arXiv subjects

Francisco S. N. Lobo

Publications and source records attributed to Francisco S. N. Lobo.

At least 73 records · Page 4Linked to original sources

arXiv2025

Vector wormholes as conduits for matter interaction

In this work, we focus on the dynamics of a massive one-form field, \textbf{B}, often referred to simply as a vector field, that is minimally coupled to standard Einstein gravity. In the framework of four-dimensional spacetimes, the theory of a massive one-form propagates three massive vector degrees of freedom. The inclusion of a self-interacting potential in this theory results in the breaking of gauge invariance. The breaking of such a fundamental symmetry in Classical Electromagnetism may introduce a ghost mode in massive vector theories, which generally leads to their instability. However, in the context of wormhole physics, the existence of at least one ghost degree of freedom turns out to be a necessary condition to support these exotic geometries within effective field theories. This requirement serves as a strong motivation for our work, wherein we explore the role and phenomenology of massive one-forms, minimally coupled to Einstein gravity, in providing the necessary conditions to sustain wormhole spacetimes. We further analyze the coupling of matter fields to such a vector field through conformal couplings and explore their impact on energy conditions and the physical viability of wormhole solutions.

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arXiv2025

Sculpting Spacetime: Thin Shells in Wormhole Physics

In this work, we employ the Darmois-Israel thin-shell formalism to construct both static and dynamic thin-shell configurations surrounding traversable wormholes. Initially, using the cut-and-paste technique, we perform a linearized stability analysis in the presence of a general cosmological constant. Our results show that for sufficiently large positive values of the cosmological constant, corresponding to the Schwarzschild-de Sitter geometry, the stability regions of the wormhole solutions are significantly enhanced compared to the Schwarzschild case. Subsequently, we construct static thin-shell solutions by matching an interior wormhole geometry to an exterior vacuum spacetime across a junction surface. In the spirit of minimizing the presence of exotic matter, we identify parameter domains in which the null and weak energy conditions are satisfied at the shell. We examine the surface stress-energy components in detail, determining regions where the tangential surface pressure is either positive or negative, interpreted, respectively, as the pressure or surface tension. Additionally, an expression describing the behavior of the radial pressure across the junction is derived. Finally, we determine key geometrical characteristics of the wormhole, including the throat radius and the junction interface radius, by imposing traversability conditions. Estimates for the traversal time and required velocity are also provided, further elucidating the physical viability of these configurations.

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arXiv2025

Accretion dynamics in black holes with spontaneous Lorentz symmetry breaking

We investigate the spherical accretion of various types of fluids onto a Schwarzschild-like black hole solution modified by a Kalb-Ramond field implementing spontaneous Lorentz symmetry violation (LV). The system is analyzed for isothermal fluids characterized by the equation of state $p=ωρ$, including ultra-stiff, ultra-relativistic, and radiation fluids. We investigate the effect of the LV parameter $l$ on the fluid density $ρ(r)$, radial velocity $u(r)$, and accretion rate $\dot{M}$. Using a Hamiltonian dynamical systems approach, we examine the behavior near critical points and identify the sonic transitions in each scenario. Our results show that the LV parameter influences the location of critical points, the flow structure, and the accretion rate, with $l>0$ ($l<0$) enhancing (suppressing) the latter. For ultra-stiff fluids, no critical points are found, and the flow remains entirely subsonic. For ultra-relativistic and radiation fluids, transonic solutions exist, with the position of the sonic point depending on the sign of $l$. We also analyze polytropic fluids $p=\mathcal{K} ρ^Γ$ with $Γ=5/3$ and $Γ=4/3$, observing similar qualitative behavior, where the sonic transition is affected by both the equation of state and the LV parameter. These findings suggest that Lorentz symmetry breaking can significantly alter accretion dynamics in black hole spacetimes.

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arXiv2025

Evolving Wormholes in a Cosmological Background

Wormholes are non-trivial topological structures that arise as exact solutions to Einstein's field equations, theoretically connecting distinct regions of spacetime via a throat-like geometry. While static traversable wormholes necessarily require exotic matter that violates the classical energy conditions, subsequent studies have sought to minimize such violations by introducing time-dependent geometries embedded within cosmological backgrounds. This review provides a comprehensive survey of evolving wormhole solutions, emphasizing their formulation within both general relativity and alternative theories of gravity. We explore key developments in the construction of non-static wormhole spacetimes, including those conformally related to static solutions, as well as dynamically evolving geometries influenced by scalar fields. Particular attention is given to the wormholes embedded into Friedmann-Lemaître-Robertson-Walker (FLRW) universes and de Sitter backgrounds, where the interplay between the cosmic expansion and wormhole dynamics is analyzed. We also examine the role of modified gravity theories, especially in hybrid metric-Palatini gravity, which enable the realization of traversable wormholes supported by effective stress-energy tensors that do not violate the null or weak energy conditions. By systematically analyzing a wide range of time-dependent wormhole solutions, this review identifies the specific geometric and physical conditions under which wormholes can evolve consistently with the null and weak energy conditions. These findings clarify how such configurations can be naturally integrated into cosmological models governed by general relativity or modified gravity, thereby contributing to a deeper theoretical understanding of localized spacetime structures in an expanding universe.

