arXiv subjects

Fernando M. Belchior

Publications and source records attributed to Fernando M. Belchior.

17 recordsLinked to original sources

arXiv2026

Circulation and operational sparsity of analogue Hawking radiation in rotating acoustic horizons

Temporal sparsity compares the mean interval between emitted quanta with a characteristic single-quantum timescale. We study this operational quantity for the rotating draining-bathtub acoustic horizon in two spatial dimensions. The Hawking temperature is fixed by the radial drain and does not depend on circulation, while the geometric capture width and the transmission spectrum do. A geometric black-body benchmark therefore predicts reduced temporal separation as circulation increases. We then solve the radial scattering problem and compute converged greybody-corrected number and energy fluxes, including the superradiant sector. At a circulation-to-drain ratio of unity, the thermal-clock sparsity is 0.44 whereas the spectrum-based sparsity is 1.82. The former crosses the temporal-overlap benchmark near a ratio of 0.40, while the latter remains above unity at all sampled circulation ratios. We also show that the binned sparsity definition is logarithmically infrared sensitive in two spatial dimensions. Rotation therefore enhances the number flux while hardening the spectrum, making temporal overlap intrinsically dependent on the operational clock used to resolve the emitted quanta.

gr-qc↗

arXiv2026

Gödel-type universes in Lorentz-violating Kalb-Ramond gravity with Ricci- and Riemann-tensor nonminimal couplings

This paper studies homogeneous \Godel-type geometries in a gravitational model where local Lorentz symmetry is spontaneously broken by the vacuum expectation value of an antisymmetric Kalb-Ramond two-form. We consider the Einstein-Hilbert action supplemented by two independent nonminimal curvature couplings: a Ricci-tensor coupling of the form $B_{a}{}^{c}B_{bc}R^{ab}$ and a Riemann-tensor coupling of the form $B^{ab}B^{cd}R_{abcd}$. In the homogeneous vacuum sector, where the Kalb-Ramond field strength and the first derivatives of the symmetry-breaking potential vanish, the field equations reduce to a closed algebraic system in the tetrad frame of the \Godel-type metric. We derive the curvature tensors, the reduced metric equations, and the Kalb-Ramond consistency equations for several inequivalent orientations of the vacuum two-form. Particular attention is given to the pseudo-magnetic background $B_{12}=b$, because it preserves the symmetry of the rotation plane and yields a transparent deformation of the usual bumblebee result. For this sector the Kalb-Ramond equation fixes $\frac{m^2}{ω^2}=\frac{2(ξ_1+3ξ_2)}{ξ_1+2ξ_2}$, where $ξ_1$ and $ξ_2$ are the Ricci and Riemann couplings. The Ricci coupling alone reproduces the noncausal \Godel value $m^2=2ω^2$, while the Riemann coupling shifts the chronology bound and allows the critical causal value $m^2=4ω^2$ for $ξ_2=-ξ_1$. We also show that the Riemann coupling can move the critical radius for closed timelike curves, but ordinary positive-energy matter severely restricts completely causal solutions. Finally, we discuss perfect fluid, scalar field, and electromagnetic sources.

gr-qc↗

arXiv2026

Electric and magnetic Penrose processes, charged-particle collisions and superradiance around a Lorentz-violating dyonic black hole

This paper is aimed at investigating electromagnetic energy extraction, charged-particle collisions, charged-field superradiance, and horizon stability in a Lorentz-violating dyonic Kalb-Ramond black hole. The geometry differs from the dyonic Reissner-Nordström solution through a modified asymptotic normalization and an effective charge combining electric and magnetic sectors. We derive the dynamics of electrically and magnetically charged probes, including monopole-induced conical motion, and formulate electric and magnetic Penrose processes as decay or collision mechanisms in which a negative-energy fragment falls into the horizon while another escapes with enhanced energy. The electric and magnetic channels are controlled by $Φ_H=Q/[(1-\ell)r_+]$ and $Ψ_H=p/[(1-2\ell)r_+]$, respectively. Negative-energy states arise from electromagnetic canonical energy rather than from a geometrical ergoregion. We also obtain escape conditions for the outgoing products, test the possibility of overcharging or overmagnetizing the black hole, and analyze the center-of-mass energy of charged-particle collisions. Nonextremal same-direction collisions remain finite, whereas head-on or near-critical configurations can become large. In the extremal limit, electrically or magnetically critical particles may generate the Bañados-Silk-West divergence when the radial reachability condition is fulfilled.

