arXiv subjects

C. R. Muniz

Publications and source records attributed to C. R. Muniz.

At least 19 recordsLinked to original sources

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

Distinguishing wormholes via Einstein rings and global curvature

In this work, we investigate the gravitational lensing properties of a static Ellis-Bronnikov wormhole embedded in a curved Friedmann-Lemaître-Robertson-Walker (FLRW) universe. By employing curvature-dependent cosmological distances, we derive the corresponding weak-field lens equation and demonstrate that the wormhole Einstein ring radius follows a characteristic cubic scaling with cosmological distances, in sharp contrast to the square-root behavior found for Schwarzschild black holes. This distinct scaling leads to a qualitatively different redshift evolution of the lensing signal, providing a model-independent geometric diagnostic to discriminate between wormhole and black hole lensing scenarios. Numerical analysis reveals that the interplay between the local wormhole geometry and the FLRW background produces an asymmetric response to spatial curvature that inverts at intermediate redshifts, exhibiting a non-negligible sensitivity even under tight modern constraints such as those from DESI 2024. We also find that Ellis-Bronnikov wormholes are substantially less efficient gravitational lenses than Schwarzschild black holes of comparable physical scale, implying that microarcsecond-scale Einstein rings require macroscopic throat radii. These results suggest that, should a population of cosmological wormholes exist, their lensing signatures could provide a sensitive, complementary probe of both exotic spacetime topology and the global geometry of the Universe.

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↗

arXiv2026

Field Sources for Dark Matter Black Holes

We investigate the field-theoretic realization of regular black holes sourced by dark matter halo profiles within nonlinear electrodynamics (NED) minimally coupled to gravity. Starting from a static, spherically symmetric geometry determined by a halo density profile $ρ(r)$, we reconstruct the associated mass function and derive the effective matter source supporting the spacetime. In the magnetic sector, the reconstruction is direct and yields a NED Lagrangian of the form $L(F)=-ρ(r(F))$, while in the electric sector the theory is obtained parametrically through the field equations. We analyze the admissibility and consistency of the reconstructed models by studying regularity at the origin, asymptotic behavior, and the relevant energy conditions. The formalism is applied to representative halo profiles, including the Einasto, Dehnen, Burkert, and pseudo-isothermal families. For halo distributions with finite central density, the resulting geometries naturally exhibit de Sitter cores and asymptotically Schwarzschild behavior, providing a controlled and physically transparent link between dark matter halo phenomenology and regular black-hole spacetimes. Our results show that a broad class of halo profiles admits an effective NED completion, offering a unified geometric and field-theoretic interpretation of regular black holes sourced by dark matter halos.

gr-qc↗

arXiv2026

Traversable double-throat wormholes in a string cloud background

This work constructs a new class of traversable wormhole solutions with a double-throat topology, modeled as a localized perturbation of the Ellis-Bronnikov metric in a string cloud background. Embedding diagrams and the analysis of curvature invariants, including the Kretschmann scalar and the Weyl invariant, illustrate the geometric transition from single to double-throat structures as a function of the perturbation amplitude. By imposing the zero-tidal condition, we derive analytical expressions for the energy density and pressures, showing an asymptotic $r^{-2}$ decay characteristic of a string cloud, endowed with the topology of a global monopole. A key finding is that the energy density converges to a positive constant at the center, with the radial pressure becoming negative. This local behavior provides the repulsive support necessary to inflate the inter-throat region with non-exotic matter, concentrating Null Energy Condition violations to the throat vicinities. These results suggest that multi-throat geometries offer a natural mechanism for localizing exotic matter while maintaining a physical asymptotic background.

gr-qc↗

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 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 the short-distance behavior exhibits a regular de Sitter-like core, demonstrating 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 perform a topological classification of the thermodynamic phase space and demonstrate that asymptotically safe effects shift the critical point while preserving the global topological number of the Schwarzschild solution.

gr-qc↗

arXiv2026

Lower-dimensional Gauss-Bonnet gravity black holes with quintessence

In this paper, we study the $D\to3$ limit of Gauss-Bonnet gravity with quintessential matter, obtaining exact solutions that extend the BTZ metric through higher-curvature terms and quintessence coupling. The solutions exhibit a single event horizon whose radius decreases with increasing quintessence parameter $ω_q$, while developing a curvature singularity at the origin for non-vanishing quintessence. The geodesic analysis reveals stable circular photon orbits exist exclusively for phantom-like quintessence ($ω_q < -1$). Thermodynamically, the system is stable, since the specific heat is positive, and with evaporation it evolves to stable remnants whose characteristic size decreases as $ω_q$ increases, with complete evaporation prevented by quintessence effects. Furthermore, we find that all physical quantities intrinsically depend on the parameter $α$ of the Gauss-Bonnet extension.These results demonstrate the profound influence of quintessential matter on both geometric and thermodynamic properties of (2+1)-dimensional black holes, offering new perspectives on gravitational theories in lower dimensions and black hole final states.

