arXiv subjects

G. Alencar

Publications and source records attributed to G. Alencar.

At least 19 recordsLinked to original sources

arXiv2026

Non-Stationary Wormholes and Matter Source Reconstruction in General Relativity

We investigate non-stationary wormhole geometries in General Relativity embedded in evolving cosmological backgrounds. We employ a Simpson-Visser-like regularization of the temporal coordinate of the $(n+1)$-dimensional FLRW metric, together with a homogeneous Cuscuton field, to construct cosmological backgrounds in which the scale factor remains finite and non-zero at the would-be initial singularity. We then consider two specific dynamical wormhole geometries: the four-dimensional defect wormhole and the topologically charged Perry-Mann type wormhole. We show that the former can be supported by a canonical scalar field with vanishing potential, while the latter requires a mimetic matter sector to support its spatial structure, with the Cuscuton governing the cosmological evolution in both cases. We subsequently generalize the analysis to a separable non-stationary spacetime and derive a scaling condition that strongly constrains the admissible matter sources. In particular, for minimally coupled scalar and nonlinear electrodynamics fields, this condition removes the functional freedom available in static reconstruction schemes. We show that, within this framework, regular, asymptotically flat wormhole geometries cannot be obtained while simultaneously satisfying the required smoothness and reflection symmetry conditions. Our results illustrate both the possibility of constructing specific non-stationary wormholes in regular cosmological backgrounds and the restrictions that arise when attempting to generalize such constructions to arbitrary spatial geometries.

gr-qc↗

arXiv2026

Geodesic dynamics and tidal forces in a quantum-regularized black hole from string T-duality

In this work, we investigate the geodesic dynamics and tidal force effects in neutral and electrically charged regular spacetimes inspired by T-duality. We first analyze the geometric properties of these solutions, including their regular central structure and horizon configurations, and show how the interplay between the T-duality scale $\ell_0$ and the electric charge $Q$ determines the transition between black hole and horizonless geometries. We then study the motion of massive and massless particles through the corresponding effective potentials, examining the existence and stability of circular orbits and the behavior of their characteristic radii. We further investigate the tidal forces experienced by observers in radial free fall, focusing on the behavior of the radial and angular components, their extrema and sign changes, and the location of these characteristic points relative to the horizon structure of the geometry. We show that the T-duality regularization renders the tidal forces finite in the central region and gives rise to different stretching and compression regimes, including a region of simultaneous compression in both the radial and angular directions, while the electric charge modifies the strength and location of these tidal features. Finally, we analyze the evolution of the geodesic deviation vector for two sets of initial conditions. We find that both the T-duality scale and the electric charge tend to suppress the accumulated radial stretching, whereas the angular deformation is considerably less sensitive to the charge. These results show how the quantum regularization induced by T-duality and the presence of electric charge modify the orbital dynamics and tidal response of these geometries in the strong-field regime. Furthermore, we demonstrate that the corresponding classical limits are recovered appropriately.

gr-qc↗

arXiv2026

Deriving Schwarzschild and Kerr from the Einstein Equivalence Principle

This manuscript demonstrates that the Schwarzschild and Kerr metrics can be derived fundamentally from the Einstein Equivalence Principle (EEP) without explicit reliance on the Einstein field equations. We show that the EEP implies a gravitational stretching of length in the direction of acceleration, a spatial counterpart to time dilation. In the static case, this effect yields the constraint $g_{00}g_{11}=-1$, from which the Schwarzschild solution is recovered by taking the Newtonian limit. For the stationary case, we employ oblate spheroidal coordinates and construct a physically motivated tetrad to implement the EP, introducing gravity through three unknown functions. By applying the EEP-induced constraints and showing that the principle requires the preservation of the spacetime volume determinant ($g = g_0$), the system is reduced to a single unknown function that yields the Kerr metric. These results suggest that classical tests of General Relativity are, at their core, fundamental validations of the EEP. We further argue that the EEP leads naturally to Unimodular Gravity, implying that the existence of the cosmological constant is a direct consequence of this foundational postulate.

physics.gen-ph↗

arXiv2026

New Improved Schwarzschild Black Hole and Its Thermodynamics and Topological Classification