gr-qc↗

arXiv2026

Nonlocal Rarita-Schwinger theory

In this paper, one constructs a nonlocal extension of the Rarita-Schwinger theory for spin-$3/2$ fermions. Two classes of analytic form factors are considered: scalar form factors $f(\Box)$ and Dirac-operator form factors $f(\slashed{\partial})$. The massless theory is treated together with a covariant nonlocal gauge fixing, which allows the propagator to be written directly in terms of the spin-$3/2$ projector. In the massive theory, we show that the free Rarita-Schwinger constraints remain intact for analytic form factors, so that the unphysical spin-$1/2$ sector does not become dynamical. For $f(\Box)$ the tensor-spinor structure of the propagator is the same as in the local theory, while the pole equation is deformed by the scalar form factor. For $f(\slashed{\partial})$ the physical modes obey a nonlocal Dirac-type equation, leading to modified dispersion relations that can be written explicitly for exponential form factors. We discuss the conditions under which the construction is ghost-free at the free level and identify the natural limitations that must be addressed before interactions are introduced.

hep-th↗

arXiv2026

Shadow, acoustic redshift, and transfer observables of Lorentz-violating rotating acoustic black holes

We develop an impact-parameter-resolved transfer analysis for the rotating acoustic black hole with Lorentz symmetry violation. The background is the $(2+1)$-dimensional Lorentz-violating draining-bathtub geometry, where the draining parameter $A$ fixes the sonic horizon, the circulation parameter $B$ controls rotation, and the Lorentz-breaking parameter $α$ deforms the effective acoustic metric. We derive the null-ray equations, the critical-impact-parameter conditions, the acoustic shadow interval, and the redshift transfer factor. We then formulate an intensity-transfer prescription for thin rings and extended disks that accounts for source emissivity, emitter motion, finite source width, and detector convolution. The resulting observables form a hierarchy: the shadow width probes Lorentz-violating broadening, the shadow centroid traces rotation, the left-right acoustic-redshift asymmetry tests branch-dependent Doppler and frame-dragging effects, and the integrated flux asymmetry measures their imprint on the observed intensity. We also construct synthetic two-dimensional acoustic screen maps, showing that the $(2+1)$-dimensional capture interval is naturally represented as a vertical strip whose displacement and brightness imbalance encode the combined effects of $B$ and $α$. We focus on the exterior-regular regime $α\geq0$, with $α=0$ retained as the Lorentz-symmetric benchmark.

gr-qc↗

arXiv2026

Hairy Kiselev black hole with quintessential matter: themodynamic properties, sparsity of Hawking radiation, and greybody factors

In this work, we investigate the thermodynamic properties, Hawking-radiation sparsity, and greybody-factor bounds of a hairy Kiselev black hole surrounded by a quintessential fluid. The spacetime geometry contains, in addition to the Schwarzschild mass term, a quintessence contribution controlled by the intensity parameter $N$ and the state parameter $ω_q$, an exponential hair correction governed by the coupling $α$ and the primary hair scale $\ell$, and a cosmological constant $Λ$. We first examined the horizon structure and derived the black hole mass from the horizon condition. The Hawking temperature, heat capacity, and Gibbs free energy were then analyzed in order to identify the influence of the hair and surrounding-field parameters on local and global thermodynamic behavior. The results show that the exponential hair mainly affects the small-horizon regime, while the quintessential sector and the cosmological constant produce significant changes in the large-scale behavior of the black hole. In particular, the heat capacity exhibits divergences associated with second-order phase transitions, and the Gibbs free energy reveals the possibility of competing thermodynamic branches. We also analyzed the sparsity of Hawking radiation and showed that the emitted flux is highly intermittent rather than continuous. The sparsity parameter is controlled by the combination of the Hawking temperature, the effective emitting area, and the greybody factors. Finally, we studied massless scalar perturbations in this background by reducing the Klein-Gordon equation to a radial equation with an effective potential.

gr-qc↗

arXiv2026

New mechanism for fermion localization in $f(T,T_G)$-brane

We investigate the localization of fermionic fields in a five-dimensional braneworld scenario within the framework of modified teleparallel gravity described by a general $f(T,T_G)$ function. Considering a non-minimal coupling between a Dirac spinor and the torsional invariants, we derive the effective Schrödinger-like equations governing the Kaluza-Klein modes. We showed that the contribution of the teleparallel Gauss-Bonnet term significantly modifies the effective potentials and, consequently, the localization properties. The zero-mode analysis reveals that only one chiral component can be localized on the brane, with the degree of confinement depending on the chosen model. In the massive sector, the spectrum is continuous, but resonant states arise due to the internal structure of the potentials. Additionally, we employ information-theoretic measures, such as Shannon entropy and relative probability, to quantify the localization mechanism. Our results show that the torsional modifications induce a nontrivial redistribution of information, exhibiting stronger localization. These findings highlight the role of higher-order torsional terms in shaping fermionic localization and resonance structures in braneworld scenarios.

