arXiv subjects

S. N. Sajadi

Publications and source records attributed to S. N. Sajadi.

At least 19 recordsLinked to original sources

arXiv2025

Boundary Conditions of Warped AdS$_3$ in New Massive Gravity

To satisfy the Cardy formula for Warped AdS$_3$ (WAdS$_{3}$) solutions in a quadratic ensemble, a specific set of boundary conditions has been proposed in \cite{Aggarwal:2020igb}. In this paper, these boundary conditions have been investigated in the three-dimensional new massive gravity (NMG) framework. The associated solution space, asymptotic symmetries, and charge algebra have been extracted. It has been shown that the surface charges are finite, but not integrable, and the integrability of the charges is obtained after restricting to a sub-sector of the original solution space. Finally, we have proved that the Cardy formula reproduces the thermodynamic entropy of a warped BTZ black hole.

hep-th↗

arXiv2024

Black hole solutions to Einstein-Bel-Robinson gravity

In this paper, we study the physical properties of black holes in the framework of the recently proposed Einstien-Bel-Robinson gravity. We show that interestingly the theory propagates a transverse and massive graviton on a maximally symmetric background with positive energy. There is also a single ghost-free branch that returns to the Einstein case when β\to 0. We find new black hole solutions to the equations, both approximate and exact, the latter being a constant curvature black hole solution, and discuss inconsistencies with metrics that were previously claimed to be approximate solutions to the equations. We obtain the conserved charges of the theory and briefly study the thermodynamics of the black hole solutions.

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arXiv2023

Gravitational repulsive effects in 3D regular black holes

In this work, we consider the effects of repulsive gravity in an invariant way for four static 3D regular black holes, using the eigenvalues of the Riemann curvature tensor, the Ricci scalar, and the strong energy conditions. The eigenvalues of the solutions are non-vanishing asymptotically (in asymptotically AdS) and increase as the source of gravity is approached, providing a radius at which the passage from attractive to repulsive gravity might occur. We compute the onsets and the regions of repulsive gravity and conclude that the regular behavior of the solutions at the origin of coordinates can be interpreted as due to the presence of repulsive gravity, which also turns out to be related with the violation of the strong energy condition. We showed that in all of the solutions for the allowed region of parameters, gravity changes its sign, but the repulsive regions only for the non-logarithmic solution are affected by the mass that generates the regular black hole. The repulsive regions for the logarithmic solutions are dependent on electric charge and the AdS$_{3}$ length. The implications and physical consequences of these results are discussed in detail.

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arXiv2023

Strongly-Coupled Anisotropic Gauge Theories and Holography in 5D Einstien-Gauss-Bonnet Gravity

In this paper, we study uncharged, non-conformal, and anisotropic systems with strong interactions using the gauge-gravity duality by considering Einstein-Quadratic-Axion-Dilaton action in five dimensions. In fact, we would like to gain insight into the influence of higher derivative gravity on the QCD system. At finite temperature, we obtain an anisotropic black brane solution to a 5D Einstein-Gauss-Bonnet-Axion-Dilaton system. The system has been investigated and the effect of the parameter of theory has been considered. The blackening function supports the thermodynamical phase transition between small/large and AdS/large black brane for suitable parameters. We also study transport and diffusion properties and observe in particular that the butterfly velocity that characterizes both diffusion and growth of chaos transverse to the anisotropic direction saturates a constant value in the IR which can exceed the bound given by the conformal value. We also determine the imaginary part of the heavy quark potential in a strongly coupled plasma dual to Gauss-Bonnet gravity.

hep-th↗

arXiv2023

Astrophysical and electromagnetic emissivity properties of black holes surrounded by a quintessence type exotic fluid in the Scalar-Vector-Tensor Modified Gravity

The astrophysical consequences of the presence of a quintessence scalar field on the evolution of the horizon and on the accretion disk surrounding a static black hole, in the Scalar-Vector-Tensor version of Modified Gravity (MOG), are investigated. The positions of the stable circular orbits of the massive test particles, moving around the central object, are obtained from the extremum of the effective potential. Detailed calculations are also presented to investigate the light deflection, shadow and Shapiro effect for such a black hole. The electromagnetic properties of the accretion disks that form around such black holes are considered in detail. The energy flux and efficiency parameter are estimated analytically and numerically. A comparison with the disk properties in Schwarzschild geometry is also performed. The quantum properties of the black hole are also considered, and the Hawking temperature and the mass loss rate due to the Hawking radiation are considered. The obtained results may lead to the possibility of direct astrophysical tests of black hole type objects in modified gravity theories.

