arXiv subjects

Reza Saffari

Publications and source records attributed to Reza Saffari.

At least 19 recordsLinked to original sources

arXiv2021

Thermodynamic geometry of static and rotating regular black holes in conformal massive gravity

A version of massive gravity, namely conformal massive gravity, is employed to study the behavior of thermodynamic geometry for both the static and the rotating regular black holes. Whereas in thermodynamic geometry singularity can be interpreted as the second phase transition, seeing such phenomena as heat capacity behavior for both cases is investigated in detail. In addition, other thermodynamic quantities like the entropy and the temperature are analyzed as well. Another important property of the black holes is the so-called stability, in which utilizing the first phase transition of the heat capacity is detailed, for both cases, say the static and the spinning black holes. It is also interestingly figured out that, the behavior of temperature of the uncharged black holes in the presence of hair parameter, $λ$, can mimic the charged ones. {The effects of scalar charge, $Q$, and hair parameters with both positive and negative signs and how it affects key parameters in the study of black holes are investigated as well.} To investigate the properties of the black holes both regular thermodynamics and thermodynamic geometry approaches are considered. Then one can observe that aiming to realize the physics of the black holes, many complicated phenomena can be analyzed dramatically easier by considering the latter approach.

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arXiv2021

Spacetime expansion in the presence of a background velocity field

In this article, we introduce a new metric assuming an additional velocity-based term in a spacetime metric. Although the inclusion of this additional phrase can indicate that the Lorentz symmetry has broken, the results of null geodesics demonstrate that the amount of variation in the speed of light is considerably smaller than what can be observed. This article delves into the primary use of this additional phrase in the expansion development of Minkowski and de Sitter spacetimes. The findings indicate that multiple versions of the velocity function may explain the universe's initial explosion and expansion behavior even before the inflation epoch, the inflation itself, and the universe's positive acceleration in late time cosmology without dark matter or dark energy. These models can also rebuild primordial black holes in the early universe as an alternative to dark matter.

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arXiv2021

Perturbed thermodynamics and thermodynamic geometry of a static black hole in $f (R)$ gravity

In this paper, we consider a static black hole in $f(R)$ gravity. We recapitulate the expression for corrected thermodynamic entropy of this black hole due to small fluctuations around equilibrium. Also, we study the geometrothermodynamics (GTD) of this black hole and investigate the adaptability of the curvature scalar of geothermodynamic methods with phase transition points of this black hole. Moreover, we study the effect of correction parameter on thermodynamic behaviour of this black hole. We observe that the singular point of the curvature scalar of Ruppeiner metric coincides completely with zero point of the heat capacity and the deviation occurs with increasing correction parameter.

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arXiv2021

Geodesic Motion in the Spacetime Of a SU(2)-Colored (A)dS Black Hole in Conformal Gravity

In this paper, we study the geodesic motion in the spacetime of a SU(2)-colored (A)dS black hole in conformal gravity, and also we investigate spacetime features, such as light spheres and horizons. Moreover, we derive the analytical solutions for the equation of motion of test particles and light rays using Weierstrass elliptic and Kleinian sigma functions. Depending on the particle energy levels and angular momentums, we classify the solutions of the geodesic equations. Furthermore, several examples of possible types of orbits illustrate the results

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arXiv2020

Particle Dynamics Around the Black String

In this paper, some dynamical properties of neutral and charged particles around a weakly magnetized five-dimensional static black string have been studied. The perturbation method was also used to calculate the Innermost Stable Circular Orbit (ISCO) of this metric in the presence of a magnetic field. The escape velocity of neutral and charged particles around the black string was derived. In the next step, the analytical solutions of the equations of motion were discussed and some possible orbits for particles in the black string space-time were plotted. Interestingly, it was found that adding an extra dimension has a slight influence on the effective potential and one term of the effective force. The magnitude of the new constant of motion $(J)$ affects both the shape of the potential and existence of the stable circular orbits. In conclusion, by comparing a black hole and a black string, it is realized that the value of a new constant of motion causes slight but interesting differences.

