arXiv subjects

Caio F. B. Macedo

Publications and source records attributed to Caio F. B. Macedo.

At least 19 recordsLinked to original sources

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.

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arXiv2026

Twinkle twinkle dark star: Oscillating profiles from dark matter scalar solitons

Real scalar fields, e.g. the axion, cannot condensate into stationary solitonic configurations to form starlike structures, eventually either dispersing or collapsing. However, by relaxing the stationarity condition on the metric, it has been shown that oscillatory solitonic solutions---known as oscillatons---exist. Oscillatons share several properties with boson stars, including comparable compactness and mass ranges. However, their time-dependent nature can lead to potentially discriminating observable signatures. In this work, we explore the observational properties of oscillatons. We find that stable oscillatory circular orbits exist, extending down to the center of the configuration, supporting the possibility of accretion disk structures within the star. We compute the deflection of light rays and verify that it is largely insensitive to the time dependence of the metric. Despite this, the oscillatory behavior of the redshift factor has a strong effect on the observed intensity profiles from accretion disks, producing a breathinglike image whose frequency depends on the mass of the scalar field. In fact, their oscillation period may lie within the observational windows of the Event Horizon Telescope for Sgr~A$^{*}$ and M87$^{*}$, suggesting that this ``twinkling'' behavior may provide a potential observable signature of time-dependent compact objects. A detailed assessment of detectability in realistic interferometric observations is left for future work.

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arXiv2026

Matter environments around black holes: geodesics, light rings, and ultracompact configurations

Astrophysical black holes are invariably embedded in matter environments whose gravitational influence can alter key strong-field features of the spacetime. In this work, we investigate the impact of spherically symmetric dark-matter distributions on black hole geometry, geodesic structure, and ringdown phenomenology. Modeling the surrounding matter through Einstein clusters, we construct self-consistent spacetimes for three widely used density profiles - the Hernquist, Navarro-Frenk-White (NFW), and Jaffe models - and examine how their near-horizon behavior modifies the location and stability of circular timelike and null geodesics, including the innermost stable circular orbit (ISCO) and light rings. In the low-compactness regime, we derive analytical expressions showing that environmental effects generically shift the ISCO inward and the principal light ring outward, leading to parametric deviations in their associated orbital frequencies and Lyapunov exponents. At higher compactness, we explore the emergence of additional light rings, marginally stable orbits, and secondary horizons, identifying the regions of parameter space in which these ultracompact configurations arise. Using time-domain evolutions of scalar perturbations, we demonstrate how such structures can imprint characteristic signatures on the ringdown signal, including long-lived trapped modes and echo-like modulations associated with multiple potential barriers. Our results provide a unified framework for assessing environmental effects around black holes and highlight the importance of matter-induced corrections for interpreting upcoming electromagnetic and gravitational-wave observations.

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arXiv2026

Multi-photon ring structure of reflection-asymmetric traversable thin-shell wormholes

We consider the observational signatures of thin accretion disks around a reflection-asymmetric traversable thin-shell wormhole. This wormhole, built in the framework of Palatini $f(R)$ gravity coupled to a Maxwell field using a junction conditions formalism, lacks horizons but features photon spheres on each side of the throat, described by different effective potentials and at different locations. This fact allows a portion of the light rays arriving to the observer's screen on one side of the throat to have explored a part of the space-time on the other side, bringing information about the geometry gathered there. In this setting we simulate the optical appearance of such an asymmetric wormhole when illuminated by thin accretion disks, investigating scenarios with either one or two (on each side of the throat) disks, revealing a rich multi-photon ring structure due to light crossing the throat, and a strong reduction in the size of the central brightness depression region. These new rings are more numerous and far more luminous in the two-disk case than in the single-disk case, and the shadow's size reduction far more acute, making a neat distinction as compared to canonical black hole images. These results highlight the potential of high-resolution imaging in providing smoking guns for the existence of ultra-compact objects distinct from black holes via their multi-ring structure.

