arXiv subjects

Paulo J. Porfírio

Publications and source records attributed to Paulo J. Porfírio.

6 recordsLinked to original sources

arXiv2026

Rotating black hole shadows in metric-affine bumblebee gravity

In this work, we investigate the structure of black hole shadows in the bumblebee gravity model formulated within the metric-affine framework, which incorporates spontaneous Lorentz symmetry breaking (LSB) through a vector field $B_μ$ with a non-zero vacuum expectation value. We analyze the influence of the dimensionless rotation parameter $a = J/M$ and the Lorentz-violating (LV) coefficient $X = ξb^2$ on the photon sphere radius, the critical impact parameter, and the shadow morphology. Using ray-tracing simulations with the GYOTO code and accretion disks, we observe that increasing values of $X$ induce progressive vertical flattening, asymmetric ``teardrop''-shaped deformations, and local collapse of the lower silhouette region, interacting with the rotational Doppler effect. These anisotropic signatures distinguish the bumblebee model from the standard Kerr metric and provide observational tests for LSB effects in strong gravity regimes, potentially detectable by the Event Horizon Telescope in sources such as M87* and Sgr A*.

gr-qc↗

arXiv2026

Slowly rotating charged BTZ black hole solutions in Palatini Chern-Simons gravity

We consider a metric-affine formulation of Chern-Simons modified gravity in 2 + 1 dimensions. The theory is built requiring projective invariance, and the structure of the equations is analyzed using a decomposition in terms of scalar, vectorial, and purely tensorial objects. This approach allows us to implement a perturbative approach to study the corrections that emerge around a given background solution, for which we consider a BTZ charged, non-rotating metric. We show that conditions on model parameters are necessary to keep perturbations under control, yielding a rotating solution with a constant angular momentum and magnetic field at the horizon, and a smooth decay further away. We comment on the possibility of going beyond the leading order in perturbations and on its dynamical implications.

gr-qc↗

arXiv2025

Shadow structure of generalized $k-n$ black-bounce metrics

The existence of black hole shadows is one of the most interesting effects of the strong field regime of general relativity (GR). Recent observations by the Event Horizon Telescope (EHT) have provided high-resolution images of the vicinity of supermassive black holes, ushering in a new era for testing gravitation on astrophysical scales. In this work, we continue the investigation initiated by \cite{furtado2025gravitational}, focusing on shadows associated with generalized $k-n$ \emph{black-bounce} type spacetimes, which smoothly interpolate between regular black holes and wormholes. We consider a generalization of the metric with free parameters $(a,k,n)$ that modify the mass function and enrich the possible phenomenology. We develop a semi-analytical study of photon orbits, obtaining the critical impact parameter and the shadow radius for different parameter combinations. Subsequently, we perform numerical ray-tracing simulations using the \textsc{GYOTO} code, incorporating optically thick accretion disks and varying the observation angle. Our results reveal characteristic signatures, including the formation of double-ring structures and deformations of the shadow radius, which can serve as observational discriminators between classical black holes and \emph{black-bounce} solutions.

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↗

arXiv2021

Ground state energy and topological mass in spacetimes with nontrivial topology

In the present paper, we investigate the ground state energy of a massless scalar field and generation of topological mass by considering a quasi-periodically identified Minkowski spacetime and the `half-Einstein Universe', that is, an Einstein Universe where the massless scalar field propagates under Dirichlet boundary condition. The analysis is performed considering one and two-loop corrections to the effective potential in both cases. Our results are compared with previous results found in literature.

hep-th↗

arXiv2020

Gaussian null coordinates, near-horizon geometry and conserved charges on the horizon of extremal non-dilatonic black $p$-branes

In this paper, we examine the emergence of conserved charges on the horizon of a particular class of extremal non-dilatonic black $p$-branes (which reduce to extremal dilatonic black holes in $D=4$ dimensions upon toroidal compactification) in the presence of a probe massless scalar field in the bulk. This result is achieved by writing the black $p$-brane geometry in a Gaussian null coordinate system which allows us to get a non-singular near-horizon geometry description. We find that the near-horizon geometry is $AdS_{p+2}\times S^2$ and that the $AdS_{p+2}$ section has an internal structure which can be seen as a warped product of $AdS_{2}\times S^{p}$ in Gaussian null coordinates. We show that the bulk scalar field satisfying the field equations is expanded in terms of non-normalizable and normalizable modes, which for certain suitable quantization conditions are well-behaved at the boundary of $AdS_{p+2}$ space. Furthermore, we show that picking the normalizable modes results in the existence of conserved quantities on the horizon. We discuss the impact of these conserved quantities in the late time regime.

hep-th↗