hep-th↗

arXiv2026

ModMax-black hole surrounded by cloud of strings in Bumblebee gravity

In this article, we investigate the optical, thermodynamic, and scattering properties of a ModMax black hole surrounded by a cloud of strings within the framework of Einstein-bumblebee gravity. We then analyze in detail the thermodynamic properties of this black hole, including the Hawking temperature, entropy, and other relevant thermodynamic quantities, and examine the outcomes. Furthermore, we study the greybody factors (GFs) associated with the emission of various perturbative fields propagating in this black hole background. In particular, we consider spin-0 scalar fields, spin-1 electromagnetic fields, and spin-2 graviton fields, and evaluate the corresponding absorption probabilities and energy emission rates. Our analysis demonstrates how the optical features, thermodynamics and GFs depend on the underlying parameters of the system, such as the Lorentz symmetry violation parameter, the cloud of strings parameter, the ModMax parameter, the electric charge, and the black hole mass, thereby providing a comprehensive understanding of the physical effects of these parameters on the radiation and scattering processes around the black hole.

gr-qc↗

arXiv2026

ModMax black hole surrounded by perfect-fluid dark matter in Lorentz-violating Kalb-Ramond gravity

We investigate a ModMax black hole surrounded by perfect-fluid dark matter within the framework of Lorentz-violating Kalb-Ramond gravity. The model combines three physically distinct contributions: nonlinear electrodynamic corrections from the ModMax sector, Lorentz-symmetry-breaking effects induced by the background Kalb-Ramond field, and environmental modifications associated with the surrounding dark matter fluid. We obtain the corresponding static and spherically symmetric black hole geometry and analyze how the charge, ModMax parameter, Kalb-Ramond coupling, and dark matter parameter affect the horizon structure and thermodynamic behavior. In particular, we study the Hawking temperature, entropy, heat capacity, and Helmholtz free energy, showing that the combined effects of nonlinear electrodynamics and Lorentz violation may shift the extremal configuration, modify the thermal stability regions, and generate nontrivial phase behavior. The perfect fluid dark matter contribution introduces an additional logarithmic correction to the geometry, becoming especially relevant at intermediate radial scales. Our results indicate that ModMax electrodynamics can effectively screen the electric sector, while the Kalb-Ramond parameter amplifies the geometric deformation and changes the thermodynamic response of the system. These features suggest that black holes in Lorentz-violating backgrounds surrounded by dark matter provide a useful arena for probing deviations from standard charged black-hole thermodynamics.

gr-qc↗

arXiv2026

Transfer observables of rotating acoustic black holes from ray tracing: shadow centroid, redshift asymmetry and flux imbalance

We construct an impact-parameter-resolved transfer framework for null acoustic rays in the rotating draining-bathtub spacetime. The formalism separates the source-independent ray geometry from the source and detector model by keeping explicit the acoustic redshift, transfer convention, emissivity, emitter velocity field, and source-to-screen mapping. The geometric capture interval provides two clean observables: a shadow centroid that shifts linearly with circulation and a shadow width that grows monotonically with circulation. Observable profiles are obtained from direct ray-source intersections, finite source width or extended-disk integration, detector convolution, and convergence checks, rather than from an approximate semi-analytic ring map. The transfer calculation shows that rotation produces a left-right redshift tilt and a branch-dependent flux imbalance, while the total flux alone remains a degenerate circulation diagnostic. The most useful diagnostics are differential quantities: the shadow centroid, branch-integrated flux asymmetry, peak asymmetry, left-right redshift asymmetry, and global redshift contrast. We also discuss how these observables respond to the transfer convention, intrinsic azimuthal emissivity, the choice of left-right split, finite resolution, and physical limitations such as dispersion, viscosity, and finite-depth corrections.

gr-qc↗

arXiv2026

Perturbations and greybody bounds of Euler-Heisenberg black holes surrounded by perfect fluid dark matter

We investigate the propagation of massless fields with spins $s=0$, $s=1$, and $s=1/2$ in the spacetime of an Euler-Heisenberg (EH) black hole surrounded by perfect fluid dark matter (PFDM). This background incorporates both the nonlinear electrodynamic correction associated with the EH effective theory and the logarithmic contribution induced by the surrounding dark matter distribution. After deriving the corresponding Schrödinger-like radial equations, we construct the effective potentials for scalar, electromagnetic, and Dirac perturbations and analyze how they are modified by the black hole charge, the EH parameter, and the PFDM parameter. The greybody factors are estimated through the rigorous Boonserm-Visser lower-bound method, and the associated partial absorption cross sections are obtained for different spin sectors using these bounds. Our results show that the nonlinear electrodynamic and dark matter parameters significantly deform the effective potential barrier, leading to potentially distinguishable changes in the transmission probabilities and absorption spectra within the model. In particular, the greybody lower bounds increase monotonically with the frequency and approach the high-frequency limit, while their low-frequency behavior is strongly affected by the geometry and by the spin of the perturbing field. Moreover, we utilize the greybody lower bound to calculate the energy emission rate. Finally, we make comparisons with the Schwarzschild, Reissner-Nordström, and pure EH limits, showing that the combined EH+PFDM background produces distinguishable corrections to black-hole scattering within the model. These results highlight greybody bounds as sensitive diagnostic probes of nonlinear electrodynamic effects and dark matter halos around compact objects.