gr-qc↗

arXiv2026

Traversable wormholes from a smoothed string fluid in 4D Einstein-Gauss-Bonnet gravity

We investigate traversable wormhole solutions in four-dimensional Einstein-Gauss-Bonnet (EGB) gravity sourced by a smoothed string fluid. Originally proposed to model regular black holes, this energy density profile is adapted here to sustain wormhole geometries by allowing for a radially varying equation of state. We obtain zero-tidal-force solutions that satisfy all traversability criteria and remain globally regular. The Gauss-Bonnet (GB) coupling $α$ plays a central role in shaping the throat geometry. We identify a parameter region ($α\geq 1$, $\varepsilon \leq 0.1$) in which the null energy condition is satisfied in the vicinity of the throat, representing a significant improvement over general relativistic counterparts. The interplay between the smoothing scale $a$ and the string density $\varepsilon$ ensures finite curvature invariants while reducing the violation of energy conditions. An analysis of the volume integral quantifier and the complexity factor further shows that strong EGB coupling simultaneously suppresses gravitational complexity and the total amount of exotic matter. These results establish a unified framework in which the same string fluid source can generate both regular black holes and stable traversable wormholes, depending on the strength of higher-curvature corrections.

gr-qc↗

arXiv2025

Regularized Black Hole Solution from a New String Cloud Source

We construct a new family of regular black hole solutions supported by the novel Letelier-Alencar string cloud and regularized through a rational Dagum-type distribution. The regulator smooths the matter profile and ensures finite curvature invariants, yielding a geometry that interpolates between a string-cloud exterior and an anti--de Sitter core. We analyze the energy conditions, identifying where the null, weak, dominant and strong conditions hold or fail across the core and exterior. The parameter space for horizon formation is mapped and the thermodynamic propertie -- mass, Hawking temperature, entropy and heat capacity -- are derived; notably, the entropy depends only on the regularization scale while the string parameter modifies temperature and heat capacity. Employing Rényi non-extensive entropy and the topological thermodynamics approach, we show the non-extensive deformation stabilizes the system and removes the standard phase transition. Finally, we compute the shadow radius and derive constraints compatible with current Event Horizon Telescope bounds for Sgr~A* and M87*.

gr-qc↗

arXiv2025

Coupled Oscillators, Frequency Transfer and the Higgs Mechanism's Teaching

In this work, we propose a simple model to teach the Higgs mechanism of spontaneous symmetry breaking in particle physics. The basic idea is associated with the analysis of two particles' motion subjected to specific potentials that, in certain situations, will reproduce oscillatory motions with a frequency transfer effect.

physics.ed-ph↗

arXiv2025

Polymer Black Hole Surrounded by Quintessence

In this paper, we study the polymer black hole solution surrounded by a quintessence field. The influence of quintessence on the polymer black hole is investigated through its thermodynamic properties, such as the Hawking temperature, entropy, and specific heat, which allow us to address the question of thermodynamic stability. We then calculate bounds on the electromagnetic greybody factors and photon emission rates of the black hole, highlighting the interplay between quintessence and quantum gravity effects in determining these phenomena. We also examine the effects of quintessence and quantum gravity on the geodesics and shadows of massless particles around the black hole. Our results are further compared with observational data of the Sagittarius A black hole from the Event Horizon Telescope (EHT) collaboration.

gr-qc↗

arXiv2025

Black holes as gravitational mirrors

Retrolensing is a gravitational lensing effect in which light emitted by a background source is deflected by a black hole and redirected toward the observer after undergoing nearly complete loops around the black hole. In this context, we explore the possibility of seeing objects of the solar system in past eras through telescope observations by using black holes as a gravitational mirror. We consider the motion of the light around Reissner-Nordström space-time and discuss the properties of the trajectories of boomerang photons. It was shown that, depending on the angle of emission and the position of the source, the photons could return to the emission point. Afterward, we explore the possibility of considering the returning photons in retrolensing geometry where the observer is between the source and the lens in which two classes of black holes are explored: The supermassive Sgr A* black hole at the galactic center and a nearby stellar black hole. For the first time in the literature, we propose the study of the returning photons of planets instead of stars in retrolensing geometry.

