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J. R. Nascimento

Publications and source records attributed to J. R. Nascimento.

At least 37 records · Page 2Linked to original sources
arXiv2025

Gravitational lensing by $k-n$ generalized black-bounce space-times

We study gravitational lensing by $k-n$ generalized black-bounce space-times both in regimes of weak and strong field approximations. These metrics interpolate between regular black holes and one-way or traversable wormholes. First, we investigate the light-like geodesic trajectories and derive an analytical expression for the deflection angle in terms of the bounce parameter in the weak-field gravitational regime. We then turn to the strong-field gravitational regime and display the behavior of the bending angle as a function of both the impact parameter and the bounce parameter. Next, using the lens equations, we analyze how the observables for \textit{Sagittarius} A* behave concerning the bounce parameter. We obtain the shadow's radii for some black-bounce metrics and plot the graph of their sizes, comparing them with the Schwarzschild one.

gr-qc↗
arXiv2025

One-loop corrections in Maxwell-metric-affine bumblebee gravity

In this paper, we consider the coupling of the metric-affine bumblebee gravity to the Abelian gauge field and obtain the effective model corresponding to the weak gravity limit of this theory. The effective bumblebee theory displays new unconventional couplings between the bumblebee field and its field strength, and the $U(1)$ gauge field along with its respective field strength, as a result of the non-metricity effects. Thus, being a new gauge-bumblebee theory, it represents an example of vector-vector couplings which are very rarely considered, if not entirely overlooked, in the Abelian case. For this theory we calculate the lower perturbative corrections. We close the paper with discussions of other possible vector-vector couplings.

hep-th↗
arXiv2025

Carroll-Field-Jackiw term in a massless Rarita-Schwinger model

We consider the massless Rarita-Schwinger (RS) LV QED. In this theory, we introduce the gauge fixing to obtain the propagator for the RS field, and calculate the Carroll-Field-Jackiw term, which turns out to be finite and ambiguous, and only in one calculation scheme, based on the nonlinear gauge framework, the gauge independence of the result is achieved.

hep-th↗
arXiv2025

Properties of an axisymmetric Lorentzian non-commutative black hole

In this work, we start by examining a spherically symmetric black hole within the framework of non-commutative geometry and apply a modified Newman-Janis method to obtain a new rotating solution. We then investigate its consequences, focusing on the horizon structure, ergospheres, and the black hole's angular velocity. Following this, a detailed thermodynamic analysis is performed, covering surface gravity, Hawking temperature, entropy, and heat capacity. We also study geodesic motion, with particular emphasis on null geodesics and their associated radial accelerations. Additionally, the photon sphere and the resulting black hole shadows are explored. Finally, we compute the quasinormal modes for scalar perturbations using the 6th-order WKB approximation.

gr-qc↗
arXiv2024

Dynamical Lorentz Symmetry Breaking in a Scale-free Theory of Gravity

This paper explores the renormalization of scale-free quadratic gravity coupled to the bumblebee field and its potential for dynamically breaking Lorentz symmetry. We conduct one-loop renormalization of the model and calculate the associated renormalization group functions. Furthermore, we compute the one-loop effective potential for the bumblebee field, demonstrating that it acquires a non-trivial vacuum expectation value (VEV) through radiative corrections -- a phenomenon known as the Coleman-Weinberg mechanism. This spontaneous breaking of scale invariance, driven by the non-zero VEV of the bumblebee field, leads to Lorentz symmetry violation. As a result, the non-minimal coupling between the bumblebee and gravitational fields induces a spontaneous generation of an Einstein-Hilbert term via radiative corrections, suggesting a possible link between the Planck scale and Lorentz violation phenomena.

