Searcharxiv⌕ Search

arXiv subjects

D. Momeni

Publications and source records attributed to D. Momeni.

At least 19 recordsLinked to original sources
arXiv2021

Exploring the Applicability of Birkhoff's Theorem in Jackiw-Teitelboim Gravity

We present a comprehensive and technically rigorous analysis of the status of Birkhoff's theorem in Jackiw-Teitelboim (JT) gravity, a paradigmatic two-dimensional model for studying semiclassical gravitational dynamics. While Birkhoff's theorem is well established in four-dimensional general relativity asserting the uniqueness and staticity of vacuum solutions under reflection symmetry remains subtle due to the absence of propagating gravitational degrees of freedom. In this work, we systematically investigate the space of symmetry under radially symmetric configurations in JT gravity using both conformal and Schwarzschild like gauges. Through analytical techniques and integral transformations, we explore the conditions under which vacuum solutions remain time-independent, identifying classes of metric dilaton configurations that either uphold or violate Birkhoff type behavior. Our findings reveal that the theorem holds only in restricted cases, depending critically on the separability of the conformal factor and the structure of the dilaton potential. These results clarify longstanding ambiguities surrounding symmetry and dynamics in two-dimensional gravity and establish JT gravity as a controlled setting for probing the breakdown of classical gravitational theorems in lower dimensional and holographic contexts. This analysis contributes to a deeper understanding of the interplay between symmetry, integrability, and geometry in quantum gravity and strongly coupled systems.

hep-th↗
arXiv2018

Mimetic Compact Stars

Modified gravity models have been constantly proposed with the purpose of evading some standard gravity shortcomings. Recently proposed by A.H. Chamseddine and V. Mukhanov, the Mimetic Gravity arises as an optimistic alternative. Our purpose in this work is to derive Tolman-Oppenheimer-Volkoff equations and solutions for such a gravity theory. We solve them numerically for quark star and neutron star cases. The results are carefully discussed.

gr-qc↗
arXiv2018

Modelling of a compact anisotropic star as an anisotropic fluid sphere in $f(T)$ gravity

In this paper, we have studied the new exact model of anisotropic star in $f(T)$ theory of gravity. The dynamical equations in $f(T)$ theory with the anisotropic fluid have been solved by using Krori-Barua solution. We have determined that all the obtained solutions are free from central singularity and potentially stable. The observed values of mass and radius of the different strange stars RX J 1856-37, Her X-1, and Vela X-12 have been used to calculate the values of unknown constants in Krori and Barua metric. The physical parameters like anisotropy, stability and redshift of the stars have been investigated in detail.

gr-qc↗
arXiv2016

Realization of Holographic Entanglement Temperature for a Nearly-AdS boundary

Computing the holographic entanglement entropy proposed by Ryu-Takayanagi shows that thermal energy near boundary region in $AdS_3$ gain maximum of the temperature. The absolute maxima of temperature is $T^{Max}_{E}= \frac{4G_3 ε_{\infty}}{l}$. By simple physical investigations it has become possible to predict a phase transition of first order at critical temperature $T_c\leq T_{E}$. As they predict a tail or root towards which the AdS space ultimately tend, the boundary is considered thermalized. The Phase transitions of this form have received striking theoretical and experimental verifications so far.

hep-th↗
arXiv2016

Generalized second law of thermodynamics in f(R,T) theory of gravity

We present a study of the generalized second law of thermodynamics in the scope of the f(R,T) theory of gravity, with R and T representing the Ricci scalar and trace of the energy-momentum tensor, respectively. From the energy-momentum tensor equation for the f(R,T) = R + f(T) case, we calculate the form of the geometric entropy in such a theory. Then, the generalized second law of thermodynamics is quantified and some relations for its obedience in f(R,T) gravity are presented. Those relations depend on some cosmological quantities, as the Hubble and deceleration parameters, and on the form of f(T).