gr-qc↗

arXiv2025

Hot Casimir wormholes in Einstein-Gauss-Bonnet gravity

In this work, we explore the thermal effects on Casimir wormholes in the context of higher-dimensional Einstein-Gauss-Bonnet gravity. Motivated by the fundamental role of EGB gravity in describing a wide range of gravitational phenomena, we investigate how thermal fluctuations affect the quantum vacuum energy density associated with the Casimir effect and its impact on the global structure of traversable wormholes. By deriving the shape function from the EGB field equations with thermally corrected Casimir energy, we verify that all necessary conditions for wormhole formation are satisfied, including asymptotic flatness and throat stability. Our results indicate that thermal corrections modify of the wormhole geometry, increasing spatial curvature in the throat region and influencing its traversability. Furthermore, we analyze gravitational Casimir effects and discuss their possible role in modified gravity theories. Expanding on the approach of reference \cite{M. Zubair1, Mushayydha, Mushayydha2}, we adopt here the appropriate formulation for Casimir wormholes in Einstein-Gauss-Bonnet gravity, taking into account the Casimir energy density in higher dimensions. This approach allows us to obtain more accurate results compared to the simplified approximation previously used.

hep-th↗

arXiv2024

Probing the Solar System for Dark Matter Using the Sagnac Effect

This study investigates the potential of the Sagnac Effect for detecting dark matter in the Solar System, particularly within the Sun. Originating from the relative delay and interference of light beams traveling in opposite directions on rotating platforms, the effect can account for how varying gravitational conditions affect its manifestation. We analyze the Sagnac time in two static, spherically symmetric spacetimes: Schwarzschild and one incorporating dark matter, in the form of a perfect fluid. Comparing the relative deviations in Sagnac time calculated for these metrics in the reference frame of satellites orbiting our star, which serve as a rotating circular platform and emit laser beams in opposite directions, with the precision of onboard atomic clocks (about $10^{-11}$), allows us to evaluate the potential for detecting dark matter's gravitational influence through this effect.

gr-qc↗

arXiv2024

Braneworld Black Bounce to Transversable Wormhole

We provide a way for embedding a 4-dimensional geometry corresponding to the Simpson Visser (SV) spacetime which is capable of representing a traversable wormhole, a one-way wormhole, or a regular black hole into a Randall-Sundrum setup. To achieve this, we linearly deform the bulk geometry and the bulk matter distribution concerning a coupling constant. These deformations induce a transition from a $5D$ vacuum AdS state to an anisotropic matter distribution. The latter results in the induced geometry on the brane transitioning from a singular Schwarzschild spacetime to a regularized SV spacetime. Since there are no sources or matter fields on the brane, we can assert that the induced SV geometry on the brane arises from the influence of geometrical and matter deformations in the bulk. Thus, the central singularity is suppressed. We determine the cases where the energy conditions are either satisfied or violated. Our spacetime is asymptotically radial AdS, which is intriguing given the absence of a global AdS box that would prevent instability under larger wavelength perturbations. Therefore, it is no longer appropriate to claim that instability exists for very small perturbations near the AdS horizon. Thus, we propose that the stability of the solution can be analyzed by examining the speed of sound due to the presence of matter fields in the energy momentum tensor.

hep-th↗

arXiv2024

Generating 4-dimensional Wormholes with Yang-Mills Casimir Sources

This work presents a new wormhole solution in General Relativity supported by the quantum vacuum fluctuations of the Casimir effect between perfect chromometallic mirrors in $(3+1)$ dimensions, which was recently fitted using first-principle numerical simulations. Initially, we employ a perturbative approach for $x = m r \ll 1$, where $m$ represents the Casimir mass. This approach has proven to be a reasonable approximation when compared with the exact case in this regime. To find well-behaved redshift functions, we impose constraints on the free parameters. As expected, this solution recovers the electromagnetic-like Casimir solution for $m = 0$. Analyzing the traversability conditions, we graphically find that all will be satisfied for $ 0 \leq m \leq 0.20$. On the other hand, all the energy conditions are violated, as usual in this context. Stability from Tolman-Oppenheimer-Volkov (TOV) equation is guaranteed for all $r$ and from the speed of sound for $0.16 \le m \le 0.18$. Therefore, for $0.16 \leq m \leq 0.18$, we will have a stable solution that satisfies all traversability conditions.

gr-qc↗