We construct a renormalization-group improved Schwarzschild-like black hole geometry using the exact new scheme running for the Newton coupling. The scale identification is implemented via a standard interpolating proper-distance function governed by an interpolation parameter $γ$ that smoothly connects the ultraviolet and infrared regimes. We present the resulting coordinate-dependent coupling and the improved metric function, analyzing its asymptotic expansions. The large-distance limit is shown to recover the classical Schwarzschild solution, while for $γ>0$ the short-distance behavior exhibits a regular de Sitter-like core, resulting in the regularization of the central singularity. We also analyze the thermodynamic properties of the solution, showing that quantum corrections significantly modify the small-radius behavior, leading to a remnant configuration and a nontrivial phase structure. Finally, we investigate the thermodynamic topology of the improved Schwarzschild black hole and show that the quantum-gravity corrections regularize the Hawking-temperature profile, giving rise to an additional thermodynamic defect with opposite winding number and changing the global thermodynamic-topological charge from the classical Schwarzschild value $W=-1$ to $W=0$.

gr-qc↗

arXiv2026

From massless wormholes to massive black bounces: Shadows and multiple light rings

We develop a general framework for constructing massive black bounce geometries from massless wormhole seeds by introducing a position-dependent mass function while preserving the underlying areal-radius profile. We apply this procedure to the generalized Ellis-Bronnikov wormhole and obtain a new family of generalized Bardeen-like black bounce space-times, which continuously interpolates between traversable wormholes and regular black holes. We investigate the motion of massive particles and photons and show that, in contrast with the simpler orbital structure of the massless generalized Ellis-Bronnikov geometry, the generalized Bardeen-like space-time can exhibit multiple circular orbits and a rich light ring structure, including configurations with two unstable circular photon orbits separated by a stable one. We determine the corresponding photon sphere and shadow radii and compare our predictions with Event Horizon Telescope observations of Sgr A*, deriving observational constraints on the parameter space of the model. We further investigate the optical appearance produced by a geometrically and optically thin accretion disk through ray tracing. Multiple light rings generate characteristic nested structures in the high-resolution intensity profiles, whose relative brightness is strongly affected by the mass deformation and gravitational redshift. However, after modeling the finite angular resolution of the EHT with a Gaussian beam convolution, these fine structures are largely washed out, revealing a strong observational degeneracy between these exotic compact objects and standard black hole geometries at current EHT resolution.

gr-qc↗

arXiv2026

Classical double copy of black strings in an Anti-de Sitter background

We study the classical double copy for static black string solutions in an Anti--de Sitter (AdS) background. By casting the black string metric into Kerr--Schild form over a cylindrical AdS geometry, we construct the corresponding single and zeroth copies. The single copy describes a gauge field satisfying Maxwell-like equations and sourced by an effective line of color charge, while the zeroth copy is given by a scalar field conformally coupled to the AdS background. We also extend the analysis to charged black strings, identifying the associated modifications in the gauge sector. These results show that the classical double copy consistently applies to extended gravitational objects in curved spacetimes.

hep-th↗

arXiv2026

Regular Black Strings and $(2+1)$ Black Hole in Unimodular Gravity Supported by Maxwell Fields

In this work, we obtain regular solutions with a dynamical cosmological function in unimodular gravity. This alternative theory to General Relativity imposes an additional condition on the spacetime volume element by fixing the metric volume density to a prescribed non-dynamical quantity, which can be represented by a constant in unimodular coordinates. In this procedure, the cosmological constant does not appear directly in the action, but rather as an integration constant of the field equations. By using the non-conservation of the energy-momentum tensor, we show that the integration constant becomes a function $Λ(x)=Λ_0+δΛ(x)$, where $Λ_0$ is the vacuum cosmological constant and $δΛ(r)$ is a matter-induced cosmological contribution determined by the geometry and the electromagnetic sector. From the definition of the geometric function $H(r)$, we verify the validity of Maxwell electrodynamics as a source for the solutions. We further show that linear combinations of the mass functions associated with the individual solutions can also be consistently supported by the Maxwell field, a property that is not generally shared by nonlinear electrodynamics. This result allows us to construct a broader family of regular solutions and investigate their physical properties. In particular, we analyze the energy conditions, curvature invariants, and thermodynamic properties of the generated solutions.

gr-qc↗

arXiv2026

Dymnikova Black Holes in Unimodular Gravity: Maxwell Sources and Vacuum Contributions

