arXiv subjects

Mehdi Shokri

Publications and source records attributed to Mehdi Shokri.

At least 19 recordsLinked to original sources

arXiv2025

Barrow entropies in black hole thermodynamics

We study the thermodynamic features of static, spherically-symmetric Schwarzschild black holes adopting different types of Barrow entropy. Specifically, in addition to the standard Barrow entropy, we consider a logarithmic-corrected type of this entropy by taking into account some loop quantum gravity effects. Moreover, we investigate the black hole thermodynamics from the viewpoint of Barrow entropy in presence of non-extensivity effects coming from the Tsallis statistics. Finally, we compare the results obtained for different Barrow-based entropies.

gr-qc↗

arXiv2024

Bekenstein bound on black hole entropy in non-Gaussian statistics

The Bekenstein bound, inspired by the physics of black holes, is introduced to constrain the entropy growth of a physical system down to the quantum level in the context of a generalized second law of thermodynamics. We first show that the standard Bekenstein bound is violated when the entropy of a Schwarzschild black hole is described in non-Gaussian statistics Barrow, Tsallis, and Kaniadakis due to the presence of the related indices $\Delta$, $q$ and $\kappa$, respectively. Then, by adding the GUP effects into the Bekenstein bound, we find that the generalized bound is satisfied in the context of the mentioned entropies through a possible connection between the entropies indices and the GUP parameter $\beta$.

hep-th↗

arXiv2024

Comparing Inflationary Models in Extended Metric-Affine Theories of Gravity

We study slow-roll inflation as a common feature in Metric-affine Theories of Gravity. In particular, we take into account extended metric, teleparallel and symmetric-teleparallel theories of gravity, based on different geometric invariants, discussing analogies and differences. The analysis for each model is performed in two approaches. First, we focus on the {\it potential-slow-roll approach} by studying the reconstructed potentials for different forms of the extended models related to the considered gravitational theory in the Einstein frame. Secondly, we investigate the {\it Hubble-slow-roll approach} for some conventional inflationary potentials related to the specific extended model in the Jordan frame. We compare all results with cosmic microwave background anisotropy observations coming from Planck 2018 and BICEP2/Keck array satellites in order to find the observational constraints on the parameters space of the models as well as their prediction from the spectral parameters. Eventually, we attempt to present a qualitative comparison between three classes of considered modified gravities using the obtained inflationary results. The aim is to select, in principle, cosmological signatures capable of discriminating among concurrent models in view to point out the representation of gravity, according with geometric invariants, which better addresses the early universe dynamics.

gr-qc↗

arXiv2024

Constant-roll inflation with a complex scalar field

We consider inflation with a constant rate of rolling in which a complex scalar field plays the role of inflaton during the inflationary epoch. We implement the inflationary analysis for an accredited angular speed $\dot{\theta}$ which satisfies our dynamical equations. Scalar and tensorial perturbations generated in the framework of constant roll inflation with a complex field are studied. In this respect, we find analytically solutions to the gauge invariant fluctuations, with which an expression for the scalar power spectrum together with its scalar index spectral in this scenario were found. By comparing the obtained results with the observations coming from the cosmic microwave background anisotropies, the constraints on the parameters space of the model and also its predictions are analyzed and discussed.

gr-qc↗

arXiv2023

Analytical Insights into Constant-Roll Condition: Extending the Paradigm to Non-Canonical Models

In this work, we explore the prospect of generalizing the constant-roll condition in canonical inflationary model to non-canonical models. To find a natural generalization, we focus on three manifestations of this condition and construct constant-roll models corresponding to each manifestation. These models are not equivalent but reduce to the familiar constant-roll model in canonical limit. To showcase the applicability of our generalized mechanism, we examine a specific class of non-canonical models, which can be viewed as extensions of k/G inflation. In these models sound speed is constant. We conduct a comparative study, and with an analytical examination of the model, specify instances when our constant-roll conditions yield dissimilar outcomes and when they exhibit analogies. We also apply our findings to scrutinize another kinetically driven inflationary model with varying sound speed. We demonstrate that each of our constant-roll conditions leads to a unique set of solutions. Afterward, we construct a four-stage constant-roll kinetically driven inflation that complies with CMB constraints, it sustains for a sufficiently long period of time, and finally gracefully exits. In this model the spectrum of curvature perturbations is enhanced in a brief phase of non-slow-roll inflationary evolution. Employing numerical methods, we analyse this scenario to elucidate how altering the constant-roll condition impacts the power spectrum and the model's dynamics.

