arXiv subjects

Manuel E. Rodrigues

Publications and source records attributed to Manuel E. Rodrigues.

At least 19 recordsLinked to original sources

arXiv2026

Accretion dynamics of thin and thick disks in charged Kalb-Ramond black holes

In this work, we investigate the accretion properties of an electrically charged, static, spherically symmetric black hole in Kalb-Ramond (KR) gravity, where a dimensionless parameter $l$ characterizes spontaneous Lorentz symmetry-breaking. We first constrain the free charge and KR parameter space, $(Q,l)$, by comparing the predicted shadow radius of this family of solutions with the bounds inferred from Event Horizon Telescope observations of Sagittarius A*. The inclusion of electric charge introduces a degeneracy between $Q$ and $l$, enlarging the region compatible with the observed shadow size and yielding the broad constraint $-0.3900 \leq l \leq 0.1898$, with part of the compatible parameter space extending into the naked-singularity sector. We then investigate relativistic accretion disks around the charged KR black hole within the above constraints. For geometrically thin disks, we compute the time averaged energy flux, temperature distribution, emission spectrum, time averaged torque, and mass-to-radiation conversion efficiency, finding that the radiative output increases for higher values of $l$, while lower values of $l$ suppress it. Furthermore, when coupled with the electric charge this effect is amplified. For geometrically thick, constant-angular momentum tori-shaped disks, Lorentz symmetry-breaking shifts the characteristic orbital radii, compresses the radial extent of equilibrium configurations, narrows the interval supporting finite equilibrium tori, and shifts the stationary-fluid topology toward open and unconfined configurations outside that interval. These results show how charge and spontaneous Lorentz symmetry-breaking jointly affect observational constraints and the structure and emission properties of black hole accretion flows, providing potential signatures of KR gravity in the strong-gravity regime.

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arXiv2026

Black bounce sourced by non-minimally coupled linear electrodynamics and a canonical scalar field through a thin shell at the throat

We construct a novel class of black bounce solutions within General Relativity, sourced by a canonical scalar field non-minimally coupled to linear electrodynamics, establishing a self-consistent framework in which regular black holes and traversable wormholes are supported by ordinary bulk matter, with the necessary exoticity confined to an infinitesimally thin defect at the bounce.

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arXiv2026

Entropy dynamics in gravitational collapse: From Minkowski breaking to de Sitter thermodynamics

The origin of black hole entropy remains one of the deepest mysteries in modern physics. Two recent developments offer complementary perspectives on this puzzle: the entropy release from regular Schwarzschild black holes with a de Sitter core (the OCK construction), and the local thermodynamics of the de Sitter vacuum (Volovik). In the OCK framework, the inner horizon carries positive Bekenstein-Hawking entropy $S_{\rm inner}=πh_c^2$ that is gradually released as the core shrinks, until a classical Minkowski breaking obstructs the evolution at $n=0$. In Volovik's framework, the total entropy of a homogeneous de Sitter region is $S_{\rm dS}= \operatorname{sgn}(H)\,π/H^{2} = \operatorname{sgn}(H)\,A/4$: expanding de Sitter ($H>0$) carries positive entropy, while contracting de Sitter ($H<0$) carries negative entropy. We show that the sign of the Hubble parameter is the unifying element linking the two pictures. The OCK core is de~Sitter only for $n>2$; the Minkowski breaking is a classical kinematic obstruction, not a dynamical transition to a contracting de Sitter phase. Any connection between the expanding and contracting regimes would require a quantum mechanism that remains speculative. Nevertheless, both frameworks embody the same thermodynamic principle, the shrinking of the interior drives the system toward increased total entropy, and together they provide a consistent picture of entropy flow during gravitational collapse. The integer nature of the OCK parameter suggests a quantized entropy spectrum and a statistical interpretation of black hole entropy, while the generalized second law is satisfied in both frameworks, and in the speculated quantum connection provided the environmental entropy is properly accounted for.