gr-qc↗
arXiv2016

Higher-Derivative $f(R,\Box R, T)$ Theories of Gravity

In literature there is a model of modified gravity in which the matter Lagrangian is coupled to the geometry via trace of the stress-energy momentum tensor $T=T_μ^μ$. This type of modified gravity is called as $f(R,T)$ in which $R$ is Ricci scalar $R=R_μ^μ$. We extend manifestly this model to include the higher derivative term $\Box R$. We derived equation of motion (EOM) for the model by starting from the basic variational principle. Later we investigate FLRW cosmology for our model. We show that de Sitter solution is unstable for a generic type of $f(R,\Box R, T)$ model. Furthermore we investigate an inflationary scenario based on this model. A graceful exit from inflation is guaranteed in this type of modified gravity.

gr-qc↗
arXiv2015

Non-equilibrium phase and entanglement entropy in 2D holographic superconductors via Gauge-String duality

An alternative method of developing the theory of non-equilibrium two dimensional holographic superconductor is to start from the definition of a time dependent $AdS_3$ background. As originally proposed, many of these formulae were cast in exponential form, but the adoption of the numeric method of expression throughout the bulk serves to show more clearly the relationship between the various parameters. The time dependence behaviour of the scalar condensation and Maxwell fields are fitted numerically. A usual value for Maxwell field on AdS horizon is $\exp(-bt)$, and the exponential $\log$ ratio is therefore $10^{-8} s^{-1}$. The coefficient $b$ of the time in the exponential term $\exp(-bt)$ can be interpreted as a tool to measure the degree of dynamical instability, its reciprocal $\frac{1}{b}$ is the time in which the disturbance is multiplied in the ratio. A discussion of some of the exponential formulae is given by the scalar field $ψ(z,t)$ near the AdS boundary. It might be possible that a long interval would elapse the system which tends to the equilibrium state when the normal mass and conformal dimensions emerged. A somewhat curious calculation has been made, to illustrate the holographic entanglement entropy for this system. The foundation of all this calculation is, of course, a knowledge of multiple (connected and disconnected) extremal surfaces. There are several cases in which exact and approximate solutions are jointly used, a variable numerical quantity is represented by a graph, and the principles of approximation are then applied to determine related numerical quantities. In the case of the disconnected phase with a finite extremal are, we find a discontinuity in the first derivative of the entanglement entropy as the conserved charge $J$ is increased.

hep-th↗
arXiv2015

Spherically symmetric solutions of light Galileon

We have been studied the model of light Galileon with translational shift symmetry $ϕ\to ϕ+c$. The matter Lagrangian is presented in the form $\mathcal{L}_ϕ= -η(\partial ϕ)^2+βG^{μν}\partial_μϕ\partial_νϕ$. We have been addressed two issues: the first is that, we have been proven that, this type of Galileons belong to the modified matter-curvature models of gravity in type of $f(R,R^{μν}T_{μν}^m)$. Secondly, we have been investigated exact solution for spherically symmetric geometries in this model. We have been found an exact solution with singularity at $r=0$ in null coordinates. We have been proven that the solution has also a non-divergence current vector norm. This solution can be considered as an special solution which has been investigated in literature before, in which the Galileon's field is non-static (time dependence). Our scalar-shift symmetrized Galileon has the simple form of $ϕ=t$, which it is remembered by us dilaton field.

