arXiv subjects

Marco Olivares

Publications and source records attributed to Marco Olivares.

At least 19 recordsLinked to original sources

arXiv2026

Study of Null Geodesics and their Stability in Kalb-Ramond Black Holes

We investigate null geodesics and their stability in four-dimensional charged Kalb-Ramond black holes with a cosmological constant. The non-vanishing vacuum expectation value of the background antisymmetric tensor field induces spontaneous Lorentz symmetry breaking, controlled by a dimensionless parameter $l$, and deforms the Reissner-Nordström-(A)dS geometry. We derive the effective potential for massless particles and obtain analytic expressions for the photon-sphere radius, critical impact parameter, capture cross section, and deflection trajectories. The limiting cases $l\rightarrow0$ recover the RN, RN-dS and RN-AdS geodesic structures, while finite values of $l$ shift the turning points, modify the bending of light and deform both first-kind and second-kind photon trajectories. In the AdS branch, we find a special limiting limaçon-type null geodesic whose angular structure is modified by the Kalb-Ramond parameter. We also compute the Lyapunov exponent of the unstable circular null orbit and show that the cosmological constant changes the instability timescale, whereas the Lorentz-violating parameter produces a non-trivial deformation of the photon-sphere instability. Our results identify null geodesics as sensitive probes of Lorentz-violating effects in charged black hole spacetimes.

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arXiv2026

Observational Tests of Regular Black Holes with Scalar Hair and their Stability

We study the geodesic structure and observable properties of asymptotically flat regular black holes sourced by a phantom scalar field characterized by a scalar charge $A$. This parameter removes the central singularity and continuously deforms the Schwarzschild geometry. The equations of motion for test particles and photons are derived, and the resulting null geodesics are analyzed, including the deflection of light, gravitational time delay, and redshift, in order to constrain $A$ using classical Solar System tests. These observations impose stringent limits on the scalar charge, confirming that $A$ must remain extremely small in the weak-field regime to ensure full consistency with general relativity. In the strong-field regime, we compute the Lyapunov exponent $λ$ associated with the photon sphere and establish its exact relations with the critical impact parameter $\mathcal{B}_u$ and the angular size of the shadow $α_{\mathrm{sh}}$, given by $\mathcal{B}_u = 1/|λ|$ and $α_{\mathrm{sh}} = 1/(r_{0}|λ|)$. These correspondences reveal that the dynamical instability of null circular orbits governs the optical appearance of the black hole. Our results show that increasing $A$ reduces the instability of photon trajectories and enlarges the angular size of the shadow, indicating that the regularization scale leaves a distinct observational imprint on the geometry of regular black holes. In addition, constraints derived from Event Horizon Telescope observations of M87* and Sgr A* further restrict the allowed range of the scalar charge, reinforcing the consistency of the model with current astrophysical observations.

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arXiv2026

Time Like Geodesics of Regular Black Holes with Scalar Hair

We investigate timelike geodesics in asymptotically flat regular black holes supported by a phantom scalar field characterized by a scalar charge $A$. This parameter removes the central singularity and continuously deforms the Schwarzschild geometry while preserving asymptotic flatness. We derive the equations of motion for massive test particles and classify bounded and unbounded trajectories in terms of the conserved energy and angular momentum. We determine circular and critical orbits, including the innermost stable circular orbit (ISCO), and analyze the transition between capture and scattering. We show that the scalar charge modifies the location of the unstable and stable circular orbits, the ISCO, and the threshold angular momentum for scattering, exhibiting a nontrivial dependence on the radial coordinate. Their physical scales are naturally described in terms of the invariant areal radius $R(r)=\sqrt{r^2+A^2}$. In the weak-field regime, we compute the perihelion precession and obtain corrections proportional to the scalar charge, allowing us to constrain the scalar charge from Solar System observations. We also analyze the motion with vanishing angular momentum and show that, while the qualitative structure of the trajectories remains connected to the Schwarzschild limit $A\to 0$, the quantitative deviations encode the geometric effects of the scalar hair.

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arXiv2025

Massive Particle Motion Around Horndeski Black Holes

The time-like structure of the four-dimensional asymptotically flat Horndeski black holes is studied in detail. Focusing on the motion of massive neutral test particles, we construct the corresponding effective potential and classify the admissible types of orbits. The equations of motion are solved analytically, yielding trajectories expressed in terms of Weierstrass elliptic functions and elementary functions. As an application, we compute the perihelion precession as a classical test of gravity within the Solar System and use it to place observational constraints on the coupling parameter between the scalar field and gravity.

