arXiv subjects

A. Dehyadegari

Publications and source records attributed to A. Dehyadegari.

10 recordsLinked to original sources

arXiv2026

Ghost Dark Energy in the Modified Kaniadakis Cosmology

We investigate ghost dark energy (GDE) in a cosmological framework derived from Kaniadakis entropy. By applying the first law of thermodynamics to the FRW apparent horizon, we obtain modified Friedmann equations that include a correction term characterized by the Kaniadakis parameter $λ$. We then study the evolution of a flat universe containing pressureless matter and interacting GDE within this modified gravity setup. Our numerical analysis reveals that the Kaniadakis correction mildly affects the dark energy equation of state and shifts the transition to cosmic acceleration. Stability analysis via the squared sound speed shows the model remains generally unstable, though the instability is moderated for larger $λ$. Statefinder diagnostics indicate that the model approaches the $Λ$CDM fixed point in the future, with deviations decreasing as $λ$ increases.

gr-qc↗

arXiv2026

Modified Entropy from Action Principle

We propose a modified gravity theory by extending the Einstein-Hilbert action with an arbitrary function of the Ricci scalar and the Kretschmann scalar invariants. The resulting modified Friedmann equations for a spatially flat FRW universe are derived, which remain free of higher-order derivatives and reduce to the standard Friedmann equations in the limiting case. Employing the gravity-thermodynamics conjecture, we investigate the thermodynamic behavior at the apparent horizon and derive the corresponding modified entropy. Using the first law of thermodynamics together with the modified Friedmann equations, we obtain a general expression for the apparent horizon entropy. This formalism allows us to compute the modified entropy for various well-known entropy models. Our approach establishes a consistent thermodynamic framework linking modified gravity theories constructed from curvature invariants to generalized entropy functions on the cosmological apparent horizon.

gr-qc↗

arXiv2026

New Black hole Solutions in $f(\mathbb{Q})$ Gravity

We investigate static and spherically symmetric vacuum solutions in the symmetric teleparallel $f(\mathbb{Q})$ modified theory of gravity. Starting from a recently proposed classification of affine connections compatible with both the symmetries of spacetime and the constraints of symmetric teleparallel geometry, we develop a systematic approach to solve the full field equations. We first identify two distinct classes of connections that satisfy the off-diagonal metric field equations and the connection constraints. For an arbitrary $f(\mathbb{Q})$ function when the non-metricity scalar $\mathbb{Q}$ vanishes, we recover exact analytical solutions equivalent to those of general relativity, including the Schwarzschild and Schwarzschild (anti)de-Sitter metrics. We then extend our analysis beyond general relativity by considering the quadratic model $f(\mathbb{Q})=\mathbb{Q}+α~\mathbb{Q}^2$ with a small parameter $α$. Using a perturbative approach, we derive asymptotically flat, analytical solutions up to second order in $α$. These solutions exhibit corrections to the standard Schwarzschild metric, characterized by new integration constants that can be interpreted as connection hair. We explore the asymptotic behavior of these solutions and disclose that the horizon radius receives corrections that can be expressed compactly using the Lambert $\mathcal{W}$ function. Our results provide new, non-trivial vacuum solutions within $f(\mathbb{Q})$ gravity and highlight the rich structure introduced by the non-metricity connection.

gr-qc↗

arXiv2017

Optical properties of Born-Infeld-dilaton-Lifshitz holographic superconductors

In this paper, we first study the Lifshitz-dilaton holographic superconductors with nonlinear Born-Infeld (BI) gauge field and obtain the critical temperature of the system for different values of Lifshitz dynamical exponent, $z$, and nonlinear parameter $b$. We find that for fixed value of $b$, the critical temperature decreases with increasing $z$. This indicates that the increase of anisotropy between space and time prevents the phase transition. Also, for fixed value of $z$, the critical temperature decrease with increasing $b$. Then, we investigate the optical properties of ($2+1$) and ($3+1$)-dimensional BI-Lifshitz holographic superconductors in the the presence of dilaton field. We explore the refractive index of the system. For $z=1$ and $(2+1)$-dimensional holographic superconductor, we observe negative real part for permittivity $\textrm{Re}[ε]$ as frequency $ω$ decreases. Thus, in low frequency region our superconductor exhibit metamaterial property. This behaviour is independent of the nonlinear parameter and can be seen for either linear ($b=0$) and nonlinear ($b\neq 0$) electrodynamics. Interestingly, for ($3+1$)-dimensional Lifshitz-dilaton holographic superconductors, we observe metamaterial behavior neither in the presence of linear nor nonlinear electrodynamics.

