arXiv subjects

Hanif Golchin

Publications and source records attributed to Hanif Golchin.

12 recordsLinked to original sources

arXiv2026

Wormhole geometries in Einstein-aether theory

We present the first analysis of traversable wormhole solutions within the framework of Einstein-aether theory. We show that the corresponding field equations admit three distinct wormhole geometries, obtained by adopting three different classes of combinations for the aether coupling constants. We examine the null and weak energy conditions for three types of wormhole shape functions. Our findings reveal that, in contrast to Einstein gravity, by choosing appropriate parameter values, wormhole geometries can satisfy the energy conditions at the wormhole throat. We also find that in one class, wormholes can satisfy the energy conditions not only at the wormhole throat but also throughout the entire spacetime. Furthermore, the requirement of energy condition satisfaction, imposes some constraints on the values of aether coupling constants. By comparing these constraints with those previously obtained from theoretical and observational analyses, we find that the satisfaction of energy conditions put more stringent limits on the allowed values of the aether couplings.

gr-qc↗

arXiv2023

Rotating black strings beyond Maxwell's electrodynamics

In this paper, we investigate the asymptotically Anti de Sitter solutions of rotating black strings coupled to Born-Infeld and Modified Maxwell non-linear electrodynamics in the context of Einsteinian, Einsteinian cubic and Einsteinian quartic gravity. By studying the near-horizon behavior of solutions, we find the mass parameter, surface gravity and accordingly the Hawking temperature. We also compute the entropy, mass, angular momentum, electric charge, and the electrostatic potential of solutions to show that, in the context of above theories and in the presence of two mentioned non-linear electrodynamics, the first law of thermodynamics for rotating black strings is also exactly hold. We also investigate extremality, thermal stability and phase transition of solutions.

gr-qc↗

arXiv2023

The effect of redshift function on the Weak Energy Conditions in f(R) Wormholes

In the present paper, we investigate traversable wormhole solutions determined by an exponential shape function and fractional redshift function in the background of four viable $f(R)$ models. Although in the absence of the redshift function $φ(r)$ the null energy condition (NEC) and weak energy condition (WEC) are violated, we find that considering the redshift function, NEC and WEC are respected by choosing the appropriate parameters in the models. We also investigate the conditions of stability and absence of anti-gravity effects for these wormholes. Our results show that in the case of $φ(r) \neq 0$ these conditions are satisfied easier than the case of $φ(r)=0$. Finally we calculate the deflection angle using the gravitational lensing effect. We show that the deflection angle increases by inserting the redshift function.

gr-qc↗

arXiv2022

Regular Black Holes: Entropy Products and Central Charges

In this paper for variety types of regular black hole solutions, we investigate the entropy product of inner and outer horizons. Similar to singular black holes, for the regular ones we find that universality (mass independence) of the entropy product is true for some solutions and it fails for some others. In the case of regular black holes that respect the universality, we read central charges of the dual CFTs from the entropy product, according to the Thermodynamics method introduced in \cite{Chen:2013rb}. For these solutions we also calculate central charges, using the asymptotic symmetry group formalism. The results of these two approaches are the same, which means that universality of the entropy product provides a simple method to find central charges of the dual CFTs.

hep-th↗

arXiv2020

Central charges from Thermodynamics method in 3D gravity

In the context of guage/gravity duality, we investigate the central charges of a number of 2-dimensional conformal field theories (CFTs) that might live on the boundary of some 3-dimensional (3D) toy models of gravity, from the thermodynamics aspect of the black holes. For many black hole solutions, the entropy product of the inner Cauchy and outer event horizons is universal (mass independent). It is proposed that for these solutions, the central charges of the left- and right-moving sectors of the dual CFTs should be the same and one may read the central charges from the universal entropy product. This provides strong motivations for investigating this prescription for BTZ and Warped AdS$_{3}$ black holes in a number of 3D gravity theories and we will show that the proposal works truly. One striking result of our analysis is that if the entropy product is not universal in any theory of 3D gravity, then the left and right central charges are not equal.

hep-th↗

arXiv2020

More on the Entropy Product and Dual CFTs

In this paper, we check the universality of the entropy product for some black hole and black ring solutions. Applying the asymptotic symmetry group (ASG) analysis, we also find the central charges of the dual CFTs for these solutions. It has been observed \cite{Chen:2013rb} that if the entropy product for a solution is universal, it is possible to read the central charges of the dual CFTs from the entropy product as $c_i \sim \frac{\partial}{\partial N_i}(S_+S_-)$, where $N_i$ is a conserved charge of the solution like angular momentum or electric charge. In this work, we consider some other families of solutions and we check that the same behavior is valid for them. Moreover, in the case of solutions containing the conical singularity, we find the central charges using the ASG analysis. We show that the central charges receive a contribution from the conical characteristic $κ$ of the solution, however it is still possible to read the central charges from the entropy product.

hep-th↗

arXiv2019

Universality of Area Product: Solutions with Conical Singularity

It has been observed that the area product of horizons for many black hole solutions is mass independent and satisfy the universality relation $A_+A_-=(8π)^2 N$, where $N$ is related to the quantized charges of the solution as angular momentum and electric charge. In this work the same analysis is done for black hole and black ring solutions with conical singularity. We find that the area product is still mass independent and regardless of the horizon topology, the conical characteristic ($κ$) of the solutions, appears in the universality relation as $κA_+A_-=(8π)^2 N$. We also check that the first law of black hole inner mechanics is satisfied for these solutions.