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arXiv2026

Sources of matter for wormholes in a k-essence theory

In this work, we analyze some matter sources associated with wormhole models within a k-essence theory coupled to the gravitational sector through a phantom scalar field. We adopt a spherically symmetric background in (3+1) dimensions and consider two types of systems: electrically and magnetically charged. In the first case, we consider the generalized Ellis-Bronnikov model, in which we fix the power of the kinetic term in the k-essence Lagrangian function to $n=1/2$ and take the parameter $m\ge 2$, which acts as a generalization factor for the geometry of the wormhole area function. From this, we obtained the expression for the scalar field, the potential, and the associated electromagnetic functions for any values of the parameter $m\geq{2}$. In the second and third models, we consider the scenario of two wormholes that are structured according to the adjustment of the parameters that define the metric component associated with the area function $Σ^2$ (the $g_{22}$ component of the line element), and in both cases we adopt $n=1/2$. We show that the violation of the null energy conditions is conditioned by the parameters of the area function. Finally, we studied the linear stability of the models through the behavior of a test scalar field using both the WKB method and the time-domain evolution method.

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arXiv2026

Entropy release from Minkowski breaking in regular Schwarzschild black holes

The classical formation of a Schwarzschild black hole from a regular, non-singular configuration has recently been shown to be impossible within general relativity: the geometry inevitably develops a discontinuity at the origin, a phenomenon termed Minkowski breaking by Ovalle, Casadio, and Kamenshchik [PRD 113 (2026), 064042]. This obstruction signals that the transition to the Schwarzschild point mass must be a discrete, quantum event. We uncover the thermodynamic footprint of this transition. Using the explicit family of regular Schwarzschild black holes with a de Sitter core, we show that the inner Killing horizon carries a formal Bekenstein-Hawking entropy $S_{\rm inner} = A_{\rm inner}/4$ that is absent in the singular Schwarzschild state. This entropy is hidden from external observers in equilibrium but, assuming the generalized second law, must be released when the inner horizon disappears. As the collapse parameter $n$ decreases, the inner horizon shrinks and its entropy is gradually released during classical evolution, until the horizon finally vanishes at $n=0$ with the Minkowski breaking. The surface gravity diverges as $n\to0^+$, with the semiclassical description breaking down at $n \sim 1/\ln(h/\ell_P)$; the final disappearance is therefore a deep quantum process. For the $n=3$ regular black hole, the stored entropy is approximately $59\%$ of $A/4$; in the semiclassical limit $n\gg1$, it approaches the full $A/4$. The integer nature of $n$ implies a quantized entropy spectrum, with the Schwarzschild black hole as the ground state within the OCK family. We discuss how the classical mass-inflation instability may be circumvented by the quantum disappearance of the Cauchy horizon, and clarify the continuous vs. discrete nature of the collapse.

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arXiv2026

Regular Black Holes in General Relativity from Nonlinear Electrodynamics with de Sitter Cores

We present new regular black hole solutions in general relativity (GR) within a static, spherically symmetric framework governed by a variable equation of state, following the approach of [Class. Quant. Grav. 42, 025024 (2025)]. The matter supporting these geometries is identified as a purely magnetic monopole configuration of the Maxwell-Faraday tensor in the context of nonlinear electrodynamics (NLED). We explicitly reconstruct the corresponding NLED Lagrangian and analyze the asymptotic and central behaviors of the solutions. The geometric structure is examined through the metric functions, the regularity of the Kretschmann scalar, and the profiles of energy density and pressures, including a discussion of the resulting energy conditions. Using Event Horizon Telescope observations of Sgr A$^*$, we constrain the model parameters by comparing the predicted size of the central dark region with the inferred observational images, taking into account the effective geometry experienced by photons in the presence of NLED. Finally, we investigate the dynamical stability of these configurations under scalar perturbations by computing the quasinormal mode spectrum and performing a time-domain analysis.