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arXiv2023

Holographic Anisotropic Background in 5D Einstien-Gauss-Bonnet Gravity

In this paper, we extend the work on the AdS/QCD model to quadratic gravity to gain insight into the influence of gravity. We obtain an anisotropic black brane solution to a $5D$ Einstein-Gauss-Bonnet-two Maxwell-dilaton system. The background is specified by an arbitrary exponent, a dilaton field, a time component of the first Maxwell field, and a magnetic component of the second Maxwell field. The system in three cases has been investigated and in each case the effect of the parameter of theory, the anisotropic parameter has been considered. The blackening function supports the thermodynamical phase transition between small/large and AdS/large black brane for a suitable chemical potential and other parameters.

hep-th↗

arXiv2022

Hairy Black holes in General Minimal Massive Gravity

In this work, we investigate the near horizon and asymptotic symmetries of static and rotating hairy$-$AdS black hole in the framework of general minimal massive gravity. We obtain energy, angular momentum and entropy of the solutions. Then we show that our results for these quantities are consistent with the first law of black hole thermodynamics. By considering the near horizon geometry of black hole, we find near horizon conserved charges and their algebra. By writing the algebra of conserved charges in terms of Fourier modes we have obtained the conserved charges in terms of zero modes.

hep-th↗

arXiv2022

Analytically Approximation Solution to Einstein-Cubic Gravity

In this work, we introduce an analytical approximate black hole solution in Einstein-Cubic gravity. To obtain complete solutions, we construct the near horizon and asymptotic solutions as the first step. Then, the approximate analytic solutions are obtained through continued fraction expansion. We also compute the thermodynamic quantities and use the first law and Smarr formula to obtain the analytic solutions for the near horizon quantities. Finally, we follow the same approach to obtain the new static black hole solutions with different metric functions.

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arXiv2022

Charged Black Holes in Einsteinian Quartic Gravity

In this paper, we studied Einsteinian quartic gravity minimally coupled to electrodynamics in four dimensions. First, by variation action, we obtain the field equations, and by integration, we obtain a nonlinear third-order differential equation for the metric function and as well as the electromagnetic potential. Then, in the context of the Maxwell field, we discussed the conditions under which the charged black hole exists. Then, we have demonstrated the thermodynamics and stability of the solution for the case of positive coupling of quartic theory. Finally, we showed that the charged black hole solutions of EQG (unlike GR and like ECG) have no inner horizon and do not conform to the extremal bound of GR. Also, the uniqueness of BH solutions in this theory does not work anymore.

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arXiv2022

Quantum Cosmology in Thick Brane

One of the interesting open problems in the cosmological framework is applying quantum physics to the whole universe, consistently. Although different conceptual aspects of quantum cosmology are still very much alive, till now, all the attempts of obtaining a unique and well-defined version of quantum cosmology have been unfeasible. Motivated by what was mentioned above, in this paper, the quantum tunneling of thick brane is investigated by deriving the Wheeler-DeWitt equation and corresponding solutions. Besides, the cosmological analysis of the brane is studied based on two special cases of the scalar field. Finally, it is found that the constant scalar potential arising from a time-dependent scalar field is supported by the so-called slow-roll inflation.