gr-qc↗

arXiv2023

WGC and WCC for charged black holes with quintessence and cloud of strings

One of the important problems with the existence of weak gravity conjecture ($WGC$) is the violation of the cosmic censorship. Such a cosmic phenomena is important and consistent in general relativity. It means that for a charged black hole in four dimensions and in the normal state, the $WGC$ cannot hold due to the violation of the weak cosmic censorship conjecture ($WCCC$). But in this paper, in order to eliminate such a violation, we try to consider RN black hole at two cases, with quintessence, quintessence and cloud of strings in asymptotically flat and non-asymptotically flat. By using such metric background and applying restrictions on its parameters, we show that there will be a good compatibility between $WGC$ and $WCCC.$ Here, we can also see that quintessence case has less freedom in establishing compatibility than quintessence and cloud of strings. Because in the second case, the number of parameters is more and there is a degree of freedom to better establish the compatibility of two theories. Also, here we explain charged black hole in \emph{presence of quintessence and cloud of string} for $Q>M$ and use some interesting figures we show the corresponding the black hole has an event horizon.

hep-th↗

arXiv2023

On the constant roll complex scalar field inflationary models

In this paper we wish to point out the possibility of using a complex scalar field in a constant roll inflationary model, as needed for observational viability. We extend the idea of real field inflaton with constant rate of roll to a complex field, showing the feasibility of solving Einstein Klein-Gordon equations constrained by an \emph{appropriate} form of constant roll definition. As compared to the well known (two-parametric class of) real field models, there is one more degree of flexibility in constant roll inflationary solutions which is represented by an arbitrary function of time, $γ(t)$. We work with an arbitrary but constant function $γ$ (where $γ=0$ refers to the corresponding real field model) and find new inflationary class of potentials. In this class of models, the behavior of real and complex field models are similar in some aspects, for example the solutions with large constant roll parameter are not stable and should be considered as early time transients. These field solutions relax at late time on a dual attractor trajectory. However, complex fields phase space trajectories reach this stable regime after real fields. We performed the stability analysis on $γ$ function space solutions and found that dynamically stable trajectories in phase space are stable under $γ$ variations. We extended this study by considering multifield models of constant roll inflation with non-canonical kinetic terms. By enlarging the size of field space, we showed that a multifield constant roll model is dynamically a single field effective theory. If field space is parametrized by $N$ non-canonical fields, there will be $N$ free parameters in the potential that can be attributed to the interaction between the fields.

gr-qc↗

arXiv2023

G-constant-roll inflation

The constant-roll inflation in the context of Galilean inflation or G-inflation is analyzed. By considering a coupling function $G(φ,χ)\propto g(φ)\,χ^n$ for the model of G-inflation, we find different expressions for a suitable development of a model inflationary in the context of constant roll inflation. In order to obtain analytical solutions, we analyze two specific cases; $g(φ)=φ$ and $n=0$, i.e., $G(φ,χ)\propto\,φ$ and when $g(φ)=$ constant and $n=1$ with which $G(φ,χ)\propto\,χ$. In both cases, we find different expressions for the reconstruction of the background variables and the cosmological perturbations in the framework the constant roll inflation. We utilize recent astronomical observations to constrain the different parameters appearing in the stage of constant roll condition as well in the coupling function $G(φ,χ)$.

gr-qc↗

arXiv2023

WGC and WCCC of black holes with quintessence and cloud strings in RPS space

In this paper, we first introduce the Riesner-Nordstrom AdS (RN-AdS) black hole in presence of quintessence and cloud of strings. And then we use the Restrict Phase Space $(RPS)$ formalism in the corresponding black holes at the critical point with equation state $ω_q =-\frac{1}{3},-\frac{2}{3},-1 $. We take advantage from the central charge and Newton's constant with form of variable and prove the compatibility between weak gravity conjecture (WGC) and weak cosmic censorship conjecture (WCCC). Here, we show that when we use the usual RN-AdS black hole with $\frac{Q}{M}>\sqrt{G}$ condition, we have weak gravity conjecture. But, in that case, the WCCC is violated by WGC conjecture. In order to avoid such violation, we consider the Riesner-Nordstrom AdS (RN-AdS) black hole in the presence of quintessence, quintessence, and cloud strings. Here, also one can say that, by using the special central charge, chemical potential, appropriate quintessence, cloud strings parameters, and the effective Newton's constant with suitable conditions one can arrive the compatibility between weak gravity conjecture and weak cosmic censorship conjecture.