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arXiv2026

Generalized Unruh temperature for dynamical thin-shell wormholes

We unify two independent notions of effective temperature in dynamical thin-shell wormholes: the local acceleration temperature of the throat and the Hawking-like particle-creation temperature governed by the peeling of null rays. The effective acceleration scale, obtained by averaging over both sides of the junction, defines the thermodynamic shell temperature, while peeling functions for each asymptotic universe yield an effective particle-creation temperature. For asymptotically exponential null-ray maps, we derive the generalized Unruh identity that equates these two temperatures. Applied to Schwarzschild-Schwarzschild wormholes, the formalism yields explicit analytical expressions for the Hawking-like spectra, renormalized quantum fluxes, and quasi-horizon limits in both symmetric and asymmetric configurations. The divergence of the shell temperature near the quasi-horizon is shown to mirror the divergence of the local Tolman temperature, both originating from the infinite gravitational blueshift at a horizon. These results provide a unified interpretation of acceleration, particle creation, and temperature in horizonless dynamical spacetimes, extending the conceptual link between the Hawking and Unruh effects beyond stationary black holes.

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arXiv2026

Thermodynamics of thin-shell wormholes

We develop a unified thermodynamic description of dynamic thin-shell wormholes, starting from the transparent (vacuum bulk) case and then relaxing the transparency condition to include bulk matter crossing the throat. For isolated shells, we show that the entropy is conserved under transparent evolution, $TdS=0$, which holds for arbitrary dynamical motion of the throat within the assumptions of the thin-shell formalism. When bulk matter is present, the generalised first law becomes $T\dot{S}=AΦ$, where $Φ$ is the net energy flux; entropy increases (decreases) when matter flows into (out of) the shell. Explicit expressions for the flux are provided for null dust and massless scalar fields, and quantum pair production (Hawking-like emission) is also discussed. A formulation of the generalised second law is presented, and it is consistent with standard thermodynamic expectations under conditions where heat flows from hotter to colder regions. As a concrete astrophysical application, we study accretion of null dust: the critical accretion rate above which the throat becomes dynamically unstable depends sensitively on the spacetime geometry and the shell equation of state, highlighting the environmental dependence of these configurations. Fluctuations in the energy flux induce a stochastic force on the throat dynamics, leading to a Langevin description with an associated fluctuation-dissipation relation. This results in diffusion of the throat radius and additional entropy production. The framework applies to a broad class of spherically symmetric thin-shell constructions within general relativity. Our results provide evidence that thin-shell wormholes admit a consistent thermodynamic description, quantify their sensitivity to accretion processes, and suggest possible avenues for probing exotic compact objects with future gravitational wave observations.

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arXiv2026

Black bounce solutions in a realistic dark matter halo from M60*

We formulate a Simpson-Visser black bounce solution embedded in a dark matter halo. The latter is modeled using an empirical density profile calibrated from observations of the elliptical galaxy NGC 4649 (M60), based on imaging from the Hubble Space Telescope, stellar velocity dispersion data, and the dynamics of globular clusters. The resulting spacetime metric, in addition to retaining dependence on the mass parameter $m$, the asymptotic circular velocity $V_c$, and the halo scale radius $a$, also depends on the regularization parameter $q_H$. It reduces to the canonical black bounce solution without a halo in the limit $V_c\to0$ (or $a\to\infty$), and to the Schwarzschild solution with a dark matter halo when $q_H\to0$. We analyze the response of fundamental geometrical and physical quantities in the presence of a halo, such as the event horizon radius, the shadow size, and some curvature invariants. In particular, we show the observational range of the shadow radius, from the imaging of Sagittarius A*, constrains the parameter space of the solution to regular black hole configurations, excluding wormhole scenarios. We study the dynamics of massless particles here through the effective potential and examine thermodynamic properties, highlighting the impact on thermodynamic potentials in terms of entropy. Finally, we extend the analysis to scenarios with electromagnetic fields non-minimally coupled to a phantom scalar field, considering configurations with either purely magnetic or purely electric charge. Our results suggest that the dark matter halo influences both the internal geometry and the observational properties of black bounces, imposing constraints on the solution's parameter space from astrophysical data. This highlights the need to include astrophysical environments in modeling regular black holes and wormholes, offering new tests of gravity in the strong-field regime.