gr-qc↗

arXiv2026

Topological and self-dual vortices in a double sigma model with Maxwell coupling

In this work, we construct a double O(3)-sigma model minimally coupled to a Maxwell field in (2+1)-dimensional spacetime and investigate the existence of self-dual magnetic vortex solutions. An analysis of the Bogomol'nyi-Prasad-Sommerfield (BPS) property reveals that both sigma fields belong to the same topological sector and that the potential assumes a periodic cosine-like form. Furthermore, the theory supports the emergence of magnetic vortices with quantized flux, described by two nonlinear O(3)-sigma sectors that effectively combine into a single topological sector in the BPS regime. In addition, we analytically verify the consistency of the BPS structure and its asymptotic behavior. Within this framework, numerical vortex solutions confirm that the field profiles are smooth and spatially localized, with both the magnetic field and the energy density remaining regular and localized.

hep-th↗

arXiv2026

Duality in $SIM(2)$ topologically massive models with $B\wedge F$ term

This paper aims to investigate the classical duality between the $SIM(2)$-Maxwell-Kalb-Ramond (MKR) theory and a self-dual non-gauge-invariant model. First, we establish the equivalence in the free-field case using two complementary methods: a direct comparison of the equations of motion and the master Lagrangian approach. In both methodologies, we verify that the classical correspondence between the MKR model and self-dual fields exhibits modifications induced by very special relativity (VSR). Moreover, we employ the master Lagrangian approach to examine the duality when the self-dual model is minimally coupled to fermionic matter. We show that the resulting MKR model contains Thirring-like interactions modified by nonlocal contributions arising from VSR.

hep-th↗

arXiv2025

Thick braneworld in $f(Q,\mathcal{T})$ gravity

In this work, we study a codimension-one thick brane within the framework of $f(Q,\mathcal{T})$ modified symmetric teleparallel gravity, where $Q$ is the nonmetricity scalar and $\mathcal{T}$ is the trace of the energy-momentum tensor. Using two specific choices of the warp factor, we construct the complete brane system, including the scalar field solution and energy density. We show how nonmetricity influences the brane profile and analyze the stability of the braneworld scenarios under small tensor perturbations.

gr-qc↗

arXiv2024

Geometrical deformation of brane matter field within $f(R,Q,P)$ gravity

In the context of braneworld scenarios, the real scalar field plays a crucial role by providing thickness for brane. In this work, we investigate a codimension-one thick brane within the framework of $f(R,Q,P)$ gravity, where $R$ represents the curvature scalar, while $Q=R_{μν}R^{μν}$ and $P=R_{μναβ}R^{μναβ}$ are quadratic geometric invariants. Our interest is to investigate the influence of these invariants on the scalar field solution and the energy density of the brane. Furthermore, we analyze the localization of spin $1/2$ fermions and the gravitino by employing a Yukawa-like coupling with the scalar field background. Such a coupling is able to produce a normalizable zero-mode for both fields.

gr-qc↗

arXiv2023

Duality between the Maxwell-Chern-Simons and self-dual models in very special relativity

This work aims to investigate the classical-level duality between the $SIM(1)$-Maxwell-Chern-Simons (MCS) model and its self-dual counterpart. Initially, our focus is on free-field cases to establish equivalence through two distinct approaches: comparing the equations of motion and utilizing the master Lagrangian method. In both instances, the classical correspondence between the self-dual field and the MCS dual field undergoes modifications due to very special relativity (VSR). Specifically, duality is established only when the associated VSR-mass parameters are the same. Furthermore, we analyze the duality when the self-dual model is minimally coupled to fermions. As a result, we show that Thirring-like interactions, corrected for non-local VSR contributions, are included in the MCS model. Additionally, we demonstrate the equivalence of the fermion sectors in both models, thereby concluding the proof of classical-level duality.

hep-th↗

arXiv2023

One-loop radiative corrections in bumblebee-Stueckelberg model

This work aims to study the radiative corrections in a vector model with spontaneous Lorentz symmetry violation, known in the literature as the bumblebee model. We consider such a model with self-interaction quadratic smooth potential responsible for spontaneous Lorentz symmetry breaking. The spectrum of this model displays a transversal nonmassive mode, identified as Nambu-Goldstone, and a massive longitudinal mode. Besides the Lorentz symmetry, this model also exhibits gauge symmetry violation. To restore the gauge symmetry, we introduce the Stueckelberg field and calculate the two-point function by employing the principal-value (PV) prescription. The result is nontransversal, leading to a massive excited mode.

hep-th↗