysical spectroscopy provides a powerful probe of spacetime variations of fundamental constants, as atomic and ionic emission and absorption lines depend sensitively on the fine-structure constant. In particular, coherent temporal oscillations induced by an ultralight scalar background produce characteristic, time-resolved signatures that can be robustly disentangled from intrinsic variability. In the Galactic Center, such scalar backgrounds can be substantially enhanced, either through the formation of dense scalar clouds powered by black hole rotational energy extraction or as ultralight scalar dark matter forming a soliton-like core. These scalar configurations generically induce oscillations of the fine-structure constant, with periods set by the scalar mass and spatial profiles determined by the scalar wavefunction and its coupling to the electromagnetic sector. We show that precise, time-resolved spectroscopy of S-stars orbiting the supermassive black hole Sgr A$^*$ provides a sensitive test of these effects, enabling constraints on quadratic scalar-photon couplings in the exceptionally high boson-density environment of the Galactic Center.

hep-ph↗
arXiv2026

Ringdown and lensing of triple systems

Triple systems have progressively been recognized as ubiquitous in our Universe and provide a good testing ground for wave generation and propagation in nontrivial environments. We study the dynamics of triple systems in a fully nonlinear setting. In particular, we analyze numerical relativity simulations of head-on collisions of black holes in the presence of a companion. We show evidence for Doppler and gravitational redshift in the ringdown, and clear signs of amplification by lensing. In certain cases, we also show the appearance of a second image, with hints of resonant mode excitation. Our results pave the way for the understanding of mergers in the vicinity of massive companions. Even in extreme setups we do not find collapse to black holes from lensed gravitational radiation.

gr-qc↗
arXiv2026

The weight of light: colliding pulses of radiation in General Relativity

We study the gravitational interaction of very strong pulses of electromagnetic radiation, up to and beyond gravitational collapse. We demonstrate the existence of very compact states close to the threshold of collapse, both for a single pulse, and a head-on collision of such pulses, and we measure their compactness to be above the maximum value proposed by the hoop conjecture. For a single pulse above the threshold of collapse, a fast black hole is produced. Depending on the amount of tuning, head-on collisions can form two horizonless states, two small black holes or one big black hole. Our results show that the gravitational interaction of massless fields can lead to nontrivial final or intermediate states, while giving rise to extremely high luminosity and compactness.

gr-qc↗
arXiv2026

Axial Oscillations of Viscous Neutron Stars

The oscillation modes of stars play an important role in observations, and on the understanding of stellar stability properties. The role of viscosity in the oscillation modes of compact stars has been so far understood very loosely only, in absence of a well posed framework. We use recent breakthroughs in the formulation of a causal and stable theory of relativistic hydrodynamics, to study oscillation modes of neutron stars. We characterize the axial spectrum of compact stars and uncover new, viscosity-driven families of modes, without a perfect fluid counterpart. Our results show mode avoidance in some of these families, and a spectrum of long-lived modes, whose role in astrophysical, dynamical processes is yet to be understood.

gr-qc↗
arXiv2026

Total absorption of tailored incoming signals by black holes

We uncover a new class of phenomena in gravitational physics, whereby resonances in the complex plane can be excited via tailored time-dependent scattering. We show that specific forms of temporal modulation of an incoming signal can lead to complete absorption for the entire duration of the scattering process. This, then, makes stars and black holes truly black. Such ``virtual absorption'' stores energy with high efficiency, releasing it once the process finishes via relaxation into the characteristic virtual absorption modes -- also known as total transmission modes -- of the object. While such modes are challenging to obtain and four-dimensional black holes have a restricted set of solutions, we also show that higher dimensional black holes have a complex and interesting structure of virtual absorption modes.

gr-qc↗
arXiv2026

The relativistic restricted three-body problem: geometry and motion around tidally perturbed black holes

We investigate the geometry of a tidally deformed, rotating black hole and timelike geodesics in its vicinity. Our framework provides a local picture of the structural evolution of a relativistic restricted three-body problem around a deformed black hole in an adiabatically evolving binary, motivated by various astrophysical settings including disk dynamics and extreme mass-ratio inspirals. As the tidal-field strength is increased, initially regular, bound geodesics undergo four stages: (i) weak chaos emerges within the bound motion; (ii) a subset of trajectories plunges into the black hole; (iii) a fraction of the remaining trajectories becomes unbound; and (iv) no bound trajectories persist. We provide semi-analytic estimates for the critical tidal amplitudes associated with each transition. Our estimates, within the idealized test-particle description, indicate that, within the frequency band of ground-based gravitational-wave detectors, the matter flow around black holes may already be depleted, whereas LISA and (B-)DECIGO could probe the earlier stages. Our results suggest that an object orbiting a tidally deformed massive black hole may remain near resonances in a long term, indicating an accumulated, non-negligible impact on the gravitational-wave phase. Another finding is that tidal perturbations can modulate nonlinear couplings among epicyclic oscillations of geodesics, and could therefore, in principle, affect resonant excitation mechanism potentially relevant to quasi-periodic oscillations in X-ray light curves from accreting black holes.

gr-qc↗