arXiv subjects

Malihe Heydari-Fard

Publications and source records attributed to Malihe Heydari-Fard.

At least 19 recordsLinked to original sources

arXiv2026

Kerr-like black holes shadow surrounded by dark matter halos: Comparison between various dark matter profiles

The study of shadows of static and rotating black holes immersed in various dark matter halo profiles has gained significant attention in recent years. In this paper, we consider the static black hole solution immersed in three dark matter halo profiles (King, Hernquist, and Moore) and, by using the Newman-Janis algorithm, obtain the corresponding rotating black hole solutions. The main goal of the paper is to study the influence of the characteristic density, characteristic radius, and the spin parameter on the inner and outer horizons, the ergoregion, and the shadow radius, and to compare the results with Kerr black holes and other dark matter profiles. Our findings indicate that the shadow radius of rotating black holes immersed in dark matter increases compared to that of the Kerr black hole in the absence of dark matter; however, the impact on the size and shape of the shadow is negligible for King, Hernquist, and Moore profiles, and for all other dark matter halo profiles considered, and is independent of whether the halo is cuspy or cored. Therefore, at least for these Kerr-like black holes, the black hole shadow is not a suitable tool for distinguishing the nature of the dark matter distribution in galactic centers.

gr-qc↗

arXiv2026

Rotating black holes in the Hernquist galactic halo and its accretion disk luminosity

Static, spherically symmetric black holes immersed in a dark matter halo with a Hernquist-type density profile have been derived by Cardoso et. al. in Ref. \redcite{Cardoso:2021wlq}. Using the Newman-Janis algorithm, we construct the metric for a stationary and axially symmetric rotating black hole in this environment. Then, we obtain the electromagnetic properties of thin accretion disks around such rotating black holes by utilizing the steady-state Novikov-Thorne model, and study the effects of spin parameter and halo compactness parameter on the disk properties. Finally, by comparison the results of the rotating Cardoso black hole with that of Kerr black hole in the absence of dark matter, we find that the presence of dark matter can not significantly affect the disk properties and thus for astrophysical black holes with large spin parameter, the distinction of rotating Cardoso black holes becomes more difficult than the Kerr black hole.

gr-qc↗

arXiv2025

Influence of plasma on the observational appearance of rotating black holes in Horndeski gravity

Exploring the influence of plasma on the light rays trajectories in the vicinity of black holes is significat since that astrophysical black holes are generally surrounded by a plasma medium. In this work, we analyze the null geodesics in the space-time of rotating hairy Horndeski black holes immersed in a plasma medium using the Hamilton-Jacobi method. By considering both uniform and non-uniform plasma distributions, we perform a comparative study of their effects on the black hole shadow. The impact of the hair parameter and the inclination angle on the black hole shadow is also investigated. Finally, the shadow results are compared with observational data of the Event Horizon Telescope for M87* to constrain the plasma parameter.

gr-qc↗

arXiv2024

Galactic black hole immersed in a dark halo with its surrounding thin accretion disk

By considering the analytic, static and spherically symmetric solution for the Schwarzschild black holes immersed in dark matter fluid with non-zero tangential pressure \cite{Jusufi:2022jxu} and Hernquist-type density profiles \cite{Cardoso}, we compute the luminosity of accretion disk. We study the circular motion of test particles in accretion disk and calculate the radius of the innermost stable circular orbits. Using the steady-state Novikov-Thorne model we also compute the observational characteristics of such black hole's accretion disk and compare our results with the usual Schwarzschild black hole in the absence of dark matter fluid. We find that the tangential pressure plays a significant role in decreasing the size of the innermost stable circular orbits and thus increases the luminosity of black hole's accretion disk.

gr-qc↗

arXiv2024

On optical appearance of Einstein-Maxwell-Æther black holes surrounded by various accretions

In this paper, we investigate the effects of the æther field and the electric charge on the observed shadow of two types of charged black holes in the Einstein-Maxwell-Æther theory. By considering that the Einstein-Maxwell-Æther black holes surrounded by the static/infalling spherical accretion flows, as well as an optically and geometrically thin disk accretion flow, we study the shadow luminosities and the observed specific intensity of the image for these various profiles of accretion flows. We find that in the thin disk accretion model the location and the emitted model of the accretion gas affect on the optical appearance of charged Einstein-Æther black holes in contrast to the spherical accretion flows. For a thin disk profile, we show that the observer will receive more intensity for an emitted model as Gaussian function when the innermost radiation radius lies in the innermost stable circular orbit. Finally, comparing the results of the charged Einstein-Æther black holes with neutral Einstein-Æther black holes, we show that the charged Einstein-Æther black holes have smaller dark area, whereas wider lensed ring and photon ring. Comparing the charged Einstein-Æther black holes with Reissner-Nordstrom black hole gives a reverse effect.

