arXiv subjects

Behrouz Mirza

Publications and source records attributed to Behrouz Mirza.

At least 19 recordsLinked to original sources

arXiv2026

A Generalized Class of Taub-NUT-Scalar Metrics

We consider a class of three-parameter metrics that was introduced in \cite{azizallahi2024three}. The class of metrics contain Zipoy-Voorhees (ZV) and Fisher-Janis-Newman- Winicour (FJNW) spacetimes at certain values of the parameters. We build an axially symmetric and asymptotically flat spacetime with gravitomagnetic effects by introducing a Newman- Unti-Tamburino (NUT) charge into this class of metrics via the Ehlers transformation. We investigate the physical features of this class of Taub-NUT-scalar (TNS) metrics including gravitational lensing, quasinormal mode (QNM) spectra, and test particle motion. Our results provide fresh insight on how mass, multipole moments, and gravitomagnetic fields behave in the presence of a scalar field and show the effect of the NUT charge on stability of orbits in this class of metrics.

gr-qc↗

arXiv2025

Accelerating FJNW Metric

We derive exact form of accelerating Fisher-Janis-Newman-Winicour (FJNW) metric by a simple perturbative method. We also argue that by using Buchdahl transformations one can obtain the same accelerating FJNW metric. We investigate singularities of the accelerating FJNW metric and study their effects on global and local structures of this spacetime. We also study geodesics and stability of circular orbits.

gr-qc↗

arXiv2025

A class of charged and charged-Taub-NUT metrics in the presence of a massless scalar field and some of their astrophysical aspects

We consider a class of exact solutions to the Einstein equations in the presence of a scalar field, recently introduced in [arXiv:2307.09328, arXiv:2307.13588], and derive their generalized form with dyonic charges using Harrison transformations. For specific parameter values, this class of metrics includes the charged Fisher Janis Newman Winicour (FJNW) and gamma metrics. We then investigate the motion of neutral particles in the background of these metrics and derive the corresponding effective potential. Next, by applying Ehlers transformations, we introduce the NUT parameter into the Reissner Nordstrom metric in the presence of the scalar field. We also examine gravitational lensing, focusing on the effects of dyonic and NUT charges, as well as the scalar field, on the deflection angle of light. Finally, we explore the quasi normal modes associated with this class of metrics.

gr-qc↗

arXiv2025

A class of charged-Taub-NUT-scalar metrics via Harison and Ehlers Transformations

We consider a class of axially symmetric solutions to Einstein's equations incorporating a $\theta$-dependent scalar field and extend these solutions by introducing electric and magnetic charges via Harrison transformations. Subsequently, we enhance the charged metrics by incorporating the NUT parameter through Ehlers transformations, yielding a novel class of charged-Taub-NUT metrics that represent exact solutions to Einstein's equations. Finally, we investigate some of astrophysical aspects of the charged-Taub-NUT metrics, focusing on phenomena such as gravitational lensing and quasi-normal modes (QNMs).

gr-qc↗

arXiv2025

Modelling the Sgr A$^$ and M87$^$ shadows by using the Kerr-Taub-NUT metrics in the presence of a scalar field

The recent unveiling of the images of Sgr A* and M87* has significantly advanced our understanding of gravitational physics. In this study, we derive a class of Kerr-Taub-NUT metrics in the presence of a scalar field (KTNS). Treating these metrics as models for supermassive objects, we constrain the parameters using shadow size estimates done by observations of M87* and Sgr A* from the Event Horizon Telescope (EHT). Comparing the obtained results with M87* data, we show an upper limit on the NUT charge $n$ such that the constraint on the shadow deviation from circularity ($ \Delta C $) will be fulfilled for $ n<0.5 $, and this allowed range changes with a variation in other parameters. Additionally, our findings reveal that fast-rotating KTNS metrics are better candidates for supermassive M87* than slowly rotating ones. We continue our study by estimating parameters using Keck and VLTI observations of Sgr A* and find that the constraint on the fraction deviation $ \delta $ is maintained within a certain range of the NUT charge such that the Keck bound is satisfied for $ n<0.41 $. In contrast, the VLTI bound can be fulfilled for $ n>0.34 $. Finally, we investigate weak gravitational lensing using the Gauss-Bonnet theorem and illustrate that all model parameters increase the deflection angle, causing light rays to deviate more significantly near fast-rotating KTNS objects.