hep-th↗

arXiv2017

Microscopic Origin of Black Hole Reentrant Phase Transitions

Understanding the microscopic behavior of the black holes ingredients has been one of the important challenges in black holes physics during the past decades. In order to shed some light on the microscopic structure of black holes, in this paper, we explore a recently observed phenomenon for black holes namely reentrant phase transition, by employing the Ruppeiner geometry. Interestingly enough, we observe two properties for the phase behaviour of small black holes that leads to reentrant phase transition. They are correlated and they are of the interaction type. For the range of pressure in which the system underlies reentrant phase transition, it transits from large black holes phase to small one which possesses higher correlation than the other ranges of pressures. On the other hand, the type of interaction between small black holes near large/small transition line, differs for usual and reentrant phase transitions. Indeed, for usual case, the dominant interaction is repulsive whereas for reentrant case we encounter with an attractive interaction. We show that in reentrant phase transition case, the small black holes behave like a Bosonic gas whereas in the usual phase transition case, they behave like a quantum anyon gas.

hep-th↗

arXiv2017

Holographic conductivity in the massive gravity with power-law Maxwell field

We obtain a new class of topological black hole solutions in $(n+1)$-dimensional massive gravity in the presence of the power-Maxwell electrodynamics. We calculate the conserved and thermodynamic quantities of the system and show that the first law of thermodynamics is satisfied on the horizon. Then, we investigate the holographic conductivity for the four and five dimensional black brane solutions. For completeness, we study the holographic conductivity for both massless ($m=0$) and massive ($m \neq 0$) gravities with power-Maxwell field. The massless gravity enjoys translational symmetry whereas the massive gravity violates it. For massless gravity, we observe that the real part of conductivity, $\mathrm{Re}[σ]$, decreases as charge $q$ increases when frequency $ω$ tends to zero, while the imaginary part of conductivity, $\mathrm{Im}[σ]$, diverges as $ω\rightarrow 0$. For the massive gravity, we find that $\mathrm{Im}[σ]$ is zero at $ω=0$ and becomes larger as $q$\ increases (temperature decreases), which is in contrast to the massless gravity. Interestingly, we observe that in contrast to the massless case, $\mathrm{Re}[σ]$ has a maximum value at $ω=0$ (known as the Drude peak) for $p=\left( n+1\right) /4$ (conformally invariant electrodynamics) where $p$ is the power parameter of the power-law Maxwell field and this maximum increases with increasing $q$. Finally, we show that for high frequencies, the real part of the holographic conductivity have the power law behavior in terms of frequency, $ω^{a}$ where $a \propto (n+1-4p)$. Some similar behaviors for high frequencies in possible dual CFT systems have been reported in experimental observations.

hep-th↗

arXiv2016

Thermodynamics, phase transitions and Ruppeiner geometry for Einstein-dilaton Lifshitz black holes in the presence of Maxwell and Born-Infeld electrodynamics

In this paper, we first obtain the ($n+1$)-dimensional dilaton-Lifshitz black hole (BH) solutions in the presence of Born-Infeld (BI) electrodynamics. We find that there are two different solutions for $z=n+1$ and $z\neq n+1$ cases ($z$ is dynamical critical exponent). We show that the thermodynamics first law is satisfied for both cases. Then, we turn to study different phase transitions (PTs) for our BHs. We start with study of Hawking-Page PT for both linearly and BI charged BHs. After that, we discuss the PTs inside the BHs. We present the improved Davies quantities and prove that the PT points shown by them coincide with Ruppeiner ones. We show that the zero temperature PTs are transitions on radiance properties of BHs by using Landau-Lifshitz theory. Next, we turn to study Ruppeiner geometry of linearly and BI charged BHs. For linearly charged case, we show that there are no PT at finite temperature for the case $z\geq 2$. For $z<2$, it is found that the number of finite temperature PT points depends on the value of BH charge and is not more than two. When we have two finite temperature PT points, there are no thermally stable BH between these two points and we have discontinues small/large BH PTs. As expected, for small BHs, we observe finite magnitude for Ruppeiner invariant which shows the finite correlation between possible BH molecules while for large BHs, the correlation is very small. Finally, we study the Ruppeiner geometry and thermal stability of BI charged Lifshtiz BHs for different values of $z$. We observe that small BHs are thermally unstable in some situations. Also, the behavior of correlation between possible BH molecules for large BHs is the same as linearly charged case. In both linearly and BI charged cases, for some choices of parameters, the BH systems behave like a Van der Waals gas near transition point.