gr-qc↗

arXiv2024

Thin accretion disk images of rotating hairy Horndeski black holes

By considering the steady-state Novikov-Thorne model, we study thin accretion disk processes for rotating hairy black holes in the framework of the Horndeski gravity. We obtain the electromagnetic properties of accretion disk around such black holes and investigate the effects of the hair parameter $h$ on them. We find that by decreasing the hair parameter from the Kerr limit, $h\rightarrow0$, the radius of the innermost stable circular orbit decreases which makes thin accretion disks around rotating hairy black holes in Horndeski gravity more efficient than that for the Kerr black hole in general relativity. Furthermore, using the numerical ray-tracing method, we plot thin accretion disk images around these black holes and investigate the effects of hair parameter on the central shadow area of accretion disk.

gr-qc↗

arXiv2023

Time-dependent wormhole solutions in conformal Weyl gravity

We present the exact time-dependent solutions on inhomogeneous spherically symmetric space-time in the conformal invariant Weyl gravity. For this purpose, the subclass of the Lemaitre-Tolman metric which is supported by an anisotropic fluid is used. For the first time, the exact solutions of the dynamical equations are obtained for two special cases. One of the exact solutions is a de Sitter space-time and other solution is a class of time-dependent wormhole geometries which can be supported by exotic matter in similar to the general relativistic solutions.

gr-qc↗

arXiv2023

Shadows and photon rings of a spherically accreting Kehagias-Sfetsos black hole

By considering Kehagias-Sfetsos black hole in the framework of the Hořava-Lifshitz gravity, we study the optical appearance of such black holes surrounded by spherical accretion flow. For the static/infalling spherical accretion flow, we compute the observed specific intensity as a function of impact parameter. We also investigate the effect of the Hořava parameter and accreting matter on the luminosity of shadows and photon rings. It is found that an increase in the Hořava parameter decreases the shadow size, while the shadows and photon rings luminosities increase. Moreover, we constrain the Hořava parameter from the observational data reported by the Event Horizon Telescope for M87* and Sgr A*.

gr-qc↗

arXiv2023

Effect of quintessence dark energy on the shadow of Hayward black holes with spherical accretion

It is expected that the astrophysical black holes are surrounded by a luminous accretion flow that is a necessary ingredient for imaging a black hole. In this paper, we study the influence of quintessence dark energy on the shadow images of a Hayward black hole surrounded by the static/infalling spherical accretion flow. We find the effect of the state parameter of quintessence matter on the horizons, photon sphere and impact parameter of the quintessence Hayward black hole. The observed specific intensity of the shadow and also the shadow and photon ring luminosities of the quintessence Hayward black hole in two different spherically accretion flow are investigated. We also use the Event Horizon Telescope observational data of Sgr A* and M87* to constrain the free parameters of quintessence Hayward black hole. Finally, by comparison the results of quintessence Hayward black holes with quintessence Schwarzschild and Hayward black holes we find that the effect of quintessence matter on the black hole shadow is more significant than the regularity effect.

gr-qc↗

arXiv2023

Thin accretion disk luminosity and its image around rotating black holes in perfect fluid dark matter

Motivated by the fact that the universe is dominated by dark matter and dark energy, we consider rotating black holes surrounded by perfect fluid dark matter and study the accretion process in thin disk around such black holes. Here, we are interested in how the presence of dark matter affects the properties of the electromagnetic radiation emitted from a thin accretion disk. For this purpose, we use the Novikov-Thorne model and obtain the electromagnetic spectrum of an accretion disk around a rotating black hole in perfect fluid dark matter and compare with the general relativistic case. The results indicate that for small values of dark matter parameter we considered here, the size of the innermost stable circular orbits would decrease and thus the electromagnetic spectrum of the accretion disk increases. Therefore, disks in the presence of perfect fluid dark matter are hotter and more luminous than in general relativity. Finally, we construct thin accretion disk images around these black holes using the numerical ray-tracing technique. We show that the inclination angle has a remarkable effect on the images, while the effect of dark matter parameter is small.

gr-qc↗

arXiv2022

On null geodesics and shadow of hairy black holes in Einstein-Maxwell-dilaton gravity

The time-like and null-like geodesics around compact objects are some of the best tools to classify and understand the structure of a space-time. In this paper, we study the null geodesics around charged static dilaton black holes in Einstein-Maxwell-dilaton gravity. The physical parameters for non-radial geodesics including the effective potential, effective force, radius of the photon sphere and impact parameter are obtained and effects of the charge parameter and dilaton coupling constant on these quantities are studied. Possible photon motions for different values of the impact parameter are analyzed and unstable circular orbits and unbounded orbits are plotted. These results are compared to that of the Schwarzschild, Reissner-Nordstrom and Gibbons-Maeda-Garfinkle-Horowitz-Strominger (GMGHS) black holes. Also, we study the shadow cast by a dilaton black hole and investigate how the dilaton coupling affects the size of the black hole shadow. Finally, as an application of null geodesics, we calculate the deflection of light and investigate the effects of the model parameters on the bending angle.

gr-qc↗

arXiv2021

Null geodesics and shadow of 4$D$ Einstein-Gauss-Bonnet black holes surrounded by quintessence