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arXiv2026

Linearized stability of T-duality quantum-inspired thin-shell wormholes

Wormholes that are traversable in principle offer fascinating insights into general relativity, yet they typically require exotic matter and suffer from stability issues. We construct a thin-shell wormhole by gluing two copies of a quantum-corrected, regular spacetime obtained from string T-duality. This regularisation replaces the classical curvature singularity with a smooth core and introduces a fundamental length scale $l_0$. For the static configuration, we derive the surface stresses and show that, unlike the Schwarzschild case, the null and strong energy conditions can be satisfied for sufficiently large throat radii. A linearised stability analysis reveals a rich landscape: close to the minimum allowed throat radius the configuration is unstable; at intermediate radii ($a \sim l_0$) the geometric stability threshold becomes negative, yielding a window of \emph{unconditional stability} where any convex surface mass function suffices; at large radii the wormhole recovers Schwarzschild-like behaviour and stability requires a stiff equation of state. The T-duality scale $l_0$ is thus not merely a regulariser but a key physical parameter that opens a novel region of unconditional stability absent in classical thin-shell wormholes. Our results suggest that quantum-gravity-motivated modifications can simultaneously cure singularities and make traversable wormholes dynamically viable, providing new targets for gravitational-wave astronomy and theoretical studies of exotic compact objects.

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arXiv2026

Black Bounce Solutions from a Self-Interacting 3-Form Field in General Relativity

We construct a new class of black-bounce solutions sourced by a self-interacting 3-form field minimally coupled to general relativity and a scalar field. The 3-form field, which naturally arises in string theory, supergravity, and cosmological models, provides the anisotropic effective stresses required to sustain regular geometries that interpolate smoothly between black holes and traversable wormholes. By exploiting the Hodge duality between a 3-form and a 1-form in four dimensions, we reduce the field equations and obtain exact solutions through the direct integration of the coupled equations of motion. In particular, the solutions are derived from algebraic combinations and manipulations of the Einstein, scalar, and 3-form field equations, starting from a complete action principle, without employing the usual reconstruction procedure in which the metric ansatz is imposed a priori and the matter sector is reconstructed afterwards. This approach reveals two distinct classes of solutions. The first one yields a globally phantom scalar field and a metric function with a characteristic arctangent dependence, reducing to the Schwarzschild-(anti) de Sitter spacetime in the limit of vanishing 3-form coupling. The second class produces a constant 3-form Lagrangian and, remarkably, a partially canonical scalar field, namely phantom only near the bounce and canonical outside the event horizon, a feature previously attainable mainly in modified theories of gravity, but which emerges here within pure general relativity. Both families are globally regular, as confirmed by the finiteness of the Kretschmann scalar, and exhibit an asymmetric horizon structure inherited from the 3-form energy-density distribution. These results demonstrate that the 3-form black-bounce framework is both mathematically consistent and observationally viable.

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arXiv2026

Lorentz-violating signatures in quasi-periodic oscillations from a magnetised Kalb-Ramond black hole

We investigate the dynamics of charged particles around a Schwarzschild-like black hole sourced by a Kalb-Ramond field and immersed in a uniform external magnetic field. The Kalb-Ramond field introduces a Lorentz violation parameter $l$ that modifies the spacetime geometry, while the magnetic field profoundly influences the trajectories through the Lorentz force, leading to a rich variety of orbital behaviours including curled (epicyclic) motion and dramatic transitions between distinct energy boundary configurations. We derive the full equations of motion, the effective potential, and the fundamental frequencies of quasi-periodic oscillations, and perform a comprehensive Monte Carlo Markov Chain (MCMC) analysis using observational data from the microquasars GRO 1655-40, XTE 1550-564, and GRS 1915+105. The pure Schwarzschild model is statistically ruled out for all three sources. For GRO 1655-40 and XTE 1550-564, only the combined effect of the magnetic field and Lorentz violation yields statistically robust models, with best-fit values $\mathcal{B}\sim 0.03$-$0.04$ and $l\sim 0.08$--$0.10$. Remarkably, for GRS 1915+105, the most massive object in our sample, the Lorentz violation parameter alone is sufficient to model the QPO frequencies, yielding an optimal fit with $ΔAIC = ΔBIC = 0$. Across all three objects, a clear trend emerges: the required value of $l$ decreases as the mass of the astrophysical object increases, suggesting a mass-dependent scaling of the Kalb-Ramond parameter. These findings establish the combined KR and magnetic field framework as a viable and statistically robust scenario for modeling QPOs in microquasars, and indicate that Lorentz-violating modifications to gravity may leave observable imprints in the strong-field regime, offering new avenues for testing quantum-gravity phenomenology with current and next-generation X-ray missions.