gr-qc↗

arXiv2019

Quasi-cosmological Traversable Wormholes in $f(R)$ Gravity

In this paper we study traversable wormholes in the context of $f(R)$ gravity. Exact solutions of traversable wormholes are found by imposing the nonconstant Ricci scalar. These solutions asymptotically match spherical, flat and hyperbolic FRW metric. By choosing some static $f(R)$ gravity models, we verify the standard energy conditions for the asymptotically spherical, flat and hyperbolic wormhole solutions. Unlike the Einstein gravity, we find that in the context of $f(R)$ modified gravity, the asymptotically spherical, flat and hyperbolic wormhole solutions can respect the null energy condition (NEC) at the wormhole throat and near that. We find that in some static $f(R)$ models, asymptotically flat and hyperbolic wormholes respect the weak energy condition (WEC) through the whole space.

gr-qc↗

arXiv2017

Interacting Dark Energy: Dynamical System Analysis

We investigate the impacts of interaction between dark matter and dark energy in the context of two dark energy models, holographic and ghost dark energy. In fact, using the dynamical system analysis, we obtain the cosmological consequence of several interactions, considering all relevant component of universe, i.e. matter (Dark and luminous), radiation and dark energy. Studying the phase space for all interactions in detail, we show the existence of unstable matter dominated and stable dark energy dominated phases. We also show that linear interactions suffer from the absence of standard radiation dominated epoch. Interestingly, this failure resolved by adding the non-linear interactions to the models. We find an upper bound for the value of the coupling constant of the interaction between dark matter and dark energy as b < 0.57 in the case of holographic model, and b < 0.61 in the case of ghost dark energy model, to result in a cosmological viable matter dominated epoch. More specifically, this bound is vital to satisfy instability and deceleration of matter dominated epoch.

gr-qc↗

arXiv2014

More on Five Dimensional EVH Black Rings

In this paper we continue our analysis of arXiv:1308.1478[hep-th] and study in detail the parameter space of three families of doubly spinning black ring solutions: balanced black ring, unbalanced ring and dipole-charged balanced black rings. In all these three families the Extremal Vanishing Horizon (EVH) ring appears in the vanishing limit of the dimensionful parameter of the solution which measures the ring size. We study the near horizon limit of the EVH black rings and for all three cases we find a (pinching orbifold) AdS$_3$ throat with the AdS$_3$ radius $\ell^2=8 G_5 M/(3π)$ where $M$ is the ring mass and $G_5$ is the 5d Newton constant. We also discuss the near horizon limit of near-EVH black rings and show that the AdS$_3$ factor is replaced with a generic BTZ black hole. We use these results to extend the EVH/CFT correspondence for black rings, a 2d CFT dual to near-EVH black rings.

hep-th↗

arXiv2013

Dual 2d CFT Identification of Extremal Black Rings from Holes

Five dimensional Einstein gravity vacuum solutions in general fall into two classes of black rings with horizon topology S^2 \times S^1, and black holes with horizon topology S^3. These solutions are specified by their mass and two spins. There are "overlapping" regions of this parameter space where one has extremal rings and holes of the same spins. We show that for such regions the hole has generically a larger entropy than the ring, and likewise, the central charge of the proposed chiral 2d CFT dual to the hole is larger than that of the ring. For special places of this overlapping region where one of the spins tends to zero, the entropies of the extremal ring and hole also tend to zero and essentially become equal. In this case we are dealing with Extremal Vanishing Horizon (EVH) black holes or rings. The near horizon geometry of the near-EVH hole and rings both contain locally AdS_3 throats, providing a basis for the EVH/CFT proposal, a 2d CFT description of the low energy excitations of EVH hole or ring. We argue how the near-EVH hole and near-EVH ring can be distinguished from this dual 2d CFT viewpoint: The hole is a thermal state with zero temperature in the left sector and finite temperature in the right, while the ring is a generic state in the ground state (of the CFT on the plane) in the left sector and a thermal state in the right. The latter is part of the Hilbert space of the 2d CFT obtained in the Discrete Light Cone Quantization (DLCQ).

hep-th↗

arXiv2008

On higher derivative corrections of tachyon action

We have examined the momentum expansion of the disk level S-matrix element of two tachyons and two gauge fields to find, up to on-shell ambiguity, the couplings of these fields in the world volume theory of N coincident non-BPS D-branes to all order of $α'$. Using the proposal that the action of D-brane-anti-D-brane is given by the projection of the action of two non-BPS D-branes with $(-1)^{F_L}$, we find the corresponding couplings in the world volume theory of the brane-anti-brane system. Using these infinite tower of couplings, we then calculate the massless pole of the scattering amplitude of one RR field, two tachyons and one gauge field in the brane-anti-brane theory. We find that the massless pole of the field theory amplitude is exactly equal to the massless pole of the disk level S-matrix element of one RR, two tachyons and one gauge field to all order of $α'$.

hep-th↗