hep-th↗

arXiv2022

RPS Thermodynamics of Taub-NUT AdS Black Holes in the Presence of Central Charge and the Weak Gravity Conjecture

We study the thermodynamics of the Taub-NUT AdS black holes by Visser holographic method using AdS radius as a constant parameter under the restricted phase space approach. Instead of the variables $P$ and $V$, we deal with the central charge and also chemical potential as a unique couple of conjugate thermodynamic variables. We study some interesting properties of the Taub-NUT black holes e.g. supercritical phase equilibrium in the $T-S$ processes, $Q-Φ$ and the Hawking-Page phase transition in the $μ-C$ processes. As a consequence, the consistency of the weak gravity conjecture of multi-charge for Taub-NUT-AdS black holes at critical points is proved. We conclude that in the presence of the central charge and assuming $C=\frac{3}{8}\ell^2$, weak gravity conjecture could be satisfied in the Taub-NUT-AdS black holes with $\hat{n}\ll 1$.

hep-th↗

arXiv2022

Weak Gravity Conjecture from Conformal Field Theory: A Challenge from Hyperscaling Violating and Kerr-Newman-AdS Black Holes

We search for a possible relation between weak gravity conjecture and conformal field theory in the hyperscaling violating and Kerr-Newman-AdS black holes. We deal with the critical points of the black hole systems through the correlation function introduced in conformal field theory. We discuss the weak gravity conjecture conditions by the imaginary part of the energy obtained from the critical points and their poles. Under the assumptions $z=1$, $d=1$ and $θ\rightarrow0^{-}$, we link weak gravity conjecture to the hyperscaling violating black holes due to the existence of $r_{H}$ larger and smaller than one. As the second black hole system, we study the conditions of weak gravity conjecture for the Kerr-Newman-AdS black hole by rotation and radius parameters. Then, we show that the conditions of weak gravity conjecture are satisfied when the charged particle near the hyperscaling violating and Kerr-Newman black holes is $\frac{1}{a}$ with a ratio $\frac{a}{\ell}\ll 1$.

hep-th↗

arXiv2022

Slow-roll inflation in $f(Q)$ non-metric gravity

We discuss the cosmological inflation in the context of $f(Q)$ non-metric gravity, where $Q$ is the non-metric scalar. After introducing conformal transformations for $f(Q)$ gravity, we first focus on the potential-slow-roll inflation by studying the corresponding potentials for different forms of the function $f(Q)$ in the Einstein frame. Secondly, we investigate the Hubble-slow-roll inflation for three classes of inflationary potentials considered for the specific form $f(Q)\propto Q^{2}$, in the Jordan frame. We compare results in both approaches with observations coming from Planck and BICEP2/Keck array satellites. Observational constraints on the parameters space of the models are obtained as well.

gr-qc↗

arXiv2022

Anisotropic constant-roll inflationary scenario with complex quintessence field and swampland conjectures

In this paper, with the presence of swampland conjecture, we use the complex form of a scalar field and investigate the anisotropic constant-roll of an inflationary scenario. Recently, the complex quintessence field has been used to describe the accelerating expansion of the universe which has had interesting results connecting with various conditions. Therefore, the Lagrangian density of the quintessence field leads to obtaining the equations of the complex scalar field. Also, we use the anisotropic constant-roll conditions and the field equation and calculate the exact solutions for some cosmology parameters such as Hubble parameter and potential. Using the exact solution of potential and refined swampland conditions, we plot some figures. In that case, the figures lead us to have challenges between three concepts as complex quintessence, anisotropic constant-roll conditions, and swampland conjectures. Then we discuss their compatibility and incompatibility with corresponding results. Finally, we will analyze the complex quintessence in examining the inflationary scenario.

gr-qc↗

arXiv2022

Warm inflation with bulk viscous pressure for different solutions of an anisotropic universe