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arXiv2022

Holographic Entanglement Entropy in $T\bar{T}$-deformed CFTs

In this work, we study the holographic entanglement entropy of two dimensional $T\bar{T}$-deformed conformal field theory. We compute the correction due to the deformation up to the leading order of the deformation parameter in the framework of New massive gravity, General minimal massive gravity, and Exotic general massive gravity. We also apply Song's prescription to obtain the entanglement entropy for deformed theories. In each cases, we find agreement between the results.

hep-th↗

arXiv2021

Conservation and Integrability in TMG

In this work, following the paper by Romain Ruzziconi and Céline Zwikel \cite{Ruzziconi:2020wrb} we extend the questions of conservation, integrability and renormalization in Bondi gauge and in GR to the theory of Topological Massive Gravity (TMG). We construct the phase space and renormalize the divergences arising within the symplectic structure through a holographic renormalization procedure. We show that the charge expressions are generically finite, conserved and can be made integrable by a field$-$dependent redefinition of the asymptotic symmetry parameters.

hep-th↗

arXiv2021

Phase Transition Between Flat Space Cosmology and Hot Flat Spacetimes in GMMG and EGMG Models

Flat Space Cosmology (FSC) spacetimes are exact solutions of 3D gravity theories. In this work, we study phase transition between FSC spacetimes and Hot Flat Spacetimes (HFS) in general minimal massive gravity and exotic general massive gravity. We show that similar to topological massive gravity the tunneling occurs between two spacetimes by comparing their free energies. We also obtain the corrections to the Bekenstein-Hawking entropy, and its effect on the phase transition is studied.

gr-qc↗

arXiv2021

Complexity Conjecture of Regular Electric Black Holes

Recently, the action growth rate of a variety of four-dimensional regular magnetic black holes in F frame is obtained in [1]. Here, we study the action growth rate of a four-dimensional regular electric black hole in P frame that is the Legendre transformation of F frame. We also investigate the action growth rates of the Wheeler-De Witt patch for such black hole configurations at the late time and examine the Lloyd bound on the rate of quantum computation. We show that although the form of the Lloyd bound formula remains unaltered, the energy modifies due to a non-vanishing trace of the energy-momentum tensor and some extra terms may appear in the total growth action. We also investigate the asymptotic behavior of complexity in two conjectures for static and rotating regular black holes.

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arXiv2020

Z2-Symmetric Thick Brane with Specific Warp Function

In this work, we investigate a new warped five-dimensional, Z2-symmetric thick brane solution in the presence of a real scalar field. We examine the different geometric aspects of the model. We discuss the stability of the solution under gravitational fluctuations and study both the graviton ground state and the continuum of Kaluza-Klein modes to find a correction to Newtons law. Then, we study the quantum version of the model by deriving the Wheeler-DeWitt equation and looking for the corresponding solution. Finally, the cosmology of the brane is studied.

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arXiv2020

Analytically Approximation Solution to Higher Derivative Gravity

We obtain analytical approximate black hole solutions for higher derivative gravity in the presence of Maxwell electromagnetic source. We construct near horizon and asymptotic solutions and then use these to obtain an approximate analytic solution using a continued fraction method to get a complete solution. We compute the thermodynamic quantities and check the first law and Smarr formula. Finally, we investigate the null and time-like geodesics of this black hole.

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arXiv2020

Physical Properties of a Regular Rotating Black Hole: Thermodynamics, Stability, Quasinormal Modes

Respecting the angular momentum conservation of torque-free systems, it is natural to consider rotating solutions of massive objects. Besides that, motivated by the realistic astrophysical black holes that rotate, we use the Newman-Janis formalism to construct a regular rotating black hole. We start with a nonlinearly charged regular static black hole in the framework of the standard general relativity and then obtain the associated rotating solution through such a formalism. We investigate the geometrical properties of the metric by studying the boundary of ergosphere. We also analyze thermodynamic properties of the solution in AdS spacetime and examine thermal stability and possible phase transition. In addition, we perturb the black hole by using of a real massless scalar field as a probe to investigate its dynamic stability. We obtain an analytic expression for the real and imaginary parts of the quasinormal frequencies. Finally, we look for a connection between the quasinormal frequencies and the properties of the photon sphere in the eikonal limit.

gr-qc↗

arXiv2020

Dynamical and Thermal Stabilities of Nonlinearly Charged AdS Black Holes

In this paper, we study an extended phase space thermodynamics of a nonlinearly charged AdS black hole. We examine both the local and global stabilities, and possible phase transition of the black hole solutions. Finally, we compute quasi-normal modes via scalar perturbations and compare the obtained results with those of RN-AdS black hole.

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