hep-th↗

arXiv2016

Holographic Conductivity for Logarithmic Charged Dilaton-Lifshitz Solutions

We disclose the effects of the logarithmic nonlinear electrodynamics on the holographic conductivity of Lifshitz dilaton black holes/branes. We analyze thermodynamics of these solutions as a necessary requirement for applying gauge/gravity duality, by calculating conserved and thermodynamic quantities such as the temperature, entropy, electric potential and mass of the black holes/branes. We calculate the holographic conductivity for a $(2+1)$-dimensional brane boundary and study its behavior in terms of the frequency per temperature. Interestingly enough, we find out that, in contrast to the Lifshitz-Maxwell-dilaton black branes which has conductivity for all $z$, here in the presence of nonlinear gauge field, the holographic conductivity do exist provided $z\leq3$ and vanishes for $z>3$. It is shown that independent of the nonlinear parameter $β$, the real part of the conductivity is the same for a specific value of frequency per temperature in both AdS and Lifshitz cases. Besides, the behavior of real part of conductivity for large frequencies has a positive slope with respect to large frequencies for a system with Lifshitz symmetry whereas it tends to a constant for a system with AdS symmetry. This behavior may be interpreted as existence of an additional charge carrier rather than the AdS case, and is due to the presence of the scalar dilaton field in model. Similar behavior for optical conductivity of single-layer graphene induced by mild oxygen plasma exposure has been reported.

hep-th↗

arXiv2016

Thermodynamics and gauge/gravity duality for Lifshitz black holes in the presence of exponential electrodynamics

In this paper, we construct a new class of topological black hole Lifshitz solutions in the presence of nonlinear exponential electrodynamics for Einstein-dilaton gravity. We show that the reality of Lifshitz supporting Maxwell matter fields exclude the negative horizon curvature solutions except for the asymptotic AdS case. Calculating the conserved and thermodynamical quantities, we obtain a Smarr type formula for the mass and confirm that thermodynamics first law is satisfied on the black hole horizon. Afterward, we study the thermal stability of our solutions and figure out the effects of different parameters on the stability of solutions under thermal perturbations. Next, we apply the gauge/gravity duality in order to calculate the ratio of shear viscosity to entropy for a three-dimensional hydrodynamic system by using the pole method. Furthermore, we study the behavior of holographic conductivity for two-dimensional systems such as graphene. We consider linear Maxwell and nonlinear exponential electrodynamics separately and disclose the effect of nonlinearity on holographic conductivity. We indicate that holographic conductivity vanishes for $z>3$ in the case of nonlinear electrodynamics while it does not in the linear Maxwell case. Finally, we solve perturbative additional field equations numerically and plot the behaviors of real and imaginary parts of conductivity for asymptotic AdS and Lifshitz cases. We present experimental results match with our numerical ones.

hep-th↗

arXiv2016

Comment on "Insight into the Microscopic Structure of an AdS Black Hole from a Thermodynamical Phase Transition"

Thermodynamic geometry analysis of interesting Letter [Phys. Rev. Lett. 115, 111302 (2015)] for charged AdS black holes, which is based on studying the Ruppeiner invariant behavior is not correct and the authors made a mistake in calculating this quantity. In the present Letter, we address the correct Ruppeiner scalar curvature and reveal the correct possible microscopic properties of 4-dimensional charged AdS black holes arise from it. Some of these properties have not been discussed in pointed out Letter.

hep-th↗