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arXiv2025

Study of Null Geodesics and their Stability in Horndeski Black Holes

We study the motion of particles in the background of a scalar-tensor theory of gravity in which the scalar field is kinetically coupled to the Einstein tensor and we present the null geodesic structure for asymptotically flat, AdS, and dS Horndeski black holes, studying the effect of the cosmological constant on the orbits. Also, we consider three classical test of the gravity in the solar system, such as, the bending of the light, the gravitational redshift, and the Shapiro time delay in order to constraint the coupling parameters of the scalar field to gravity. Calculating the Lyapunov exponent we explore the stability of these geodesics for various values of the cosmological constant.

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arXiv2024

Light propagation around a Kerr-like black hole immersed in an inhomogeneous anisotropic plasma in Rastall gravity: Analytical solutions to the equations of motion

In this paper, we explore the behavior of light ray trajectories in the exterior geometry of a rotating black hole within the Rastall theory of gravity, which is surrounded by an inhomogeneous anisotropic electronic cold plasma. By specifying the plasma's frequency profile, we derive fully analytical solutions for the temporal evolution of spacetime coordinates using elliptic integrals and Jacobi elliptic functions. These solutions illustrate various possible orbits. Throughout the study, we compare the results with those in the vacuum case, emphasizing the influence of plasma. Additionally, we utilize the analytical solutions to establish the lens equation for the considered spacetime. The investigation also addresses the significance of spherical photon orbits on critical trajectories, by presenting several examples.

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arXiv2023

Timelike geodesics for five-dimensional Schwarzschild and Reissner-Nordström Anti-de Sitter black holes

The timelike structure of the five-dimensional Schwarzschild and Reissner-Nordström Anti-de Sitter black holes is studied in detail. Different kinds of motion are allowed and studied by using an adequate effective potential. Then, by solving the corresponding equations of motion, several trajectories and orbits are described in terms of Weierstrass elliptic functions and elementary functions for neutral particles.

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arXiv2023

Spherical photon orbits around a rotating black hole with quintessence and cloud of strings

In this paper, we calculate the analytical solutions for the radii of planar and polar spherical photon orbits around a rotating black hole that is associated with quintessential field and cloud of strings. This includes a full analytical treatment of a quintic that describes orbits on the equatorial plane. Furthermore, The radial profile of the impact parameters is studied and the radii corresponding to the extreme cases are derived. For the more general cases, we also discuss the photon regions that form around this black hole. To simulate the orbits that appear in different inclinations, we analytically solve the latitudinal and azimuth equations of motion in terms of the Weierstrassian elliptic functions, by considering the radii of spherical orbits, in their general form, as the initial conditions. The period and the stability conditions of the orbits are also obtained analytically.

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arXiv2022

Study of null and time-like geodesics in the exterior of a Schwarzschild black hole with quintessence and cloud of strings

Recently, an analytical study of radial and circular orbits for null and time-like geodesics that propagate in the spacetime produced by a Schwarzschild black hole associated with cloud of strings, in a universe filled by quintessence, has been done in Ref. \cite{Mustafa:2021}. In this paper, we complete the aforementioned study by investigating possible analytical solutions to the equations of motion for other types of bound orbits, besides taking into account the cases of unbound orbits. This requires an extensive study of the corresponding effective potentials that categorize the test particle motion. We follow the standard Lagrangian dynamics to parametrize the radial and angular geodesics and the resultant (hyper-)elliptic integrals of motion are treated accordingly. We also simulate the orbits which correspond to different levels of energy in the effective potentials.

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arXiv2021

Analytical study of light ray trajectories in Kerr spacetime in the presence of an inhomogeneous anisotropic plasma

We calculate the exact solutions to the equations of motion that govern the light ray trajectories as they travel in a Kerr black hole's exterior that is considered to be filled with an inhomogeneous and anisotropic plasmic medium. This is approached by characterizing the plasma through conceiving a radial and an angular structure function, which are let to be constant. The description of the motion is carried out by using the Hamilton-Jacobi method, that allows defining two effective potentials, characterizing the evolution of the polar coordinates. The elliptic integrals of motion are then solved analytically, and the evolution of coordinates is expressed in terms of the Mino time. This way, the three-dimensional demonstrations of the light ray trajectories are given respectively.