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arXiv2025

Relativistic structure of a supermassive black hole embedded in the dark matter halo of NGC 4649 (M60)

We construct a static, spherically symmetric black hole (BH) solution embedded within a dark matter (DM) halo, formulated as a non-vacuum extension of the Schwarzschild spacetime. The DM distribution is modeled via an empirical density profile calibrated to observations of the elliptical galaxy NGC 4649 (M60), incorporating Hubble Space Telescope (HST) imaging, stellar velocity dispersion data, and globular cluster dynamics. The resultant spacetime metric depends on three independent parameters: the black hole mass $M$, the asymptotic circular velocity $V_c$, and the halo scale radius $a$, and smoothly reduces to the Schwarzschild limit as $V_c \to 0$ and $a \to 0$. We analyze the influence of the halo on key geometric and physical quantities, including the event horizon radius, photon sphere, shadow size, and curvature invariants. The Kretschmann scalar exhibits an enhanced sensitivity to halo-induced modifications, particularly in the near-horizon regime. Thermodynamic properties of the solution are also examined. In the extremal limit, characterized by a vanishing surface gravity, the model supports a finite tangential pressure, implying a non-trivial extension of standard black hole thermodynamics. These results highlight the relevance of incorporating astrophysical environments into BH modeling and offer new avenues for testing strong-field gravity through precision observational data.

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arXiv2025

Black hole solutions in Cotton gravity coupled to nonlinear electrodynamics

The framework of General Relativity (GR) has recently been expanded through the introduction of Cotton Gravity (CG), a theoretical extension proposed by J. Harada. This modified approach integrates the Cotton tensor into the gravitational field equations, naturally encompassing all conventional GR solutions while allowing the cosmological constant to emerge as an integration constant. In this study, we delve into the implications of CG when coupled with nonlinear electrodynamics (NLED), constructing and analyzing three distinct static, spherically symmetric configurations. Our investigation centers on the horizon structure, metric characteristics, and the underlying NLED Lagrangian density of each model. We also confront the theoretical predictions with observational data by comparing the calculated shadow radii of these solutions to constraints imposed by the Event Horizon Telescope's (EHT) measurements of Sgr A$^{*}$. The results reveal an extensive spectrum of spacetime geometries, ranging from multi-horizon structures to naked singularities. Furthermore, the agreement between the predicted shadow sizes and EHT observations reinforces the viability of these models in describing the astrophysical image of Sgr A$^{*}$ within certain parameter bounds.

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arXiv2025

Constraints on the $γ$-parameter for the vacuum solution of Cotton gravity with geodesics and shadows

We consider a recently introduced extension of General Relativity dubbed as Cotton gravity (CG), based on the use of the Cotton tensor, to estimate the size of a new constant $γ$ appearing within a spherically symmetric, vacuum solution of the theory. Taking into account its non-asymptotically flat character, we use the inferred size of the central brightness depression of the supermassive object at the heart of the Milky Way galaxy (Sgr A*) by the Event Horizon Telescope to constrain at $2σ$ the CG parameter as $γM \approx 3.5 \times 10^{-12}$. We study the potential observational consequences from the smallness of such a value using exact and numerical expressions for the deflection angle, optical images from optically and geometrically thin accretion disks, isoradials, and instability scales (Lyapunov index) of nearly bound geodesics associated to photon rings. Our results point towards the impossibility to distinguish between these two geometries using current and foreseeable techniques in the field of interferometric detection of optical sources.