In this work, we investigate the Dymnikova regular black hole within the framework of unimodular gravity, emphasizing the role of the effective vacuum sector in the regularization of the geometry. By allowing a controlled violation of the covariant conservation of the energy--momentum tensor, the cosmological contribution emerges dynamically as a radial-dependent function, $Λ=Λ(r)$. We first reinterpret the Dymnikova spacetime as a charged configuration supported by nonlinear electrodynamics and derive the corresponding electric and magnetic sources. Subsequently, we demonstrate that the same geometry can be consistently generated by standard Maxwell electrodynamics in unimodular gravity. In this construction, the resulting electric field is everywhere regular and corresponds to a localized charge distribution with vanishing asymptotic charge, indicating that the spacetime does not behave as an asymptotically charged object.

gr-qc↗

arXiv2026

Electrically Charged Non-Abelian Black String in Anti-de Sitter Space

We construct a new family of static, electrically charged, cylindrically symmetric black-string solutions of four-dimensional Einstein--Yang--Mills theory with a negative cosmological constant, supported by a genuinely non-Abelian $SU(2)$ vortex field. The coexistence of electric and vortex sectors renders the standard single-function Lemos metric inconsistent with the Einstein equations, requiring a three-function metric compatible with the anisotropic Yang--Mills stress tensor. Regular near-horizon and asymptotic AdS expansions provide boundary data for numerical integration of the complete field equations. Two limiting sectors are recovered: vanishing electric horizon datum yields the neutral Lemos black string with zero Yang--Mills field strength, while switching off the vortex component gives the embedded $U(1)\subset SU(2)$ charged Lemos--Zanchin solution. Perturbations about the latter show that genuinely non-Abelian hair appears at linear order through the commutator field strength, whereas its backreaction on the electric profile and geometry begins at quadratic order. The horizon equations yield the analytic local non-degeneracy bound $R_1^{\rm crit}=s_0α\sqrt{3/(4πG)}$. The asymptotic geometry exhibits a quadratic correction absorbable into an effective Abelian charge and a cubic coefficient giving the first irreducible asymptotic signature of the non-Abelian hair. The entropy obeys the Bekenstein--Hawking area law, while the temperature difference from the embedded Abelian solution arises through the asymptotic lapse normalization. The fully backreacted numerical solutions are nonlinear realizations of this transverse non-Abelian deformation.

gr-qc↗

arXiv2026

Analyzing Quasi-normal modes spectrum in asymptotically de Sitter black bounces and their optical appearance

This work explores asymptotically de Sitter black-bounce spacetimes, obtained as a regular extension of the Schwarzschild-de Sitter geometry via the Simpson-Visser prescription. The resulting family of solutions provides a smooth interpolation between regular black holes, extremal configurations, and traversable wormholes within a single geometrical framework. In particular, we verify the regularity of the spacetime by showing that (some of) curvature invariants, including the Kretschmann and Ricci scalars, remain finite at the origin for any non-vanishing value of the regularization parameter $a$, ensuring geodesic completeness. An analysis of the effective stress-energy tensor reveals a violation of the null energy condition, which can be consistently interpreted in terms of a composite source involving a phantom scalar field coupled to nonlinear electrodynamics. Subsequently, we investigate the observational signatures of these geometries by studying: i) null geodesic propagation and ii) constructing the associated optical appearance. In particular, we determine the shadow radius and characterize the structure of photon rings through numerical ray-tracing procedure. Finally, we compute the spectrum of axial gravitational perturbations using a pseudospectral approach, obtaining the quasinormal mode frequencies and their overtones. Our results show a non-trivial interplay between the cosmological constant and the regularization scale $a$, which imprints itself on both the light-ring structure and the quasinormal spectrum. These features suggest potential observational discriminants that could distinguish these regular spacetimes from their singular counterparts in future gravitational-wave and high-resolution electromagnetic observations.

gr-qc↗

arXiv2026

Embedding Wormholes and Dyonic Black Strings in Warped Braneworlds via Local Sum Rules

Building on our previous work [1], where the Local Sum Rules (LSR) were established, we investigate the construction of compact objects in Randall-Sundrum braneworlds supported by matter fields that are dynamically consistent and localizable. We begin by revisiting the Chamblin et al. black string, highlighting its role as a foundational higher-dimensional solution. We then show that the Ellis-Bronnikov wormhole can be consistently embedded in this framework via a localized free scalar field, providing a simple yet nontrivial example of a braneworld compact object. Finally, we derive two novel black string solutions sourced by a localized nonlinear electrodynamics (NED) theory with Lagrangian $\mathcal{L}(\mathcal{F}) = -β\sqrt{\mathcal{F}}$, corresponding to purely magnetic and dyonic configurations. The purely magnetic solution reproduces the classical Letelier string cloud on the brane, while the dyonic solution generalizes it to include electric charge, closely paralleling the Letelier-Alencar construction. Both NED solutions reduce smoothly to the Chamblin et al. black string in the limit $β\to 0$, illustrating how localized higher-dimensional matter fields can consistently support braneworld compact objects and connect higher-dimensional physics with well-known four-dimensional solutions.

