arXiv subjects

Carlos F. S. Pereira

Publications and source records attributed to Carlos F. S. Pereira.

12 recordsLinked to original sources

arXiv2026

Optical Signatures of Black Holes Surrounded by a Generalized Cloud of Strings

In this work, we investigate the optical signatures of black holes surrounded by a generalized cloud of strings, described by the Letelier--Alencar spacetime. We first analyze null geodesics, showing that the string parameters can either increase or decrease the photon sphere radius and the critical impact parameter relative to their Schwarzschild values. Since the spacetime is not asymptotically flat, the critical impact parameter does not directly coincide with the physical shadow radius. We use this distinction to derive constraints on the model parameters from the shadow radius bounds of Sgr A* and M87*. In the weak-field regime, we calculate the light deflection angle using both the Gauss--Bonnet method and a perturbative expansion of the null geodesic equations, obtaining consistent results that contain a contribution from the asymptotically conical geometry in addition to the local gravitational deflection. We also examine the Shapiro time delay and identify a distance dependent contribution associated with the non Minkowskian asymptotic background. Finally, we study the optical appearance of geometrically and optically thin accretion disks and construct celestial sphere images through backward ray tracing. The transfer functions and lensing images show that the generalized cloud of strings modifies the null geodesic mapping in a nonuniform manner rather than producing a simple rescaling of the Schwarzschild image. In particular, configurations with a smaller shadow may still produce an outward displacement of the main Einstein ring image, demonstrating that different optical observables probe distinct aspects of photon propagation in this geometry.

gr-qc↗

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.

gr-qc↗

arXiv2025

LQG inspired spacetimes as solutions of the Einstein equations

Black bounces are compact objects with a wormhole structure hidden behind an event horizon. This type of metric can be obtained through general relativity by considering the presence of exotic matter. Such spacetimes can also arise within the framework of effective theories inspired by loop quantum gravity. In this work, we verify the possibility of obtaining black bounce models inspired by loop quantum gravity as solutions of general relativity. For this, we examine which sources can generate these solutions and the consequences of using these types of sources. We find that the sources can be expressed as a combination of a phantom scalar field and nonlinear electrodynamics. Once we obtain the sources in terms of fields, we analyze the energy conditions for each field separately to verify which of the fields is responsible for the violation of the energy conditions.

gr-qc↗

arXiv2025

Black-bounce solutions in a k-essence theory under the effects of bumblebee gravity

In the present study, we analyze the effects of violation of Lorentz symmetry for black-bounce solutions in a $k$-essence theory that has the form of a power law for the configuration $n=1/3$. We perform such analysis for a known model explored in previous work $Σ^2=x^2+a^2$ and complement the proposal with a new black-bounce model for the area functions $Σ^2_1=\sqrt{x^4+d^4}$. This model has the Schwarzschild-de Sitter asymptomatic behavior for $x\to{-\infty}$, and we investigate the scalar field, potential, and energy conditions for both models. We have shown that the violation of Lorentz symmetry can be generated through $k$-essence without the need for an additional field. These results corroborate the validation of other previously investigated wormhole solutions.

gr-qc↗

arXiv2025

Magnetically charged black-bounce solution via nonlinear electrodynamics in a k-essence theory

In the present work, we obtain and analyze a new class of analytical solutions of magnetically charged black bounces in k-essence theory, spherically symmetric in (3+1)-dimensions, coupled to nonlinear electrodynamics (NED). We consider two metric models, Simpson-Visser and Bardeen, for the k-essence configurations n = 1/3 and n = 1/5. We obtain in an analytical way which scalar field, field potential, and Lagrangian NED are necessary to support the metrics. We analyze the behavior of these quantities and the energy conditions due to the scalar field and the NED.

gr-qc↗

arXiv2024

Rotation effect for chiral magnetic cosmic string in the Kaluza-Klein theory

We have investigated bound state solutions for a scalar particle described in the spacetime of a chiral cosmic string subjected to rotational effects in a Kaluza-Klein theory. We saw that combinations between the parameters that describe the generalized spacetime impose upper and lower limits for the radial coordinate leading to well-established boundary conditions.

hep-th↗

arXiv2024

New sources of ghost fields in $k$-essence theories for black-bounce solutions

In the present study, we generalize the possible ghost field configurations within the framework of $k$-essence theory to the Simpson-Visser metric area function $Σ^2=x^2+a^2$. Our analysis encompasses field configurations for the region-defined metric function $dA_\pm$ as well as the general solution that asymptotically behaves as Schwarzschild-de Sitter for $x\to-\infty$. Specifically, we investigate two scalar field configurations and define the associated potential for each one. Through rigorous calculations, we verify that all equations of motion are satisfied. Notably, our findings indicate that even when proposing new configurations of ghost scalar fields, the energy conditions remain unchanged. This result serves to validate the wormhole solutions obtained in previous studies.

gr-qc↗

arXiv2024

On the Klein-Gordon oscillators in Eddington-inspired Born-Infeld gravity global monopole spacetime and a Wu-Yang magnetic monopole