gr-qc↗
arXiv2015

Inflation driven by q-de Sitter

We propose a generalised de Sitter scale factor for the cosmology of early and late time universe, including single scalar field is called as inflaton. This form of scale factor has a free parameter $q$ is called as nonextensivity parameter. When $q=1$, the scale factor is de Sitter. This scale factor is an intermediate form between power-law and de Sitter. We study cosmology of such families. We show that both kinds of dark components, dark energy and dark matter simultaneously are described by this family of solutions. As a motivated idea, we investigate inflation in the framework of $q$-de Sitter. We consider three types of scenarios for inflation. In a single inflation scenario, we observe that, inflation ended without any specific ending inflation $ϕ_{end}$, the spectral index and the associated running of the spectral index are %$ n_\mathrm{s} - 1 \sim -2ε, \quad α_\mathrm{s} \equiv 0 $. To end the inflation: we should have $q=\frac{3}{4}$. We deduce that the inflation ends when the evolution of the scale factor is $a (t) =e_ {3/4} (t) $. With this scale factor there is no need to specify $ϕ_{end}$. As an alternative to have inflation with ending point, We will study q-inflation model in the context of warm inflation. We propose two forms of damping term $Γ$. In the first case when $Γ=Γ_0$, we show the scale invariant spectrum, (Harrison-Zeldovich spectrum, i.e. $n_s=1$) may be approximately presented by ($q=\frac{9}{10},~~N=70$). Also there is a range of values of $R$ and $n_s$ which is compatible with the BICEP2 data where $q=\frac{9}{10}$. In case $Γ=Γ_1V(ϕ)$, it is observed that small values of a number of e-folds are assured for small values of $q$ parameter. For $q=\frac{9}{10}$ a range of values of $R$ and $n_s$ is compatible with the BICEP2 data.

physics.gen-ph↗
arXiv2015

Cosmological viable Mimetic $f(R)$ and $f(R,T)$ theories via Noether symmetry

Extended $f(R)$ theories of gravity have been investigated from the symmetry point of view. We briefly has been investigated Noether symmetry of two types of extended $f(R)$ theories: $f(R,T)$ theory, in which curvature is coupled non minimally to the trace of energy momentum tensor $T_{μν}$ and mimetic $f (R) $ gravity, a theory with a scalar field degree of freedom, but ghost-free and with internal conformal symmetry. In both cases we write point -like Lagrangian for flat Friedmann-Lemaitre-Robertson-Walker (FLRW) cosmological background in the presence of ordinary matter. We have been shown that some classes of models existed with Noether symmetry in these viable extensions of $f(R)$ gravity. As a motivated idea, we have been investigating the stability of the solutions and the bouncing and $Λ$CDM models using the Noether symmetries. We have been shown that in mimetic $f(R)$ gravity bouncing and $Λ$CDM solutions are possible. Also a class of solutions with future singularities has been investigated.

gr-qc↗
arXiv2015

Anisotropic Compact Stars in $f(G)$ Gravity

This paper is devoted to study the possibility of forming anisotropic compact stars in modified Gauss-Bonnet, namely called as $f(G)$ theory of gravity which is one of the strong candidates,responsible for the accelerated expansion of the universe. For this purpose, we have used analytical solution of Krori and Barua metric to the Einstein field equations with anisotropic form of matter and power law model of $f(G)$ gravity. To determine the unknown constants in Krori and Barua metric, we have used the sample of compact stars, 4$U$1820-30, Her X-1, SAX J 1808-3658. The physical behavior of these stars have been analyzed with the observational data. In this setting, we have checked all the regularity conditions and stability of the compact stars 4$U$1820-30, Her X-1, SAX J1808-3658.

gr-qc↗
arXiv2015

Universality of critical magnetic field in holographic superconductor

In this letter we study aspects of the holographic superconductors analytically in the presence of a constant external magnetic field. We show that the critical temperature and critical magnetic field can be calculated at nonzero temperature. We detect the Meissner effect in such superconductors. A universal relation between black hole mass $ M$ and critical magnetic field $H_c$ is proposed as $\frac{H_c}{M^{2/3}}\leq 0.687365$. We discuss some aspects of phase transition in terms of black hole entropy and the Bekenstein's entropy to energy upper bound.

hep-th↗
arXiv2014

Tolman-Oppenheimer-Volkoff Equations in Modified Gauss-Bonnet Gravity

Based on a stringy inspired Gauss-Bonnet (GB) modification of classical gravity, we constructed a model for neutron stars. We derived the modified forms of Tolman-Oppenheimer-Volkoff (TOV) equations for a generic function of $f(G)$ gravity. The hydrostatic equations remained unchanged but the dynamical equations for metric functions are modified due to the effects of GB term.

gr-qc↗
arXiv2014

Analytical holographic superconductors in $AdS_N$ Lifshitz topological black holes