gr-qc↗

arXiv2024

Efficiency of Higher Dimensional Black Holes as Particle Accelerators

The center-of-mass energy of two colliding particles could be arbitrarily high in the vicinity of event horizons of the extremal Myers-Perry black holes if the angular momentum of colliding particles is fine-tuned to the critical values. We investigate the maximum efficiency of two colliding particles in four and six dimensions. The efficiency of collision for two particles near the four-dimensional Kerr black holes is 130 %. We show that the efficiency increases to 145 % for collision in six dimensions. We also show that the region for the polar angle in which the particle can reach the high energy is larger when the dimension of space-time increases.

hep-th↗

arXiv2024

A class of Taub-NUT-scalar metrics via Ehlers transformations

We derive a class of Taub-NUT metrics in the presence of a scalar field (TNS) by using Ernst equations and potential, as well as using Ehlers transformations on the exact solutions that was recently introduced in Azizallahi et al. (Nucl Phys B 998:116414, https://doi.org/10.1016/j.nuclphysb.2023.116414, arXiv:2307.09328 [gr-qc], 2023) and Mirza et al. (Eur Phys J C 83:1161, https://doi.org/10.1140/epjc/s10052-023-12255-7, arXiv:2307.13588 [gr-qc], 2023). Furthermore, we investigate the effective potential, geodesics, topological charge, quasinormal modes (QNMs) and the deflection angle of light in a gravitational lensing for the obtained class of TNS metrics. We also use conformal transformations to generate a new class of exact solutions of the Einstein-conformal-scalar theory by using the obtained TNS solutions as seed metrics. Finally we compare QNMs of the class of exact solutions.

gr-qc↗

arXiv2024

Nonperturbative thermodynamic extrinsic curvature of the anyon gas

Thermodynamic extrinsic curvature is a new mathematical tool in thermodynamic geometry. By using the thermodynamic extrinsic curvature, one may obtain a more complete geometric representation of the critical phenomena and thermodynamics. We introduce nonperturbative thermodynamic extrinsic curvature of an ideal two dimensional gas of anyons. Using extrinsic curvature, we find new fixed points in nonperturbative thermodynamics of the anyon gas that particles behave as semions. Here, we investigate the critical behavior of thermodynamic extrinsic curvature of two-dimensional Kagome Ising model near the critical point $ \beta_{c} =({{k_{B} T_{c}}})^{-1}$ in a constant magnetic field and show that it behaves as $ \left| {\beta- \beta_{c} } \right|^{\alpha} $ with $ \alpha=0 $, where $ \alpha $ denotes the critical exponent of the specific heat. Then, we consider the three dimensional spherical model and show that the scaling behavior is $ \left| {\beta- \beta_{c} } \right|^{\alpha} $ , where $ \alpha =-1 $. Finally, using a general argument, we show that extrinsic curvature $ K $ have two different scaling behaviors for positive and negative $ \alpha $. For $\alpha> 0$, our results indicate that $ K \sim \left|{\beta- \beta_{c} } \right|^{{\frac{1}{2}} (\alpha-2)} $. However, for $ \alpha <0$, we found a different scaling behavior, where $ K\sim \left| {\beta- \beta_{c} } \right|^{\alpha} $.

cond-mat.stat-mech↗

arXiv2023

A class of rotating metrics in the presence of a scalar field

We consider a class of three parameter static and axially symmetric metrics that reduce to the Janis-Newman-Winicour (JNW) and $ \gamma$-metrics in certain limits of the parameters. We obtain rotating form of the metrics that are asymptotically flat, stationary and axisymmetric. In certain values of the parameters, the solutions represent the rotating JNW metric, rotating $ \gamma$-metric and Bogush-Gal'tsov (BG) metric. The singularities of rotating metrics are investigated. Using the light-ring method, we obtain the quasi normal modes (QNMs) related to rotating metrics in the eikonal limit. Finally, we investigate the precession frequency of a test gyroscope in the presence of the rotating metrics.

gr-qc↗

arXiv2023

Three parameter metrics in the presence of a scalar field in four and higher dimensions