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arXiv2025

Horizons, throats and bounces in hybrid metric-Palatini gravity with a non-zero potential

This work conducts an in-depth exploration of exact electrically charged solutions, including traversable wormholes, black holes, and black bounces, within the framework of the scalar-tensor representation of hybrid metric-Palatini gravity (HMPG) with a non-zero scalar potential. By integrating principles from both the metric and Palatini formulations, HMPG provides a flexible approach to addressing persistent challenges in General Relativity (GR), such as the late-time cosmic acceleration and the nature of dark matter. Under the assumption of spherical symmetry, we employ an inverse problem technique to derive exact solutions in both the Jordan and Einstein conformal frames. This method naturally leads to configurations involving either canonical or phantom scalar fields. A thorough examination of horizon structures, throat conditions, asymptotic behaviour, and curvature regularity (via the Kretschmann scalar) reveals the intricate causal structures permitted by this theoretical model. The analysis uncovers a diverse range of geometric configurations, with the phantom sector exhibiting a notably richer spectrum of solutions than the canonical case. These solutions encompass traversable wormholes, black universe models, where the interior of a black hole evolves into an expanding cosmological phase rather than a singularity, as well as black bounce structures and multi-horizon black holes. The results demonstrate that introducing a non-zero scalar potential within HMPG significantly expands the array of possible gravitational solutions, yielding complex causal and curvature properties that go beyond standard GR. Consequently, HMPG stands out as a powerful theoretical framework for modelling extreme astrophysical environments, where deviations from classical gravity are expected to play a crucial role.

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arXiv2025

Quantum-inspired wormholes from String T-Duality

The intrinsic non-perturbative features of string-theoretic corrections, particularly those arising from T-duality, have been shown to naturally introduce an effective ultraviolet (UV) cutoff into the gravitational framework. This cutoff, often referred to as the zero-point length in the context of path integral duality, acts as a fundamental minimal length scale that regulates short-distance divergences. Using the established correspondence between T-duality and path integral duality, it has been shown that the static Newtonian potential becomes regularized at small distances. Building upon this regular behavior, we proceed to construct self-consistent, spherically symmetric, and electrically neutral wormhole solutions, which remain free of curvature singularities and embodies the effects of this duality-induced UV completion. We explore wormhole configurations with the aim of minimizing violations of the null energy condition (NEC). In fact, solutions with a constant redshift function or specific shape functions generally exhibit NEC violations throughout the entire spacetime. To address this, we explore two distinct thin shell constructions: (i) wormholes formed by matching an interior wormhole geometry to an exterior Schwarzschild vacuum spacetime, thereby confining exotic matter to a localized region; and (ii) standard thin-shell wormholes generated by gluing two identical black hole spacetimes across a timelike hypersurface situated outside their event horizons. In both cases, the NEC violations are minimised and restricted to a finite region, improving the physical plausibility and traversability of the resulting configurations.

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arXiv2025

Dyonic regular black bounce solutions in General Relativity

This work explores dyonic black bounce (BB) solutions within the framework of General Relativity (GR), coupled with nonlinear electrodynamics (NLED) and scalar fields (SFs). Previous research has employed NLED and SFs to obtain BB solutions in GR; however, these solutions typically assume the presence of either magnetic monopoles or electric charges exclusively as components of the Maxwell-Faraday tensor. In this study, we examine static and spherically symmetric BB solutions that incorporate both magnetic and electric components, forming what are known as dyon solutions. A dyon is a particle characterized by the coexistence of both magnetic and electric charges. We determine the NLED Lagrangian density and the scalar field potential that produce these solutions and analyze the associated gravitational configurations, focusing on horizons, the behavior of the metric function, and spacetime regularity as described by the Kretschmann scalar. Notably, we present the first BB solution derived from the coupling of a linear electromagnetic Lagrangian and a scalar field with an associated potential as the matter source. This work broadens the class of non-singular geometries in the literature and opens new avenues for investigating dyonic BB solutions within the context of other modified gravity theories.

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arXiv2025

Charged black hole solutions in $f(R,T)$ gravity coupled to nonlinear electrodynamics

In this work, we investigate static and spherically symmetric black hole solutions in $f(R,T)$ gravity, where $R$ is the curvature scalar and $T$ is the trace of the energy-momentum tensor, coupled to nonlinear electrodynamics (NLED). To construct our solutions, we adopt a linear functional form, $f(R,T) = R + βT$. In the limit $β= 0$, the theory reduces to General Relativity (GR), recovering $f(R,T) \approx R$. We propose a power-law Lagrangian of the form $\mathcal{L} = f_0 + F + αF^p$, where $α=f_0= 0$ corresponds to the linear electrodynamics case. Using this setup, we derive the metric functions and determine an effective cosmological constant. Our analysis focuses on specific cases with $p = 2$, $p = 4$, and $p = 6$, where we formulate analytic expressions for the matter fields supporting these solutions in terms of the Lagrangian as a function of $F$. Additionally, we verify the regularity of the solutions and study the structure of the event horizons. Furthermore, we examine a more specific scenario by determining the free forms of the first and second derivatives $\mathcal{L}_F(r)$ and $\mathcal{L}_{FF}(r)$ of the Lagrangean of the nonlinear electromagnetic field. From these relations, we derive the general form of $\mathcal{L}_{\text{NLED}}(r)$ using consistency relations. This Lagrangian exhibits an intrinsic nonlinearity due to the influence of two constants, $α$ and $β$. Specifically, $α$ originates from the power-law term in the proposed Lagrangian, while $β$ arises from the assumed linear function $f(R,T)$. The interplay of these constants ensures that the nonlinearity of the Lagrangian is governed by both $α$ and $β$, rather than $α$ alone.