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arXiv2019

Thermodynamic geometry of a black hole surrounded by perfect fluid in Rastall theory

In this paper, we study thermodynamics and thermodynamic geometry of a black hole surrounded by the perfect fluid in Rastall theory. In particular, we calculate the physical quantity like mass, temperature and heat capacity of the system for two different cases. From the resulting heat capacity, we emphasize stability of the system. Following Weinhold, Ruppiner, Quevedo and HPEM formalism, thermodynamic geometry of this black hole in Rastall gravity is also analyzed. We find that the singular points of the curvature scalar of Ruppeiner and HPEM metrics entirely coincides with zero points of the heat capacity. But there is another divergence of HPEM metric which coincides with the singular points of heat capacity, so we can extract more information of HPEM metric compared with Ruppeiner metric. However, we are unable to find any physical data about the system from the Weinhold and Quevedo formalism.

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arXiv2017

Analytical solutions of the geodesic equation in the (rotating) black string-(anti-) de sitter spacetime

In this paper we add a compact dimension to Schwarzschild-(anti-) de sitter and Kerr-(anti-) de sitter spacetimes, which describes (rotating) black string-(anti-) de sitter spacetime. We study the geodesic motion of test particles and light rays in this spacetime. We present the analytical solutions of the geodesic equations in terms of Weierstrass elliptic and Kleinian sigma hyperelliptical functions. We also discuss the possible orbits and classify them according to particle's energy and angular momentum. Moreover, the obtained results, are compared to Schwarzschild-(anti-) de sitter and Kerr-(anti-) de sitter spacetimes.

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arXiv2017

Shadow of a Charged Rotating Black Hole in $f(R)$ Gravity

We study the shadow of a charged rotating black hole in $f(R)$ gravity. This black hole is characterized by mass, $M$, spin, $a$, electric charge, $Q$ and $R_{0}$ which is proportional to cosmological constant. We analyze the image of the black hole's shadow in four types 1) at $r\rightarrow\infty$, 2) at $r\rightarrow r_{o}$, in vacuum, 3) at $r\rightarrow\infty$, 4) at $r\rightarrow r_{o}$, for an observer at the presence of plasma. Moreover, we investigate the effect of spin, charge and modfication of gravity on the shape of shadow. In addition, we use two observables, the radius $R_{s}$ and the distortion parameter $δ_{s}$, characterizing the apparent shape. We show that for all cases, the shadow becomes smaller with increasing electric charge. Also, by increasing the rotation parameters, circular symmetry of the image of black hole's shadow will change. Furthermore, in the presence of plasma, plasma parameter also effects on size of the shadow.

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arXiv2016

Shadow of a Kerr-Sen dilaton-axion Black Hole

We analyze the shadow of charged stationary axially symmetric space-time (Kerr-Sen dilaton-axion black hole). This black hole is defined by a mass $M$, a spin $a$ and $r_α=Q^{2}/M$, where $Q$ is the electric charge. Shadows are investigated in two conditions, i) for an observer at infinity in vacuum and ii) for an observer at infinity in the presence of plasma with radial power-low density. In vacuum, the shadow of this black hole depends on charge and spin parameter. We can see that, increasing electric charge, $Q$ decreases the size of shadow. Also, increasing spin parameter $a$ decreases the size of shadow. However, in existence of plasma, parameter of plasma like refraction index, $n$, playing an important role on shadows. In fact, decreasing refraction index decreases the size of shadow.

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arXiv2016

Analytic treatment of complete geodesics in a static cylindrically symmetric conformal spacetime

We consider the motion of test particles and light rays in a static cylindrically symmetric conformal spacetime given by Said et al [1]. We derive the equations of motion and present their analytical solutions in terms of the Weierstrass {\wp} function and the Kleinian σ function. Using parametric diagrams and effective potentials we analyze the possible orbits and characterize them in terms of the energy and the angular momentum of the test particles. Finally we show some examples of orbits.

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arXiv2016

Geodesic motion in the spacetime of a static charged black hole in $f(R)$ gravity

In the present paper we study the geodesic motion of test particles and light rays in the spacetime of a static charged black hole in $f(R)$ gravity. The complete set of analytic solutions of the geodesic equations in the spacetime of this black hole are presented. The geodesic equations are solved in terms of Weierstrass elliptic $\wp$ function and derivatives of Kleinian $σ$ function. With the help of parametric diagrams and effective potentials we analyze the geodesic motion and give a list of all possible orbit types. The different types of the resulting orbits are characterized in terms of the conserved energy, angular momentum, charge and cosmological constant.