We present the analytic Lifshitz solutions for a scalar field model minimally coupled with the abelian gauge field in $N$ dimensions. We also consider the presence of cosmological constant $Λ$. The Lifshitz parameter $z$ appearing in the solution plays the role of the Lorentz breaking parameter of the model. We investigate the thermodynamical properties of the solutions and discuss the energy issue. Furthermore, we study the hairy black hole solutions in which the abelian gauge field breaks the symmetry near the horizon. In the holographic picture, it is equivalent to a second order phase transition. Explicitly we show that there exists a critical temperature which is a function of the Lifshitz parameter $z$. The system below the critical temperature becomes superconductor, but the critical exponent of the model remains the same of the usual holographic superconductors without the higher order gravitational corrections, in agreement with Ginzburg-Landau theories.

hep-th↗
arXiv2014

Condensation of the scalar field with Stuckelberg and Weyl Corrections in the background of a planar AdS-Schwarzschild black hole

We study analytical properties of the Stuckelberg holographic superconductors with Weyl corrections. We obtain the minimum critical temperature as a function of the mass of the scalar field $m^2$. We show that in limit of the $m^2=-3$,$T^{Min}_c\approx0.158047\sqrt[3]ρ$ which is close to the numerical estimate $T_c^{Numerical}\approx 0.170\sqrt[3]ρ$. Further we show that the mass of the scalar field in bounded from below by the $ m^2>m_c^2$ where $m_c^2=-5.40417$. This lower bound is weaker and different from the previous lower bound $m^2=-3$ predicted by stability analysis. We show that in the Breitenlohner-Freedman bound, the critical temperature remains finite. Explicitly, we prove that here there is exist a linear relation between $ $ and the chemical potential.

physics.gen-ph↗
arXiv2014

Observational constraints on models of the Universe with time variable Gravitational and Cosmological constants along MOG

The subject of this paper is to investigate the weak regime covariant scalar-tensor-vector gravity (STVG) theory, known as the MOdified gravity (MOG) theory of gravity. First, we show that the MOG in the absence of scalar fields is converted into $Λ(t),G(t)$ models. Time evolution of the cosmological parameters for a family of viable models have been investigated. Numerical results with the cosmological data have been adjusted. We've introduced a model for dark energy (DE) density and cosmological constant which involves first order derivatives of Hubble parameter. To extend this model, correction terms including the gravitational constant are added. In our scenario, the cosmological constant is a function of time. To complete the model,interaction terms between dark energy and dark matter (DM) manually entered in phenomenological form. Instead of using the dust model for DM, we have proposed DM equivalent to a barotropic fluid. Time evolution of DM is a function of other cosmological parameters. Using sophisticated algorithms, the behavior of various quantities including the densities, Hubble parameter, etc. have been investigated graphically. The statefinder parameters have been used for the classification of DE models. Consistency of the numerical results with experimental data of $SneIa+BAO+CMB$ are studied by numerical analysis with high accuracy.

gr-qc↗
arXiv2013

The instabilities and (anti)-evaporation of Schwarzschild-de Sitter black holes in modified gravity

We investigate the future evolution of Nariai black hole which is extremal limit of Schwarzschild-de Sitter one in modified gravity. The perturbations equations around Nariai black hole are derived in static and cosmological patches for general $F(R)$-gravity. The analytical and numerical study of several realistic $F(R)$-models shows the occurence of rich variety of scenarios: instabilities, celebrated Hawking evaporation and anti-evaporation of black hole. The realization of specific scenario depends on the model under consideration. It is remarkable that the presence of such primordial black holes at current universe may indicate towards the modified gravity which supports the anti-evaporation as preferrable model. As some generalization we extend the study of Nariai black hole evolution to modified Gauss-Bonnet gravity. The corresponding perturbations equations turn out to be much more complicated than in the case of $F(R)$ gravity. For specific example of modified Gauss-Bonnet gravity we demonstrate that Nariai solution maybe stable.

gr-qc↗