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arXiv2026

Thermodynamics, Phase Transitions, and Geodesic Structure of F(R)-Phantom Banados-Teitelboim-Zanelli (BTZ) Black Holes

This paper investigates phantom BTZ black holes within the high-curvature gravity theory framework, specifically using a special case of power-Maxwell theory, which functions as a nonlinear electrodynamics source called F(R)-conformally invariant Maxwell gravity. We examine how the phantom or anti-Maxwell field affects the structure of these black holes and how the theory's parameters influence their horizon structure. Additionally, we derive the conserved and thermodynamic potentials associated with these black holes, thereby establishing their conformance to the foundational first law of thermodynamics. Next, the stability characteristics of BTZ black holes endowed with phantom and Maxwell fields are explored under canonical and grand canonical ensemble conditions by inspecting their heat capacity and Gibbs free energy profiles. This assessment reveals how the phantom field and scalar curvature affect these stability regions. We then perform a rigorous analytical verification of the Ehrenfest equations to determine whether the critical behavior of the phantom BTZ black hole corresponds to a second-order phase transition. Our results demonstrate adherence to both Ehrenfest relations, thereby confirming the occurrence of a second-order phase transition within the black hole system concurrent with the critical point. Furthermore, we explore the geodesic structure of the obtained solutions to analyze the motion of massive and massless test particles in the $F(R)$-phantom BTZ spacetime. The analysis demonstrates that stable timelike circular orbits exist only in the phantom regime for negative curvature backgrounds, while the phantom configuration also allows for stable circular photon orbits. These results underscore the significant influence of the phantom field and the F(R) correction on the spacetime geometry and orbital dynamics.

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arXiv2026

Scalar-Electromagnetic Couplings as Source of Deformed Black Hole: From Shadows to Thermodynamic Topology

We reconstruct a static and spherically symmetric black hole geometry originally proposed as an effective metric by identifying a consistent matter source derived from a fundamental action. The space-time is supported by a magnetically charged nonlinear electrodynamics (NED) field non-minimally coupled to a scalar field. Dimensional consistency reduces the parameter space to a single magnetic charge, and the inverse construction formalism yields a one-parameter family of electromagnetic Lagrangians $\mathcal{L}(F)=F^{n+1}/(n+1)$, encompassing both linear and nonlinear electrodynamics. We analyze the horizon structure and determine the critical magnetic charge separating black hole and horizonless configurations. The photon sphere and the corresponding shadow radius are computed, and observational bounds from the Event Horizon Telescope for Sagittarius A* constrain the allowed range of the magnetic charge. In the extended phase space thermodynamics, the solution satisfies the first law and the Smarr relation, exhibits a Hawking-Page phase transition, and presents a single change in stability without van der Waals-type critical behavior. We also investigate the topological properties of both the photon sphere and the thermodynamic parameter space. The photon sphere carries a total topological charge $Q_{\text{tot}}=-1$, while the thermodynamic vector field yields a global winding number $W=0$, placing the solution in the same topological class as the one of the Reissner-Nordström black hole. We finally discuss the versatility of this non-minimal coupling framework in both providing theoretical support to previously introduced solution and also to connect them to observational settings within strong-field gravity.