gr-qc↗

arXiv2026

The scalar--Maxwell--$Λ(x)$ system: Wormhole spacetimes without nonlinear electrodynamics in unimodular gravity

In General Relativity, supporting known traversable wormhole geometries with electromagnetic fields typically requires complex Non-Linear Electrodynamics (NED). We demonstrate that Unimodular Gravity (UG) elegantly resolves this limitation. By relaxing energy-momentum conservation, UG introduces a dynamical cosmological term, $Λ(x)$, enabling a semi-classical energy exchange between matter and the vacuum. Exploiting this mechanism, we show how established exact wormhole spacetimes can be fully supported by a phantom scalar field coupled to standard linear Maxwell electrodynamics. We demonstrate that, provided the shape function $b(r)$ satisfies specific geometric conditions, this non-conservative framework naturally accommodates these solutions. By providing a new physical matter source mechanism that entirely bypasses the need for NED, our main contribution highlights UG as a powerful framework for sustaining established non-trivial topologies with simplified, well-understood classical fields.

gr-qc↗

arXiv2026

On regular black string spacetimes in nonlinear electrodynamics

In this work, we investigate the coupling of General Relativity with Nonlinear Electrodynamics (NED), governed by a general Lagrangian $\mathcal{L}(\mathcal{F})$, to address the axial singularity of four-dimensional black strings. Through a model-independent analysis, we scrutinize the viability of regular configurations by extending no-go theorems, originally formulated for spherical spacetimes, to cylindrical symmetries. We provide a comprehensive mathematical proof that regular, purely electric black strings cannot be generated by any NED Lagrangian that recovers the Maxwell limit in the weak-field regime, establishing a fundamental constraint for cylindrical topologies. Despite these limitations, we employ specific mathematical frameworks to construct new exact solutions for black strings, including cylindrical analogues of the well-known Bardeen and Hayward regular black hole classes. Each solution is analytically derived, and we demonstrate that their curvature invariants remain finite everywhere, effectively replacing the axial singularity with a regular core. Furthermore, we evaluate the physical consistency of these new metrics by subjecting them to stringent causality and unitarity constraints. Our results provide a comprehensive classification of the conditions under which NED can regularize cylindrical spacetimes and offer new insights into how topological differences between spherical and axial symmetries influence the global structure and the physical viability of non-singular gravitational objects in nonlinear gauge theories.

gr-qc↗

arXiv2026

Horizon formation from effective matter profiles in static spacetimes

The formation of event horizons is traditionally studied as the endpoint of gravitational collapse, where the matter distribution and the spacetime geometry are evolved simultaneously toward a black hole state. In this work, we consider the inverse problem: given a static geometry containing the simplest causally exposed singular structure, namely a timelike naked singularity, what are the minimal conditions on the surrounding matter required for the emergence of an event horizon? Within classical general relativity, we derive sufficient conditions for horizon formation in terms of the radial organization, compactness and finiteness of the matter distribution. These conditions are summarized by a simple geometric criterion that determines whether a static configuration becomes causally inaccessible to external observers. We further identify situations in which horizon formation necessarily fails, thereby characterizing both the existence and obstruction of causal cloaking in static spacetimes. Our results show that the emergence of an event horizon is not controlled solely by the total amount of matter, but rather by the way mass-energy is radially accumulated. This provides a minimal and geometrically transparent framework for understanding horizon formation as a causal transition in static spacetimes, independent of gravitational collapse or dynamical evolution.

gr-qc↗

arXiv2026

The End of the Road for Bulk Fields in Warped Randall-Sundrum Braneworlds

In this manuscript we generalize Ref. [1] and derive a complete set of local consistency conditions for bulk fields in braneworld scenarios with an arbitrary number of dimensions. This provides the first fully local and dimension-independent generalization of all known criteria for bulk fields. Within this framework, we show that a free scalar field is consistent and localized, whereas minimally and non-minimally coupled Maxwell fields violate the conditions, leading to a no-go theorem valid in any dimension. For nonlinear electrodynamics, we find that only the model $L(F)=b\sqrt{F}$ admits a consistent and normalizable zero mode, and that among p-forms, consistency occurs solely for the free 0-form. We also demonstrate that Dirac fermions, with or without Yukawa terms, are inconsistent within this framework and therefore cannot propagate in the bulk. Our local approach makes explicit that these conclusions do not depend on any particular internal geometry or warp factor: previously known results arise merely as special cases of a broader and strictly local structure, highlighting the universality of the constraints derived here.