We consider Klein-Gordon (KG) particles in a global monopole (GM) spacetime within Eddington-inspired Born-Infeld gravity (EiBI-gravity) and in a Wu-Yang magnetic monopole (WYMM). We discuss a set of KG-oscillators in such spacetime settings. We propose a textbook power series expansion for the KG radial wave function that allows us to retrieve the exact energy levels for KG-oscillators in a GM spacetime and a WYMM without EiBI-gravity. We, moreover, report some \textit{conditionally exact}, closed form, energy levels (through some parametric correlations) for KG-oscillators in a GM spacetime and a WYMM within EiBI-gravity, and for massless KG-oscillators in a GM spacetime and a WYMM within EiBI-gravity under the influence of a Coulomb plus linear Lorentz scalar potential. We study and discuss the effects of the Eddington parameter $κ$, GM-parameter $α$, WYMM strength $σ$, KG-oscillators' frequency $Ω$, and the coupling parameters of the Coulomb plus linear Lorentz scalar potential, on the spectroscopic structure of the KG-oscillators at hand. Such effects are studied over a vast range of the radial quantum number $n_r\geq 0$ and include energy levels clustering at $κ>>1$ (i.e., extreme EiBI-gravity), and at $|σ|>>1$ (i.e., extreme WYMM strength).

gr-qc↗

arXiv2023

Black-bounce solution in $k$-essence theories

In the present work, we construct black-bounce configurations in the context of $k$-essence theory. The solutions have a regular metric function at the origin. The area metric function is linked to the black-bounce area initially considered by Simpson-Visser, $Σ^2=x^2+a^2$. Subsequently, the expressions for the scalar field and scalar potential corresponding to the found solutions are determined, exhibiting phantom behavior everywhere due to violation of Null Energy Condition $(NEC^ϕ)$. The Kretschmann scalar is regular throughout spacetime, and the geodesics are complete. The energy conditions are analyzed, verifying that the null $(NEC^ϕ_1)$ and dominant energy conditions $(DEC^ϕ_1)$ are violated inside and outside the event horizon. Finally, the extrinsic curvature method was applied to determine the sign of the mass on the junction surface.

gr-qc↗

arXiv2023

Renormalization group and spectra of the generalized Pöschl-Teller potential

We study the Pöschl-Teller potential $V(x) = α^2 g_s \sinh^{-2}(αx) + α^2 g_c \cosh^{-2}(αx)$, for every value of the dimensionless parameters $g_s$ and $g_c$, including the less usual ranges for which the regular singularity at the origin prevents the Hamiltonian from being self-adjoint. We apply a renormalization procedure to obtain a family of well-defined energy eigenfunctions, and study the associated renormalization group (RG) flow. We find an anomalous length scale that appears by dimensional transmutation, and spontaneously breaks the asymptotic conformal symmetry near the singularity, which is also explicitly broken by the dimensionful parameter $α$ in the potential. These two competing ways of breaking conformal symmetry give the RG flow a rich structure, with phenomena such as a possible region of walking coupling, massive phases, and non-trivial limits even when the anomalous dimension is absent. We show that supersymmetry of the potential, when present, is also spontaneously broken, along with asymptotic conformal symmetry. We use the family of eigenfunctions to compute the S-matrix in all regions of parameter space, for any value of anomalous scale, and systematically study the poles of the S-matrix to classify all bound, anti-bound and metastable states, including quasi-normal modes. The anomalous scale, as expected, changes the spectra in non-trivial ways.

hep-th↗

arXiv2020

Some novel considerations about the collective coordinates approximation for the scattering of $ϕ^4$ kinks

The collective coordinates approximation for the kink/anti-kink scattering in the $1+1$ dimensional $ϕ^4$ model is considered and we discuss how the results found in the current literature on the topic can be improved by giving the analytical expression for the lagrangian in the space of parameters. A comprehensive discussion of the role of the collective coordinates approximation in this particular situation is given for completeness.

hep-th↗

arXiv2020

Real scalar field kink(-antikink)s and their perturbation spectra in a closed universe

In this paper we demonstrate that solitons of a simple real scalar field model that are {\it static and linearly stable} do exist when considered in a (3+1)-dimensional, spatially compact space-time background, the static Einstein universe, which is a good approximation to the observed universe for sufficiently small time intervals. We study the properties of these solutions for a $Φ^4$-potential and demonstrate that next to the fundamental solutions, excited configurations exist. We also investigate general perturbations about the solitons, determine their eigenfrequency spectra and compare them with those of the perturbations about the vacua of the model. We find that the degeneracy with respect to the multipoles of the perturbation, which is present for the vacua, does no longer exist in the presence of the soliton. Moreover, specific perturbations correspond to zero modes of the system. Our results have applications in condensed matter physics as well as computations of quantum effects (e.g. the Casimir energy) in spatially compact space-times in the presence of soliton-like objects.

hep-th↗