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arXiv2021

Probing the parameters of a Schwarzschild black hole surrounded by quintessence and cloud of strings through four standard astrophysical tests

In this paper, we concern about applying general relativistic tests on the spacetime produced by a static black hole associated with cloud of strings, in a universe filled with quintessence. The four tests we apply are precession of the perihelion in the planetary orbits, gravitational redshift, deflection of light, and the Shapiro time delay. Through this process, we constrain the spacetime's parameters in the context of the observational data, which results in about $\sim 10^{-9}$ for the cloud of strings parameter, and $\sim 10^{-20}$ m$^{-1}$ for that of quintessence. The response of the black hole to the gravitational perturbations is also discussed.

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arXiv2021

Ergosphere, photon region structure, and the shadow of a rotating charged Weyl black hole

In this paper, we explore the photon region and the shadow of the rotating counterpart of a static charged Weyl black hole, which has been previously discussed according to null and time-like geodesics. The rotating black hole shows strong sensitivity to the electric charge and the spin parameter, and its shadow changes from being oblate to being sharp by increasing in the spin parameter. Comparing the calculated vertical angular diameter of the shadow with that of M87*, we found that the latter may possess about $10^36$ protons as its source of electric charge, if it is a rotating charged Weyl black hole. A complete derivation of the ergosphere and the static limit is also presented.

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arXiv2021

Gravitational Rutherford scattering of electrically charged particles from a charged Weyl black hole

Considering electrically charged test particles, we continue our study of the exterior dynamics of a charged Weyl black hole which has been previously investigated regarding the motion of mass-less and (neutral) massive particles. In this paper, the deflecting trajectories of charged particles are designated as being gravitationally Rutherford-scattered and detailed discussions of angular and radial particle motions are presented.

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arXiv2021

Time like geodesics in three-dimensional rotating Hořava AdS black hole

We study the motion of particles in the background of a three-dimensional rotating Hořava AdS black hole that corresponds to a Lorentz-violating version of the BTZ black hole and we analyze the effect of the breaking of Lorentz invariance in such motion by solving analytically the geodesic equations. Mainly, we find that Lorentz-violating version of the BTZ black hole posses a more rich geodesic structure, where the planetary and circular orbits are allowed, which does not occurs in the BTZ background.

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arXiv2021

Null geodesics in five-dimensional Reissner-Nordström anti-de Sitter black hole

The study of the motion of photons around massive bodies is one of the most useful tools to know the geodesic structure associated with said gravitational source. In the present work, different possible paths projected in an invariant hyperplane are investigated, considering five-dimensional Reissner-Nordström anti-de Sitter black hole. Also, we study some observational test such as the bending of light and the Shapiro time delay effect. Mainly, we found that the motion of photons follows the hippopede of Proclus geodesic, which is a new type of trajectory of second kind, being the Limaçon of Pascal their analogue geodesic in four-dimensional Reissner-Nordström anti-de Sitter black hole.

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arXiv2020

Motion of massive particles around a charged Weyl black hole and the geodetic precession of orbiting gyroscopes

The advanced state of cosmological observations constantly tests the alternative theories of gravity that originate from Einstein's theory. However, this is not restricted to modifications to general relativity. In this sense, we work in the context of Weyl's theory, more specifically, on a particular black hole solution for a charged massive source, which is confronted with the classical test of the geodetic precession, to obtain information about the parameters associated with this theory. To fully assess this spacetime, the complete geodesic structure for massive test particles is presented.

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arXiv2020

Constraints on Scalar-Tensor Theory of Gravity by Solar System Tests

We study the motion of particles in the background of a scalar-tensor theory of gravity in which the scalar field is kinetically coupled to Einstein tensor. We constrain the value of the derivative parameter $z$ through solar system tests. By considering the perihelion precession we obtain the constrain $\sqrt{z}/m_p > 2.6\times 10^{12}$ m, the gravitational red-shift $\frac{\sqrt{z}}{m_{p}}>2.7\times10^{\,10}$ m, the deflection of light $\sqrt{z}/m_p > 1.6 \times 10^{11}$ m, and the gravitational time delay $\sqrt{z}/m_p > 7.9 \times 10^{12}$ m; thereby, our results show that it is possible to constrain the value of the $z$ parameter in agreement with the observational tests that have been considered.

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arXiv2020

Motion and trajectories of photons in a three-dimensional rotating Hořava AdS black hole

We study the motion of photons in the background of a three-dimensional rotating Hořava AdS black hole that corresponds to a Lorentz-violating version of the BTZ black hole and we analyze the effect of the breaking of Lorentz invariance on the null geodesics structure, by solving analytically the equations of motion. Mainly we find that, through a fine tuning of the parameters of the theory, new kinds of orbits are allowed, such as unstable circular orbits and trajectories of first kind. Also, we show that an external observer will see that photons arrive at spatial infinity in a finite coordinate time.

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