We study a warm inflationary model for different expansions assuming an anisotropic universe described by Bianchi I metric. The universe is filled with a scalar field or inflaton, radiation, and bulk viscous pressure. We carry out the inflationary analysis for different solutions of such universe in two different cases of the bulk viscosity coefficient $ξ$ and the dissipation coefficient $Γ$ as constant and variable parameters, respectively. We compare the obtained results with the recent observations, in order to find the observational constraints on the parameters space of the models. Moreover, we attempt to present a better judgment among the considered models by calculation of the non-linear parameter $f_{NL}$ describing the non-Gaussianity property of the models. Additionally, we investigate the warm inflationary models with viscous pressure from the Weak Gravity Conjecture approach, considering the swampland criteria.

gr-qc↗

arXiv2021

Quintessential constant-roll inflation

We investigate a single field model in the context of the constant-roll inflation in which inflaton moves down to the minimum point of the potential with a constant rate of rolling. We use a quintessential inflationary model obtained by a Lorentzian function which is dependent on the number of e-folds. We present the inflationary analysis for the model and find the observational constraints on the parameters space using the observations of CMB anisotropies \textit{i.e.} the Planck and Keck/array datasets. We find the observationally acceptable values of the Width of the Lorentzian function as $0.3<Γ\leq0.5$ at the $68\%$ CL and $Γ\leq0.3$ at the $95\%$ CL when $ξ=120$, $|β|=0.02$ and $N=60$. Also, we acquire the observationally favoured values of the amplitude of the Lorentzian function as $400<ξ\leq600$ at the $68\%$ CL and $ξ\leq400$ at the $95\%$ CL when $Γ=0.1$, $|β|=0.02$ and $N=60$. Moreover, we study the model from the Weak Gravity Conjecture approach using the swampland criteria.

astro-ph.CO↗

arXiv2021

Constant-roll $f(R)$ inflation compared with Cosmic Microwave Background anisotropies and swampland criteria

Inflationary models derived from $f(R)$ gravity, where the scalaron rolls down with a constant rate from the top to the minimum of the effective potential, are considered. Specifically, we take into account three $f(R)$ models \textit{i.e.}\,Starobinsky $R^{2}$, $R^{2p}$ and the logarithmic corrected models. We compare the inflationary parameters derived from the models with the observational data of CMB anisotropies \textit{i.e.} the Planck and Keck/array datasets in order to find observational constraints on the parameters space. We find that although our $f(R)$ constant-roll models for $γ=0$ show observationally acceptable values of $r$, they do not predict favoured values of the spectral index. In particular, we have $n_{s}>1$ for the Starobinsky $R^{2}$ and $R^{2p}$ models and $0.996<n_{s}<0.999$ for logarithmic model. Finally, we study the models from the point of view of Weak Gravity Conjecture adopting the swampland criteria.

gr-qc↗

arXiv2021

Nonminimal coupling inflation with constant slow roll

We study a single field inflationary model modified by a non-minimal coupling term between the Ricci scalar $R$ and the scalar field $φ$ in the context of constant-roll inflation. The first-order formalism is used to analyse the constant-roll inflation instead of the standard methods used in the literature. In principle, the formalism considers two functions of the scalar field, $W=W(φ)$ and $Z=Z(φ)$, which lead to the reduction of the equations of motion to first-order differential equations. The approach can be applied to a wide range of cosmological situations since it directly relates the function $W$ with Hubble parameter $H$. We perform the inflationary analysis for power-law and exponential couplings, separately. Then, we investigate the features of constant-roll potentials as inflationary potentials. Finally, we compare the inflationary parameters of the models with the observations of CMB anisotropies in view of realize a physically motivated model.

gr-qc↗

arXiv2021

The generalized $sl(2, R)$ and $su(1, 1)$ in non-minimal constant-roll inflation

In the present work, we consider the non-minimal coupling inflationary model in the context of the constant-roll idea which is investigated by the first-order formalism. We attempt to find the hidden symmetries behind the model by the Lie symmetry method. We supply this aim by using the symmetry features of the Heun function instead of Killing vector approach. We show that the hidden symmetries of the non-minimal constant-roll inflation in the cases of power-law and exponential couplings are characterized as a generalized form of $sl(2, R)$ and $su(1, 1)$ algebra, respectively.

gr-qc↗