hep-th↗
arXiv2024

Generalized Chern-Pontryagin models

We formulate a new class of modified gravity models, that is, generalized four-dimensional Chern-Pontryagin models, whose action is characterized by an arbitrary function of the Ricci scalar $R$ and the Chern-Pontryagin topological term ${}^*RR$, i.e., $f(R, {}^*RR)$. Within this framework, we derive the gravitational field equations and solve them for a particular model, $f(R, {}^*RR)=R+β({}^*RR)^2$, considering two ansatzes: the slowly rotating metric and first-order perturbations of Gödel-type metrics. For the former, we find a first-order correction to the frame-dragging effect boosted by the parameter $L$, which characterizes the departures from general relativity results. For the latter, Gödel-type metrics hold unperturbed. We conclude this paper by displaying that generalized four-dimensional Chern-Pontryagin models admit a scalar-tensor representation, whose explicit form presents two scalar fields: $Φ$, a dynamical degree of freedom, while the second, $\vartheta$, a non-dynamical degree of freedom. In particular, the scalar field $\vartheta$ emerges coupled with the Chern-Pontryagin topological term ${}^*RR$, i.e., $\vartheta {}^*RR$, which is nothing more than Chern-Simons term.

gr-qc↗
arXiv2024

Derivative four-fermion model, effective action and bumblebee generation

In this paper, we study the one-loop effective potential of a derivative four-fermion model. As a result, an exact bumblebee-like potential is radiatively generated. Afterwards, we generalize our study for a finite temperature case and explicitly demonstrate the possibility of phase transitions allowing for the restoration of the Lorentz symmetry. We also investigate the low-energy effective action, from which we obtain the usual kinetic term and the corresponding bumblebee potential.

hep-th↗
arXiv2024

Effects of non-commutative geometry on black hole properties

In this study, we investigate the signatures of a non-commutative black hole solution. Initially, we calculate the thermodynamic properties of the system, including entropy, heat capacity, and Hawking radiation. For the latter quantity, we employ two distinct methods: surface gravity and the topological approach. Additionally, we examine the emission rate and remnant mass within this context. Remarkably, the lifetime of the black hole, after reaching its final state due to the evaporation process, is expressed analytically up to a grey-body factor. We estimate the lifetime for specific initial and final mass configurations. Also, we analyze the tensorial quasinormal modes using the 6th-order WKB method. Finally, we study the deflection angle, i.e., gravitational lensing, in both the weak and strong deflection limits.

gr-qc↗
arXiv2024

An exact stationary axisymmetric vacuum solution within a metric--affine bumblebee gravity

Within the framework of the spontaneous Lorentz symmetry breaking, we consider a metric--affine generalization of the gravitational sector of the Standard--Model Extension (SME), including the Lorentz--violating (LV) coefficients $u$ and $s^{μν}$. In this model, we derive the modified Einstein field equations in order to obtain a new axisymmetric vacuum spinning solution for a particular bumblebee's profile. Such a solution has the remarkable property of incorporating the effects of Lorentz symmetry breaking (LSB) through the LV dimensionless parameter $X=ξb^2$, as the LSB is turned off, $X=0$, we recover the well--established result, the Kerr solution, as expected. Afterwards, we calculate the geodesics, the radial acceleration and thermodynamic quantities for this new metric. We also estimate an upper bound for $X$ by using astrophysical data of the advance Mercury's perihelion.

gr-qc↗
arXiv2024

On metric-affine bumblebee model coupled to scalar matter

We consider the coupling of the metric-affine bumblebee gravity model to scalar matter and calculate the lower-order contributions to two-point functions of bumblebee and scalar fields in the weak gravity approximation. We also obtain the one-loop effective potentials for both scalar and vector fields.

hep-th↗
arXiv2024

Two-loop corrections to the Carroll-Field-Jackiw term in a CPT-odd Lorentz-violating scalar QED

In this study, we systematically calculate one-loop corrections to the Lorentz-violating vertices within the framework of CPT-odd Quantum Electrodynamics, encompassing scalar and photon fields in arbitrary gauge. Additionally, we ascertain the finite two-loop corrections to the Carroll-Field-Jackiw term. Furthermore, we analyze the UV divergent component of the two-loop Lorentz-violating correction in the self-energy of the scalar field.