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arXiv2026

Dyonic Ellis-Bronnikov wormholes from warped extra dimensions

We investigate traversable wormhole solutions within a four-dimensional effective theory derived from a five-dimensional Einstein-Maxwell-Chern-Simons action with a non-minimally coupled scalar field. A warped Kaluza-Klein compactification yields an Einstein-frame theory containing a phantom dilaton, a canonical axion, a Maxwell field, and a Kaluza-Klein vector, with the couplings fixed by the higher-dimensional origin. Focusing on the Ellis-Bronnikov geometry, we construct solutions that incorporate both dyonic Maxwell and Kaluza-Klein fields. For exponential gauge couplings, the Einstein equations determine the scalar kinetic term and the combined potentials, while the remaining field equations reduce to algebraic relations fixing the individual potentials and the radial behaviour of the electric charges. We obtain a systematic classification of configurations, ranging from the pure phantom-supported wormhole to fully coupled dilaton-axion-gauge configurations. The Kaluza-Klein sector enriches the solution space with additional structure while preserving analytic tractability. These results show that regular, asymptotically flat traversable four-dimensional wormholes arise naturally from higher-dimensional scalar-tensor theories.

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arXiv2026

Three dimensional black bounces in $f(R)$ gravity

We investigate the existence of black bounce solutions in $2+1$ dimensions within the framework of $f(R)$ gravity. We analyze whether black bounce geometries originally obtained in general relativity can be consistently generalized to $f(R)$ theories and identify the matter sources capable of supporting such solutions. We also construct a new class of solutions by imposing a vanishing curvature scalar. In the matter sector, we consider models involving a coupling between a scalar field and nonlinear electrodynamics, while in the gravitational sector we analyze both the Starobinsky model and more general forms of $f(R)$. We further examine the viability conditions of the $f(R)$ models that give rise to these spacetimes, including the behavior of the scalaron mass. Finally, we study the associated energy conditions, in order to assess the degree of exoticity of the matter content required to sustain these black bounce solutions and how the $f(R)$ theory modifies the energy conditions.

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arXiv2026

Born-Infeld AdS Black Holes Surrounded by Perfect Fluid Dark Matter

We obtain exact charged AdS black hole solutions in Einstein Lambda gravity including the effects of Born Infeld nonlinear electrodynamics and Perfect Fluid Dark Matter. The influence of the PFDM and BI parameters on the event horizon is analyzed. We compute the conserved and thermodynamic quantities and verify that they satisfy the first law of thermodynamics. Thermal stability is studied in the canonical ensemble using the heat capacity and Helmholtz free energy showing how PFDM and BI parameters affect local and global stability regions. We further investigate the thermodynamics in the extended phase space by treating the cosmological constant as thermodynamic pressure obtaining consistent conserved quantities and confirming the first law. The Ehrenfest equations are analytically verified demonstrating that the critical behavior corresponds to a second order phase transition. Heat engines associated with these black holes are also constructed to examine how PFDM and BI parameters influence their efficiency. Finally we analyze the geodesic structure through timelike and null trajectories using the effective potential determining conditions for stable and unstable circular orbits the innermost stable circular orbit and the photon sphere. PFDM significantly modifies the orbital structure while BI corrections are weaker.

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arXiv2025

Topological Mod(A)Max AdS black holes

In this work, we construct new classes of topological black hole solutions in anti-de Sitter (AdS) spacetime using a novel model of nonlinear electrodynamics called Modification Maxwell (ModMax) and Modification phantom or Modification anti-Maxwell (ModAMax). We then evaluate the thermodynamic quantities and verify the first law of thermodynamics. Our study examines how the parameters of the ModMax and ModAMax fields, as well as the topological constant, affect the black hole solutions, thermodynamic quantities, and local and global thermal stabilities. Furthermore, within the framework of extended phase space thermodynamics, we analyze the Joule-Thomson expansion process and determine the inversion curves. This analysis reveals that the ModMax and ModAMax parameters significantly alter the cooling and heating behavior of these AdS black holes, depending on their topology. Finally, by treating these topological Mod(A)Max AdS black holes as heat engines, we assess their efficiencies, demonstrating that the parameters of nonlinear electrodynamics and horizon topology play crucial roles in enhancing or suppressing the system's thermodynamic performance.

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