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arXiv2025

Periodical orbits and waveforms with spontaneous Lorentz symmetry-breaking in Kalb-Ramond gravity

In this paper, we study time-like geodesics around a spherically symmetric black hole in Kalb-Ramond (KR) gravity, characterized by the parameter $l$, which induces spontaneous Lorentz symmetry breaking. The geodesic equations and effective potential are derived to investigate the influence of $l$. We calculate the marginally bound orbits and innermost stable circular orbits, analyzing the parameter's impact. Periodic orbits are computed numerically and classified within the standard taxonomy, revealing significant effects of $l$ on their momentum and energy. Additionally, we explore an extreme mass ratio inspiral system under the adiabatic approximation to derive gravitational waveforms emitted by an object orbiting a supermassive black hole in KR gravity. These waveforms reflect the distinctive characteristics of periodic orbits and highlight the influence of $l$. With advancements in gravitational wave detection, these results offer insights into black holes influenced by Lorentz symmetry-breaking fields.

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arXiv2025

Supermassive black hole in NGC 4649 (M60) with a dark matter halo: Impact on shadow measurements and thermodynamic properties

We investigate black hole (BH) solutions embedded in a dark matter (DM) halo, modeled as extensions of the Schwarzschild metric. The DM density profile is constrained by Hubble Space Telescope data, stellar dynamics, and globular cluster (GC) measurements of the elliptical galaxy NGC 4649 (M60). Using this profile, we construct two distinct spacetime solutions characterized by the black hole mass ($M$), critical velocity ($V_c$), and core radius ($a$), all reducing to the Schwarzschild case in the limit $V_c=0$ and $a=0$. Our results show that the DM halo modifies essential BH features, such as the event horizon radius and spacetime curvature, as reflected in the Kretschmann scalar. We also derive an approximate analytical expression for the BH shadow radius, which increases slightly due to the halo's influence. Comparisons with two observational datasets further validate the analysis. Thermodynamic properties are examined across the two models. In the first, a generalized Smarr formula is obtained via two additional variables, $γ$ and $a$. The second incorporates halo effects through $V_c=0$ and $a=0$. These results underscore the role of DM in shaping both geometric and thermodynamic aspects of BHs.

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arXiv2025

Novel electrically charged wormhole, black hole and black bounce exact solutions in hybrid metric-Palatini gravity

This paper presents a systematic exploration of exact solutions for electrically charged wormholes, black holes, and black bounces within the hybrid metric-Palatini gravity (HMPG) framework. HMPG combines features of the metric and Palatini formulations of modified gravity, offering a powerful approach to address challenges in General Relativity, particularly those related to cosmic acceleration and dark matter. We examine configurations characterized by a zero scalar potential under spherical symmetry, and present solutions in both the Jordan and Einstein conformal frames. A diverse set of solutions emerges, including traversable wormholes, black holes with double horizons, and ``black universe'' models in which spacetime beyond the horizon leads to an expanding cosmological solution rather than a singularity. Each configuration is categorized according to the properties of the scalar field, with an in-depth analysis of the horizon and throat structures, asymptotic behaviour, and singularities. These findings underscore the versatility of HMPG in capturing complex gravitational phenomena and broadens the scope of the theory, offering a robust framework for modelling gravitational phenomena across a range of astrophysical contexts. Future work will benefit from extending these solutions to include scalar potentials, addressing both early-universe inflation and late-time acceleration, and applying observational data, such as gravitational lensing and gravitational wave measurements.