hep-th↗
arXiv2023

Lorentz-breaking Rarita-Schwinger model

In this paper, we formulate the Lorentz-breaking extension for the spin-3/2 field theory and couple it to the Abelian gauge field in a Lorentz violating (LV) manner. Next, we calculate the lower LV quantum corrections, that is, the Carroll-Field-Jackiw (CFJ) term, which, being superficially divergent, turns out to be finite but ambiguous, and also the higher-derivative CFJ term. Besides, we compute the aether term, being the lowest CPT-even LV term, involving the second order in the LV vector.

hep-th↗
arXiv2023

Vacuum solution within a metric-affine bumblebee gravity

We consider a metric-affine extension to the gravitational sector of the Standard-Model Extension for the Lorentz-violating coefficients $u$ and $s^{μν}$. The general results, which are applied to a specific model called metric--affine bumblebee gravity, are obtained. A Schwarzschild-like solution, incorporating effects of the Lorentz symmetry breaking through coefficient $X=ξb^2$, is found. Furthermore, a complete study of the geodesics trajectories of particles has been accomplished in this background, emphasizing the departure from general relativity. We also compute the advance of Mercury's perihelion and the deflection of light within the context of the weak field approximation, and we verify that there exist two new contributions ascribed to the Lorentz symmetry breaking. As a phenomenological application, we compare our theoretical results with observational data in order to estimate the coefficient $X$.

gr-qc↗
arXiv2023

Non-Abelian Carroll-Field-Jackiw term term in a Rarita-Schwinger model

In this paper, we demonstrate the possibility of generating a non-Abelian Carroll-Field-Jackiw (CFJ) term in the theory of a non-Abelian gauge field coupled to a spin-3/2 field in the presence of the constant axial vector field. Applying two regularization schemes, we prove that this term is finite and ambiguous, particularly vanishing within the 't Hooft-Veltman scheme.

hep-th↗
arXiv2023

On aether terms in a space-time with a compact extra dimension

In this paper, we explicitly calculate the aether-like corrections for the electromagnetic field in the case when the space-time involves an extra compact spatial dimension besides of usual four dimensions. Our methodology is based on an explicit summation over the Kaluza-Klein tower of fields which turns out to be no more difficult than the finite-temperature calculations. We found some new terms which can be treated as certain five-dimensional generalizations of the aether-like terms $κ^{mnpq}F_{mn}F_{pq}$ and $\barψc^{mn}γ_m\partial_nψ$ which in four dimensions are treated as important ingredients of the Lorentz-breaking extension of the standard model. We have shown that these new terms become to dominate in the limit where the extra dimension is very small, allowing us to suggest that there will be essential modifications of quantum corrections if our space indeed has a small extra spatial dimension. Moreover, we have shown that in the CPT-even case we have extra new particles as a consequence of the extra dimension.

hep-th↗
arXiv2023

Gödel-type solutions within the $f(R,Q,P)$ gravity

In this work, we scrutinize the consistency of spacetime homogeneous Gödel-type metrics within $f(R,Q,P)$ theories of gravity for well-motivated matter sources. As it is well known, such geometries allow for causality violation. We provide general conditions to engender completely causal solutions in a manner completely different from general relativity. We take some specific models, for instance, $f(R,Q,P)=R -\dfrac{μ^{4n+2}}{\left(a R^2+b Q+c P\right)^n}$, to illustrate the general results. Notably, we also find an unusual completely causal vacuum solution in the presence of a non-trivial cosmological constant which corresponds to the case $m^2=4ω^2$.

gr-qc↗
arXiv2023

Axion-Photon Interaction from Nonminimal Dimension-5 Lorentz-Violating Operators

In this paper, we discuss various possible schemes for the perturbative generation of the axion-photon interaction term in different Lorentz-breaking extensions of QED, involving operators with mass dimensions up to 5. We demonstrate explicitly that there are only a few schemes allowing to generate a finite axion-photon interaction term from one-loop radiative corrections, and within all these schemes, the generated term turns out to be ambiguous.

hep-ph↗