We investigate a class of three parameter metrics that contain both the $\gamma$-metric and Janis-Newman-Winicour (JNW) metric at special values of the parameters. To see the effect of the scalar field we derive some properties of this class of metrics such as curvature invariants, the effective potential, and epicyclic frequencies. We also introduce the five and higher dimensional forms of the class of metrics in the presence of a scalar field.

gr-qc↗

arXiv2023

Nonlinear Yang-Mills black holes

This paper is devoted to investigating the nonlinear non-abelian Yang-Mills black holes. We consider three Born-Infeld, exponential, and logarithmic nonlinear Yang-Mills theories with $SO(n-1)$ and $SO(n-2,1)$ semi-simple groups, which n is the dimension of spacetime, and obtain a new class of nonlinear Yang-Mills (NYM) black hole solutions. Depending on the values of dimension $n$, Yang-Mills charge $e$ and the mass $m$ and nonlinear parameters $β$, our solutions can lead to a naked singularity, a black hole with two horizons, an extreme or a Schwarzschild-type black hole. We also investigate the thermodynamic behaviors of the NYM black holes. For small charge values, the NYM solutions may be thermally stable in the canonical ensemble, if we consider an AdS spacetime with spherical $k=+1$ and hyperbolic $k=-1$ coordinates or a flat one with $k=+1$. However, there are no stable regions in the grand canonical ensemble in higher dimensions. For the NYM black hole, we observe a reentrant phase transition between large and small black holes in the BI-branch with small $β$, which cannot be visible for the nonlinear Reissner-Nordstrom AdS black hole in the higher dimension. For the limit $β\rightarrow\infty$, the critical ratio $\frac{P_{c} v_{c}}{T_{c}}$ tends to the constant value $3/8$ for each dimension $n$, while it depends on the dimension for the case of nonlinear electrodynamics black holes.

gr-qc↗

arXiv2022

Thermodynamic geometry of pure Lovelock black holes

In this paper, we study thermodynamic geometry for pure Lovelock black holes. The thermodynamics scalar curvature contains information about the interaction of microstates that might be repulsive or attractive. We obtain critical exponents and critical amplitudes of scalar and extrinsic curvatures for small and large black holes for various dimensions. We demonstrate that this model thermodynamic Ricci scalar scaling behavior has a universal behavior for different dimensions. Moreover, we determine the order of phase transition by using Ehrenfest equations and the Prigogine Defay ratio. We also consider the extended phase space and investigate the critical behavior of the pure Lovelock black holes for various dimensions.

hep-th↗

arXiv2022

Novel way to the metric of higher dimensional rotating black holes

We wish to carry forward to higher dimensions the insightful and novel method of obtaining the Kerr metric proposed by one of us [Gen. Relativ. Gravit. 45, 2383 (2013)] for deriving the Myers-Perry rotating black hole metric. We begin with a flat spacetime metric written in oblate spheroidal coordinates (ellipsoidal geometry) appropriate for the inclusion of rotation, and then introduce arbitrary functions to introduce a gravitational potential due to mass, which are then determined by requiring that a massless particle experiences no acceleration, while a massive particle feels Newtonian acceleration at large r. We further generalize the method to include the cosmological constant Λ to obtain the MyersPerry de Sitter/antide Sitter black hole metric.

gr-qc↗

arXiv2022

Entangled vacuum state for accelerated observers

Entanglement of Dirac fields has been studied and it is known to decrease with increasing acceleration when a quantum state is shared between users in non-inertial frames. A new form of an entangled vacuum state observed by the accelerated observer is postulated in which it is assumed that entanglement is present between the modes of the quantum field with sharp and opposite momenta defined in two causally disconnected regions of space-time. We find that this assumption does not affect the entanglement of the system.

quant-ph↗

arXiv2022

$γ$-Metrics in Higher Dimensions

We introduce five and higher dimensional $γ$-metrics. The higher dimensional metrics are exact solutions of the vacuum field equations and represent new types of singularities. For dimensions $d>5$ we have obtained $γ$-metrics in flat coordinates. We obtain singularities of metrics and for a better understanding of geometrical and physical properties of the five dimensional metric, stable circular orbits are determined by means of the effective potential. Effect of the deformed parameter ($γ$) on redshift of the $γ$-metric are calculated. Interior solution for the five-dimensional $γ$-metric is also obtained.