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arXiv2026

Spectral instability of horizonless compact objects within astrophysical environments

Recent non-modal analyses have uncovered spectral instabilities in the quasinormal-mode spectrum of black holes; a phenomenon that intriguingly extends to spherically-symmetric exotic compact objects. These results point to a sensitivity of the spectrum with potentially far-reaching implications for black-hole spectroscopy. At the same time, growing attention has turned to astrophysical environments around compact objects and their role in shaping gravitational-wave astrophysics. In this work, we establish a direct link between spectral instabilities and environmental effects by modeling matter as a localized bump outside the light ring of a spectrally-unstable exotic compact object with a purely reflective surface. We find that while such environments can destabilize the fundamental quasinormal modes of loosely-compact exotic objects, the fundamental modes of ultra-compact horizonless objects remain remarkably robust. In contrast, overtones are shown to develop spectral instabilities in the presence of the bump. By tracking both interior modes, trapped between the light ring and the surface of the exotic compact object, and exterior modes, confined between the bump and the light ring, we uncover an overtaking instability in which ``unperturbed'' exterior overtones metamorphose into ``perturbed'' fundamental modes as the bump moves outward. Finally, we demonstrate that environmental effects, while capable of further amplifying spectral instabilities, cannot induce next-to-leading-order perturbations strong enough to trigger a modal instability.

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arXiv2025

Relativistic tidal divergences in circular orbits and the dynamics of light rings

Tidal forces acting on orbiting bodies arise from inhomogeneities in the gravitational field, generating stresses that can deform or even disrupt these objects. In this work, we analyze relativistic tidal forces associated with ultracompact objects described by static and spherically symmetric spacetimes, focusing on observers in circular geodesic motion. We show that, in contrast to the case of radial geodesics, tidal forces diverge as the orbit approaches null circular geodesics. As illustrative examples, we study two uniform-density stellar models: one isotropic and another supported purely by tangential stresses. We conjecture that the divergence of tidal forces near light rings may play a role in the nonlinear stability of ultracompact, horizonless objects.

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arXiv2025

Black holes in higher-derivative Weyl conformal gravity

We obtain exact black hole solutions for static and spherically symmetric sources in a Weyl conformal gauge theory of gravity. We consider a quadratic gravitational action built from the Weyl tensor within a dilation geometry. In a post-Riemannian formulation, we derive a Weyl conformal action for a scalar-vector-tensor theory, where the scalar degree of freedom originates from the high-curvature terms and the vectorial one stems from the Weyl non-metricity condition. Adopting a static, spherically symmetric geometry, the vacuum field equations for the gravitational, scalar, and Weyl fields are obtained. Under these conditions, we find a Mannheim-Kazanas-type black hole solution, whose Rindler acceleration term depends on the Weyl gauge coupling constant. Furthermore, we show that the original theory can recover the Einstein-Hilbert action with a positive cosmological constant plus a higher-derivative term with Horndeski-like terms through a spontaneous symmetry breaking triggered by the vacuum expectation value of the scalar field. The new solution in the theory without conformal symmetry presents new terms introduced by the residual Weyl symmetry corrections. We demonstrate that in a regime where the Planck mass suppresses the higher-derivative term, the Rindler term persists in the low-energy limit.

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arXiv2025

Time evolution of perturbations in quasi-Schwarzschild black holes

Parametric deviations of quasinormal modes~(QNMs) is a common feature of beyond General Relativity (GR) theories. For theories with additional degrees of freedom, such as scalars and vectors, new family of modes might appear, usually called scalar-led and vector-led modes. Although a power series expansion in terms of the new parameters entering the potential is usually suitable to describe the frequency of the modes, the time-evolution of signals might present a richer structure, with different behavior in the tail, the presence of new modes (such as massive modes), or even instabilities. All these features are not explicitly exposed by a pure frequency domain analysis and might give hints of new physics. In this paper, we investigate the time evolution of signals considering potentials that slightly deviate from the ones coming from GR, looking into scalar, vector and metric perturbations. We focus on deviations that can be parametrized by a sum of $\sim 1/r^j$ terms in the effective potential, analyzing the effect of each term on the time domain profile.

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arXiv2025

Black hole spectral instabilities in the laboratory: Shallow water analogue

Small deviations in the spacetime around black holes can lead to instabilities in the underlying quasinormal mode spectrum, potentially altering the hierarchy of its overtones. A practical way to induce such spectral instability is by introducing small modifications to the effective potential governing the dynamics of fluctuations in the black hole spacetime. While finding a physically meaningful interpretation for such ad hoc modifications in an astrophysical context can be challenging, analogue black hole models provide an alternative framework to explore their effects and study the instabilities. In this work, we consider an analogue black hole modeled by a draining bathtub flow and demonstrate that vorticities in the fluid introduce a small bump in the effective potential of the wave equation. This naturally realizes a physically motivated version of the elephant and the flea configuration. We analyze the spectrum using two complementary approaches: direct mode computation via two distinct frequency-domain methods and time evolution of initial perturbations. As in astrophysical black holes, the vorticities destabilizes the QNM spectrum of the analogue system, possibly yielding time evolution with long-lived ringing effects, akin to those observed for massive fields in curved spacetimes.