We study the properties of the null geodesics of four-dimensional Einstein-Gauss-Bonnet black holes surrounded by quintessence matter. Due to the quintessence correction, we discuss the radial and non-radial geodesics. For the case of non-radial geodesics we obtain the effective potential, the photon sphere and the impact parameter associated to the null geodesics in details. We analyze the different possible orbits of massless particles (i.e. photons) such as unstable circular orbits and unbounded orbits and investigate the role of the Gauss-Bonnet coupling $α$ and the quintessence parameter $q$ on the null geodesic trajectories. Moreover, we also study the effects of the model parameters on the shadow cast by theses black holes. The results are compared with the Schwarzschild black hole with and without quintessence matter and four-dimensional Einstein-Gauss-Bonnet black holes without quintessence. As a physical application of the null geodesics, the deflection angle is calculated and the effect of the quintessence matter on it is investigated.

gr-qc↗

arXiv2021

Thin accretion disks around rotating black holes in $4D$ Einstein-Gauss-Bonnet gravity

Recently, Kumar and Ghosh have derived Kerr-like rotating black hole solutions in the framework of four-dimensional Einstein-Gauss-Bonnet theory of gravity and investigated the black hole shadow. Using the steady-state Novikov-Thorne model, we study thin accretion disk processes for such rotating black holes including the energy flux, temperature distribution, emission spectrum, energy conversion efficiency as well as the radius of the innermost stable circular orbit. We also study the effects of the Gauss-Bonnet coupling parameter $α$ on these quantities. The results are compared to slowly rotating relativistic Kerr black holes which show that for a positive Gauss-Bonnet coupling, thin accretion disks around rotating black holes in four-dimensional Einstein-Gauss-Bonnet gravity are hotter and more efficient than that for Kerr black holes with the same rotation parameter $a$, while for a negative coupling they are cooler and less efficient. Thus the accretion disk processes may be considered as tools for testing Einstein-Gauss-Bonnet gravity using astrophysical observations.

gr-qc↗

arXiv2021

Bending of light in novel 4$D$ Gauss-Bonnet-de Sitter black holes by Rindler-Ishak method

We study the bending of light in the space-time of black holes in four-dimensional Einstein-Gauss-Bonnet theory of gravity, recently proposed by Glavan and Lin \cite{Glavan}. Using Rindler-Ishak method, the effect of Gauss-Bonnet coupling on the bending angle is studied. We show that a positive Gauss-Bonnet coupling gives a negative contribution to the Schwarzschild-de Sitter deflection angle, as one would expect.

gr-qc↗

arXiv2020

Thin accretion disks and charged rotating dilaton black holes

Einstein-Maxwell-dilaton theory is an interesting theory of gravity for studying scalar fields in the context of no-hair theorem. In this work, we consider static charged dilaton and charged, slowly rotating dilaton black holes in Einstein-Maxwell-dilaton gravity. We investigate the accretion process in thin disks around such black holes, using the Novikov-Thorne model. The electromagnetic flux, temperature distribution, energy conversion efficiency and also innermost stable circular orbits of thin disks are obtained and effects of dilaton and rotation parameters are studied. For the static and slowly rotating black holes the results are compared to that of Schwarzschild and Kerr, respectively.

gr-qc↗

arXiv2019

Higher-order geodesic deviations and orbital precession in a Kerr-Newman space-time

A novel approximation method in studying the perihelion precession and planetary orbits in general relativity is to use geodesic deviation equations of first and high-orders, proposed by Kerner et.al. Using higher-order geodesic deviation approach, we generalize the calculation of orbital precession and the elliptical trajectory of neutral test particles to Kerr$-$Newman space-times. One of the advantage of this method is that, for small eccentricities, one obtains trajectories of planets without using Newtonian and post-Newtonian approximations for arbitrary values of quantity ${G M}/{R c^2}$.

gr-qc↗

arXiv2012

Effect of bulk Lorentz violation on anisotropic brane cosmologies

The effect of Lorentz invariance violation in cosmology has attracted a considerable amount of attention. By using a dynamical vector field assumed to point in the bulk direction, with Lorentz invariance holding on the brane, we extend the notation of Lorentz violation in four dimensions to a five-dimensional brane-world. We obtain the general solution of the field equations in an exact parametric form for Bianchi type I space-time, with perfect fluid as a matter source. We show that the brane universe evolves from an isotropic/anisotropic state to an isotropic de Sitter inflationary phase at late time. The early time behavior of anisotropic brane universe is largely dependent on the Lorentz violating parameters $β_i, i = 1, 2, 3$ and the equation of state of the matter, while its late time behavior is independent of these parameters.

physics.gen-ph↗

arXiv2012

Virial mass in warped DGP-inspired L(R) gravity

A version of the virial theorem is derived in a brane-world scenario in the framework of a warped DGP model where the action on the brane is an arbitrary function of the Ricci scalar, L(R). The extra terms in the modified Einstein equations generate an equivalent mass term (geometrical mass), which give an effective contribution to the gravitational energy and offer viable explanation to account for the virial mass discrepancy in clusters of galaxies. We also obtain the radial velocity dispersion of galaxy clusters and show that it is compatible with the radial velocity dispersion profile of such clusters. Finally, we compare the result of the model with L(R) gravity theories.

physics.gen-ph↗