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arXiv2025

Baryon asymmetry from higher-order matter contributions in gravity

We investigate the observed asymmetry between matter and antimatter by incorporating higher-order matter contributions in gravity, specifically analyzing gravitational baryogenesis within the framework of $ f(R,\mathcal{T}^2) $ gravity, where $ R $ is the Ricci scalar and $ \mathcal{T}^2 \equiv T_{μν}T^{μν} $. We further explore the impact of high-order matter contributions by considering an interaction term analogous to those in gravitational and spontaneous baryogenesis, constructed using $ \mathcal{T}^2 $. The cutoff energy scale of the new interaction term is presented and its implications to Big Bang Nucleosynthesis (BBN) are discussed. The properties and implications of this term are analyzed within the frameworks of General Relativity and $f(R, \mathcal{T}^2)$ gravity. Furthermore, a connection to Big Bang Nucleosynthesis (BBN) is established, providing an observational constraint on the functional form of $f(R, \mathcal{T}^2)$. By introducing $\mathcal{T}^2$ into the gravitational action, we propose that these modifications could significantly influence the early Universe's dynamics, thereby altering the conditions necessary for baryogenesis to occur.

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arXiv2025

Wormhole geometries in $f\left(R,T^2\right)$ gravity satisfying the energy conditions

We explore the properties of traversable wormhole spacetimes within the framework of energy-momentum squared gravity, also known as $f(R,T^2)$ gravity, where $R$ represents the Ricci scalar, $T_{ab}$ is the energy-momentum tensor, and $T^2 = T_{ab}T^{ab}$. Adopting a linear functional form $f(R,T^2) = R + γT^2$, we demonstrate the existence of a wide range of wormhole solutions that satisfy all of the energy conditions without requiring fine-tuning of the model parameters. Due to the inherent complexity of the field equations, these solutions are constructed through an analytical recursive method. However, they lack a natural localization, requiring a junction with an external vacuum region. To address this, we derive the corresponding junction conditions and establish that the matching must always be smooth, precluding the formation of thin shells at the interface. Using these conditions, we match the interior wormhole geometry to an exterior Schwarzschild solution, yielding localized, static, and spherically symmetric wormholes that satisfy all the energy conditions throughout the entire spacetime. Finally, we extend our analysis to more intricate dependencies on $T^2$, demonstrating that the methodology remains applicable as long as no mixed terms between $R$ and $T^2$ are introduced.

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arXiv2025

Novel charged black hole solutions in conformal Killing gravity

In this paper, we investigate static spherically symmetric solutions in the context of Conformal Killing Gravity, a recently proposed modified theory of gravity that offers a new approach to the cosmological constant problem. Coupling this new theory with nonlinear electrodynamics, we derive the corresponding field equations and study their behavior under different parameter choices. We analyze three different models, each focusing on different key parameters. Our results reveal a rich causal structure with multiple horizons and transitions between extreme and non-extreme solutions depending on the parameter values. Moreover, we compute the nonlinear Lagrangian density for each model and find that it agrees with Maxwell theory in the limit $F \rightarrow 0$. We also confirm the existence of a central curvature singularity via the Kretschmann scalar. To connect our theoretical results with observational prospects, we study the black hole shadows associated with each model. The analysis shows that the calculated shadow size and shape of the three proposed models are consistent with the data for the supermassive object at the center of our galaxy and are therefore possible candidates for modeling this structure.

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arXiv2024

Spontaneous Lorentz symmetry-breaking constraints in Kalb-Ramond gravity

In this work, we study timelike and lightlike geodesics in Kalb-Ramond (KR) gravity around a black hole with the goal of constraining the Lorentz symmetry-breaking parameter $l$. The analysis involves studying the precession of the S2 star periastron orbiting Sgr A* and geodesic precession around the Earth. The ratio of precession frequencies for General Relativity (GR) and KR gravity is computed, with Event Horizon Telescope (EHT) results providing a parameter range for the spontaneous symmetry-breaking of $-0.185022 \leq l \leq 0.060938$. Utilizing the geodesic precession frequency from the Gravity Probe B (GP-B), the $l$ parameter is further constrained to $-6.30714 \times 10^{-12} \leq l \leq 3.90708 \times 10^{-12}$, which is consistent with the Schwarzschild limits. Moreover, for timelike geodesics, the innermost circular orbit (ICO) and innermost stable circular orbit (ISCO) are determined and analyzed to illustrate the impact of the symmetry breaking term. Zoom-whirl obstructions are compared with the Schwarzschild solution. Lower and upper limits of the photon sphere for lightlike geodesics are established to demonstrate the influence of KR gravity on the photon sphere. Additionally, the shadow radius is determined for two observers, one situated at a finite distance from the KR black hole, and the other located at an infinite distance, to constrain the symmetry-breaking parameter $l$, with comparisons made to EHT results. The bounds for $l$ derived from constraints on the photon sphere radius for lightlike geodesics yield $-0.0700225 \leq l \leq 0.189785$ using EHT data. The findings of this paper align with experimental results in the $l \rightarrow 0$ limit.

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