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arXiv2024

Extreme mass-ratio inspirals in ultra-light dark matter

Previous works have argued that future gravitational-wave detectors will be able to probe the properties of astrophysical environments where binary coalesce, including accretion disks, but also dark matter structures. Most analyses have resorted to a Newtonian modelling of the environmental effects, which are not suited to study extreme-mass-ratio inspirals immersed in structures of ultra-light bosons. In this letter, we use relativistic perturbation theory to consistently study these systems in spherical symmetry. We compute the flux of scalar particles and the rate at which orbital energy (and angular momentum) is dissipated via gravitational radiation and depletion of scalars, i.e. dynamical friction. Our results suggest that the Laser Inteferometer Space Antenna will be able to probe ultra-light dark matter structures in the Galaxy by tracking the phase of extreme-mass-ratio inspirals.

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arXiv2024

Scattering effects of bumblebee gravity in metric affine formalism

In this work, we explore a Schwarzschild-like black hole within the framework of metric--affine bumblebee gravity. First, we investigate the behavior of the Kretschmann scalar and singularities in this modified gravity approach. Next, we introduce a newly defined time coordinate related to a stationary asymptotically flat spacetime. We also analyze the scattering effects and numerically calculate and comprehensively examine the partial and total absorption cross sections. At the high--frequency approximation, we find that the absorption cross section tends to the geodesic capture cross section. The continued fraction method is applied to investigate the quasinormal modes, and we explore the deviations of both the real and imaginary terms of the quasinormal modes from the Schwarzschild case in detail. We verify the relation between the shadow radius and the real part of the quasinormal frequencies at the eikonal limit within this modified gravity framework. Finally, we examine the energy emission rate.

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arXiv2024

Quasibound state reminiscent in de-Sitter black holes: Quasinormal modes and the decay of massive fields

Massive perturbations in asymptotic flat black holes leave a distinct signature in their late-time evolution `tail': an oscillatory behavior modulated by the Compton wavelength of the field, which can be associated with the so-called quasibound state spectrum. In asymptotically de-Sitter spacetimes, however, the massive perturbations always leak to the cosmological horizon, which indicates the absence of a quasibound part of the spectrum. In this work, we show that an additional mode exists in asymptotically de-Sitter black holes that produces an imprint similar to that of the quasibound states in the late-time behavior of massive scalar perturbations. If the Compton wavelength is larger than a certain critical value (which depends on the cosmological constant), the oscillatory behavior of the tail turns into an exponential decay due to the fact that the de-Sitter mode is purely imaginary. Even for black holes with typical length scales small in comparison to the size of the cosmological horizon, the late-time tail behavior of the massive perturbations is modified as compared to the usual $t^{-5/6}$ for Schwarzschild black holes, thus being a distinctive feature induced by the presence of a cosmological constant.

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arXiv2024

Axial perturbations of hairy black holes in generalised scalar-tensor theories

Gravitational wave observations can test the validity of General Relativity (GR) in the strong field regime. Certain classes of scalar-tensor theories indeed predict that compact objects can exhibit significant deviations from their GR counterparts. Here we explore the quasinormal modes of axial perturbations in spherically symmetric black holes in three such classes: (i) dilatonic black holes with an additional scalar-Ricci coupling (EdRGB), (ii) spontaneously scalarized black holes (EsRGB) with a quadratic coupling to the Gauss-Bonnet invariant and the Ricci scalar, (iii) spontaneously scalarized black holes with a quadratic and a quartic coupling to the Gauss-Bonnet invariant.

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arXiv2024

Observational imprints of gravastars from accretion disks and hot-spots

In this work, we analyze the observational properties of thin-shell gravastars under two astrophysical frameworks, namely surrounded by optically-thin accretion disks and orbited by hot-spots. We consider the thin-shell gravastar model with two free parameters, the gravastar radius and ratio of mass allocated at the thin-shell, and produce the corresponding observables via the use of numerical backwards ray-tracing codes. Regarding the observations of accretion disks, our results indicate that, due to the absence of a strong gravitational redshift effect, smooth gravastar configurations cannot reproduce shadow observations when internal emission is assumed. We thus expect such models to be excluded as candidates for supermassive objects in galactic cores. Nevertheless, thin-shell gravastars with a large portion of their total mass allocated at the surface can produce such an effect and are thus adequate candidates for black-hole mimickers. In the context of hot-spot orbits, the astrometrical observational properties of ultra-compact gravastars resemble closely those of other ultra-compact objects e.g. fluid stars and bosonic stars. However, for low-compacticity configurations, the time-integrated fluxes feature additional contributions in the form of a high-intensity plunge through image. These qualitative differences in the observational properties of gravastars in comparison with black-hole spacetimes could potentially be discriminated by the next generation of interferometric experiments in gravitational physics.