gr-qc↗

arXiv2026

Geometrically Regular Black Object Solutions in Lower-Dimensional Gauss-Bonnet Gravity and Its Unimodular Extension

We investigate the construction of regular compact objects in the recently proposed lower-dimensional Einstein--Gauss--Bonnet (EGB) gravity obtained through regularized dimensional reduction. Unlike the standard BTZ black hole, the corresponding vacuum EGB solution develops a genuine curvature singularity at the origin, providing an interesting setting in which higher-curvature corrections deteriorate the ultraviolet behavior of spacetime. To address this issue, we reconstruct matter sectors capable of restoring regularity while preserving the BTZ-like asymptotic structure. First, we derive regular black-hole solutions supported by nonlinear electrodynamics and determine the corresponding electromagnetic Lagrangians directly from the field equations. We then extend the analysis to Simpson--Visser black-bounce geometries, obtaining smooth throat configurations with finite curvature invariants throughout the spacetime. As an alternative regularization mechanism, we formulate a unimodular extension of lower-dimensional EGB gravity and show that standard Maxwell fields can support regular geometries through a dynamical exchange between the vacuum and matter sectors mediated by a spacetime-dependent cosmological function. We further investigate the thermodynamic properties of the regular black-hole and black-bounce solutions, showing that the matter sector modifies the evaporation process, allows for remnant formation, and produces nontrivial phase transitions. In the black-bounce case, the thermodynamic quantities smoothly recover the EGB-BTZ behavior in the appropriate limit. These results demonstrate that lower-dimensional EGB gravity provides a useful laboratory for exploring the interplay between higher-curvature corrections, regular compact objects, nonlinear electrodynamics, and unimodular gravity.

gr-qc↗

arXiv2026

Topological Thermodynamics of Black Holes: Revisiting the methods of winding numbers calculation

In this paper, the equivalence between two methods for computing winding numbers is established: the approach of $ϕ$-mapping topological current and the residue method. The methods are shown to be equivalent when the condition $M'' S' - S'' M' \neq 0$ holds, while deviations appear when this relation fails, signaling subtle connections between mass $M(r_h)$, entropy $S(r_h)$, and topological structure, with $r_h$ being the horizon radius. We first verify this equivalence to Schwarzschild and Reissner-Nordstr"om black holes, recovering known classifications and confirming the consistency of our approach with respect to the validity of the above condition. We then extend the analysis to four-dimensional black strings, regarded as cylindrically symmetric black hole solutions in asymptotically AdS spacetimes. Our results show that both neutral and charged black strings possess the same global topological number, $W = +1$, implying that electric charge does not influence their topological classification. This insensitivity to charge mirrors earlier findings for BTZ black holes in three dimensions, suggesting that it may represent a universal property of cylindrically symmetric black holes in AdS backgrounds.

gr-qc↗

arXiv2026

Field Sources for Generalized Ellis-Bronnikov Wormhole

The so-called generalized Ellis-Bronnikov wormhole is a modification of the standard Ellis-Bronnikov solution, in which a parameter $m>2$ is introduced-recovering the original Ellis-Bronnikov geometry when $m=2$. In this work, we investigate the properties of this spacetime by analyzing its embedding diagrams and how they are affected by variations in the parameter $m$. Furthermore, we study the accretion of dust onto this geometry, showing that, unlike in black hole scenarios, the radial infall velocity of the dust decreases as it approaches the wormhole throat, with this deceleration becoming increasingly abrupt for larger values of $m$. Our results also demonstrate that the mass of the wormhole generally decreases due to the accretion process, a finding that aligns with recent works in the literature for Ellis-Bronnikov-type geometries. This mass loss, coupled with the characteristic accumulation of matter near the throat, highlights the unique dynamical response of traversable wormholes to baryonic influx. As a main result, we demonstrate that this geometry arises as an exact solution of General Relativity when considering the combined presence of a phantom scalar field and a magnetic or electric source.

gr-qc↗