gr-qc↗

arXiv2021

Lifshitz and hyperscaling violated Yang-Mills-dilaton black holes

We obtain two types of Yang-Mills-dilaton black hole solutions in both Lifshitz and hyperscaling violation spacetimes. We must consider at least three Yang-Mills gauge fields that interact with a scalar field and either $SO(n)$ or $SO(n-1,1)$ gauge symmetry groups, where $n+1$ denotes the dimension of the spacetime. They lead to the spherical and hyperbolic solutions. The obtained solutions in the hyperscaling violation spacetime fall into two categories for $z\neq n-3-\frac{n-5}{n-1}θ$ and $z= n-3-\frac{n-5}{n-1}θ$, where $θ=0$ represents the Lifshitz spacetime. In order to have a real asymptotic behavior for the hyperscaling violated black hole solutions, we should consider a negative value for the hyperscaling violation parameter as $θ<0$. We also evaluate the thermodynamic quantities of the mentioned black holes and probe their thermal stability in the grand canonical ensemble. For $z\geq2$, the hyperscaling violated solutions are not thermally stable for $z\leq n-3-\frac{n-5}{n-1}θ$, while they are stable for large $r_{+}$ with $z>n-3-\frac{n-5}{n-1}θ$. We also check out the critical behavior of the obtained black holes and obtain a Smarr relation for the solutions. The results also announce of a first-order small-large phase transition for both black holes in the case $T>T_{C}$.

hep-th↗

arXiv2021

Yang-Mills black holes in Quasitopological gravity

In this paper, we formulate two new classes of black hole solutions in higher curvature quartic quasitopological gravity with nonabelian Yang-Mills theory. At first step, we consider the $SO(n)$ and $SO(n-1,1)$ semisimple gauge groups. We obtain the analytic quartic quasitopological Yang-Mills black hole solutions. Real solutions are only accessible for the positive value of the redefined quartic quasitopological gravity coefficient, $μ_{4}$. These solutions have a finite value and an essential singularity at the origin, $r=0$ for space dimension higher than $8$. We also probe the thermodynamic and critical behavior of the quasitopological Yang-Mills black hole. The obtained solutions may be thermally stable only in the canonical ensemble. They may also show a first order phase transition from a small to a large black hole. In the second step, we obtain the pure quasitopological Yang-Mills black hole solutions. For the positive cosmological constant and the space dimensions greater than eight, the pure quasitopological Yang-Mills solutions have the ability to produce both the asymptotically AdS and dS black holes for respectively the negative and positive constant curvatures, $k=-1$ and $k=+1$. This is unlike the quasitopological Yang-Mills theory which can lead to just the asymptotically dS solutions for $Λ>0$. The pure quasitopological Yang-Mills black hole is not thermally stable.

hep-th↗

arXiv2021

Joule-Thomson expansion of the quasitopological black holes

In this paper, we investigate the thermal stability and Joule-Thomson expansion of some new qusitopological black hole solutions. We first study the higher-dimensional static quasitopological black hole solutions in the presence of Born-Infeld, exponential and logarithmic nonlinear electrodynamics. The stable regions of these solutions are independent of the types of the nonlinear electrodynamics. The solutions with the horizons relating to the positive constant curvature, $k=+1$, have a larger region in thermal stability, if we choose positive quasitopological coefficients, $μ_{i}>0$. We also have a review on the power Maxwell quasitopological black hole. Then, we obtain the five-dimensional Yang-Mills quasitopological black hole solution and compare with the quasitopological Maxwell solution. For large values of the electric charge, $q$, and the Yang-Mills charge, $e$, we showed that the stable range of the Maxwell quasitopological black hole is larger than the Yang-Mills one. This is while thermal stability for small charges has the same behavior for these black holes. In the following, we obtain the thermodynamic quantities for these solutions and then study the Joule-Thomson expansion. We consider the temperature changes in an isenthalpy process during this expansion. The obtained results show that the inversion curves can divide the isenthalpic ones into two parts in the inversion pressure, $P_{i}$. For $P P_{i}$. As the values of the nonlinear parameter, $β$, the electric and Yang-Mills charges decrease, the temperature goes to zero with a small slope and so the heating phenomena happens slowly.

gr-qc↗