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arXiv2024

Optical appearance of black holes surrounded by a dark matter halo

Black holes in General Relativity are described by space-time metrics that are simpler in comparison to non-vacuum compact objects. However, given the universality of the gravitational pull, it is expected that dark matter accumulates around astrophysical black holes, which can have an impact in the overall gravitational field, especially at galactic centers, and induce non-negligible effects in their observational imprints. In this work we study the optical appearance of a spherically symmetric black hole both when orbited by isotropically emitting light sources and when surrounded by a (geometrically and optically thin) accretion disk, while immersed in a dark matter halo. The black hole geometry plus the dark matter halo come as a solution of Einstein's field equations coupled to an anisotropic fluid whose density component follows a Hermquist-type distribution. Even in situations in which the geodesic description differs profoundly from the isolated black hole case, we find minor modifications to the primary and secondary tracks of the isotropic orbiting sources, and to the width, location, and relative luminosity of the corresponding photon rings as compared to the Schwarzschild black hole at equal black hole mass and emission models. This fact troubles distinguishing between both geometries using present observations of very-long baseline interferometry.

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arXiv2024

Parametrized black hole quasinormal ringdown. I. Decoupled equations for nonrotating black holes

Black hole solutions in general relativity are simple. The frequency spectrum of linear perturbations around these solutions (i.e., the quasinormal modes) is also simple, and therefore it is a prime target for fundamental tests of black hole spacetimes and of the underlying theory of gravity. The following technical calculations must be performed to understand the imprints of any modified gravity theory on the spectrum: 1. Identify a healthy theory; 2. Find black hole solutions within the theory; 3. Compute the equations governing linearized perturbations around the black hole spacetime; 4. Solve these equations to compute the characteristic quasinormal modes. In this work (the first of a series) we assume that the background spacetime has spherical symmetry, that the relevant physics is always close to general relativity, and that there is no coupling between the perturbation equations. Under these assumptions, we provide the general numerical solution to step 4. We provide publicly available data files such that the quasinormal modes of {\em any} spherically symmetric spacetime can be computed (in principle) to arbitrary precision once the linearized perturbation equations are known. We show that the isospectrality between the even- and odd-parity quasinormal mode spectra is fragile, and we identify the necessary conditions to preserve it. Finally, we point out that new modes can appear in the spectrum even in setups that are perturbatively close to general relativity.

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arXiv2023

Eternal binaries

The two-body problem is extensively studied in open systems and asymptotically flat spacetimes. However, there are many systems where radiation is trapped: they range from radiating charges in cavities to low-energy excitations of massive degrees of freedom, to anti-de Sitter spacetimes. Here, we study the problem of motion of a pointlike particle orbiting a massive compact object inside a cavity. We first show that - assuming circular motion - there are initial conditions for which the self-force vanishes and the binary is eternal. We then consider the evolution of the system under radiation reaction in a toy model which we argue captures the essentials of orbiting particles. We show that eternal circular binaries may exist. We also show that the presence of cavity modes leads to chaos in regimes of strong coupling or when the system is initialized close enough to a resonance. Our results have implications for physics in anti-de Sitter spacetimes and possibly for binaries evolving within dark matter haloes, if it consists on massive fundamental fields.

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arXiv2023

Imaging compact boson stars with hot-spots and thin accretion disks

In this work we consider the observational properties of compact boson stars with self-interactions orbited by isotropically emitting (hot-spot) sources and optically thin accretion disks. We consider two families of boson stars supported by quartic and sixth-order self-interaction potentials, and choose three samples of each of them in growing compactness; only those with large enough compactness are capable to hold light-rings, namely, null bound orbits. For the hot-spots, using inclination angles $θ=\{20^\circ, 50^\circ, 80^\circ \}$ we find a secondary track plunge-through image of photons crossing the interior of the boson star, which can be further decomposed into additional images if the star is compact enough. For accretion disks we find that the latter class of stars actually shows a sequence of additional secondary images in agreement with the hot-spot analysis, a feature absent in typical black hole space-times. Furthermore, we also find a shadow-like central brightness depression for some of these stars in both axial observations and at the inclination angles above. We discuss our findings in relation to the capability of boson stars to effectively act as black hole mimickers in their optical appearances as well as potential observational discriminators.

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