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arXiv2016

Geodesic equations in the static and rotating dilaton black holes: Analytical solutions and applications

In this paper, we consider the timelike and null geodesics around the static [GMGHS (Gibbons, Maeda, Garfinkle, Horowitz and Strominger), magnetically charged GMGHS, electrically charged GMGHS] and the rotating (Kerr-Sen dilaton-axion) dilaton black holes. The geodesic equations are solved in terms of Weierstrass elliptic functions. To classify the trajectories around the black holes, we use the analytical solution and effective potential techniques and then characterize the different types of the resulting orbits in terms of the conserved energy and angular momentum. Also, using the obtained results we study astrophysical applications.

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arXiv2016

Detailed study of geodesics in the Kerr-Newman-(A)dS spactime and the rotating charged black hole spacetime in $f(R)$ gravity

We perform a detailed study of the geodesic equations in the spacetime of the static and rotating charged black hole corresponding to the Kerr-Newman-(A)dS spacetime. We derive the equations of motion for test particles and light rays and present their solutions in terms of the Weierstrass $\wp$, $ζ$ and $σ$ functions as well as the Kleinian $σ$ function. With the help of parametric diagrams and effective potentials we analyze the geodesic motion and classify the possible orbit types. This spacetime is also a solution of $f(R)$ gravity with a constant curvature scalar.

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arXiv2016

Study of the geodesic equations of a spherical symmetric spacetime in conformal gravity

Set of analytic solutions of the geodesic equation in a spherical conformal spacetime is presented. Solutions of this geodesics can be expressed in terms of the Weierstrass {\wp} function and the Kleinian σ function. Using conserved energy and angular momentum we can characterize the different orbits. Also, considering parametric diagrams and effective potentials, we plot some possible orbits. Moreover, with the help of analytical solutions, we investigate the light deflection for such an escape orbit.

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arXiv2016

Thermodynamic geometry of black holes in f(R) gravity

In this paper, we consider three types (static, static charged and rotating charged) of black holes in f(R) gravity. We study the thermodynamical behavior, stability conditions and phase transition of these black holes. It will be shown that, the number and type of phase transition points are related to different parameters, which shows the dependency of stability conditions to these parameters. Also, we extended our study to different thermodynamic geometry methods (Ruppeiner, Weinhold and GTD). Next, we investigate the compatibility of curvature scalar of geothermodynamic methods with phase transition points of the above balck holes. In addition, we point out the effect of different values of spacetime parameters on stability conditions of mentioned black holes.

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arXiv2016

A study of geodesic motion in a (2+1)-dimensional charged BTZ black hole

This study is purposed to derive the equation of motion for geodesics in vicinity of spacetime of a (2 + 1)-dimensional charged BTZ black hole. In this paper, we solve geodesics for both massive and massless particles in terms of Weierstrass elliptic and Kleinian sigma hyper-elliptic functions. Then we determine different trajectories of motion for particles in terms of conserved energy and angular momentum and also using effective potential.

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arXiv2015

Thermodynamics of rotating black holes in conformal gravity

In this paper we consider a metric of a rotating black hole in conformal gravity. We calculate the thermodynamical quantities for this rotating black hole including Hawking temperature and entropy in four dimensional space-time, as we obtain the effective value of Komar angular momentum. The result is valid on the event horizon of the black hole, and at any radial distance out of it. Also we verify that the first law of thermodynamics will be held for this type of black hole.

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arXiv2015

Analytical solutions of the geodesic equation in the spacetime of a black hole in f(R) gravity

We consider the motion of test particles in the spacetime of a black hole in f(R) gravity. The complete set of analytic solutions of the geodesic equation in the spacetime of this black hole are presented. The geodesic equations are solved in terms of Weierstrass elliptic functions and derivatives of Kleinian sigma functions. The different types of the resulting orbits are characterized in terms of the conserved energy and angular momentum as well as the cosmological constant $Λ$ and the real constant $β$.

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