arXiv subjects

J. Sadeghi

Publications and source records attributed to J. Sadeghi.

At least 37 records · Page 2Linked to original sources

arXiv2022

Traversable wormhole in logarithmic $f(R)$ gravity by various shape and redshift functions

We study the traversable wormhole solutions for a logarithmic corrected $f(R)$ model by considering two different statements of shape $b(r)$ and redshift $Φ(r)$ functions. We calculate the parameters of the model including energy density $ρ$, tangential pressure $P_{t}$ and radial pressure $P_{r}$ for the corresponding forms of the functions. Then, we investigate different energy conditions such as null energy condition, weak energy condition, dominant energy condition and strong energy condition for our considered cases. Finally, we explain the satisfactory conditions of energy of the models by related plots.

gr-qc↗

arXiv2022

Smeared Mass Source Wormholes in Modified $f(R)$ Gravity with the Lorentzian Density Distribution Function

Wormholes are speculative structures linking disparate space-time points. Their geometry can be obtained by solving Einstein equations with tolerating the violation of null energy conditions. Recently, many researchers have studied different wormholes according to different criteria, and they achieved remarkable results. In this paper, we investigate a series of exact solutions of the static wormhole with smeared mass source geometry in modified $f(R)$ gravity theories. In fact, we consider the Lorentzian density distribution which is coming from a particle-like source. To be more specific, the modified gravity models we consider here are some power laws. We compute resulting solutions according to the wormhole field equations. We also specify parameters such as the radial pressure and transverse pressure as well as various energy conditions such as null energy conditions, weak energy conditions and strong energy conditions. Finally, by plotting some figures, in addition to identifying the wormhole throat, we describe the results of either the violation or the satisfaction of the energy conditions completely.

gr-qc↗

arXiv2022

Tsallis and Kaniadakis holographic dark energy with Complex Quintessence theory in Brans-Dicke cosmology

In this paper, by considering the complex form of the quintessence model, we study two different dynamic structures of holographic dark energy as Tsallis and Kaniadakis in the framework Brans-Dicke cosmology. In each setup, we employ non-interacting and interacting cases and calculate some cosmological parameters such as the equation of state $ω$. We also discuss the $ ω- ω'$ behavior. By modifying the potential and studying the scalar field dynamics, we examine the complex quintessence cosmology. In addition, considering the two parts of the quintessence field effects, i.e., real and complex, and considering the fractional energy density $ Ω_{D} $, we examine whether it can describe a real universe or not. We also specify that the fractional energy density can not be arbitrary between 0 and 1. In other words, it depends on the Tsallis, Kaniadakis, and Brans-Dicke cosmology free parameters. We create a relationship between the fractional energy density and other parameters introduced in the text such as $δ$, $b^{2}$, $α$ and $β$ for each model separately. Finally, we compare the obtained results of models to each other and the latest observable data.

gr-qc↗

arXiv2022

Pleasant behavior of swampland conjectures in the face of specific inflationary models

Recently researchers have been studying various conditions as swampland criteria in cosmological implications. They have studied many inflation models with different swampland conditions. Occasionally these conjectures are modified and lead to fascinating points. The swampland conjecture is in contradiction with the slow-roll single-field inflation model. So in this paper, we will briefly describe resolving this contradiction with the special method; we consider a small parameter λ with respect to a coupling function in inflation and resolve this important antithesis. Then we will discuss a new point of view in studying inflation models according to the swampland criteria. Therefore we introduce some inflation models and challenge the swampland criteria. We specify the allowable range of cosmological parameters as the tensor-to-scalar ratio (r) the scalar spectral index (ns) according to the latest observable data. Next, we study the new constraints to determine the compatibility or incompatibility of the corresponding model with the swampland conjectures. Finally, we express the results and compare them with the latest observational data by plotting some figures.

astro-ph.CO↗

arXiv2022

The correlation of WGC and Hydrodynamics bound with $R^4$ correction in the charged AdS$_{d+2}$ black brane

In this paper, we focus on the possible correlation between conjectures KSS bound and weak gravity conjecture (WGC). The hydrodynamic values KSS bound and weak gravity conjecture constraint the low-energy effective field theory. These conjectures identify UV complete theories. We give four, six and eight order derivative corrections to corresponding action and employ the hyperscaling violating charged AdS$_{d+2}$ black brane solution. These corrections lead us to find correlation between conjectures KSS bound and weak gravity conjecture. We see that, with increasing perturbation correction, this correlation is more likely to appear. We consider dynamical constant $z=1$, $d=5$ and obtain the range of hyperscaling violation exponent $d+z-2\leqθ\leq d+z-1$ for the above mentioned black brane. Here, we show that higher derivative corrections reduce the ratio of $\frac{M}{Q}$ to extremal black holes. Likewise, we also obtain the universal relaxation bound $τ\geq \frac{1}{πT}$ and KSS bound $\fracη{s}\geq \frac{1}{4π}$ for our model. The results indicate that there is a possibility of a relationship between the two conjectures. Our studies also show the consistency of the WGC and the KSS bound conjectures for all corrections (except curvature-cubed, $β_2$) in the extremal and near-extremal condition.

hep-th↗

arXiv2022

Weak Gravity Conjecture, Black Branes and Violations of Universal Thermodynamic Relation

The universal thermodynamic relations between corrections to entropy and extremality for various black holes solutions have been studied. In this regard, we hereby consider a number of different black brane solutions in different structures for perturbative corrections to general relativity. These are, namely, black brane solution in Rastall AdS massive gravity, Einstein-Yang-Mills AdS black brane solution in massive gravity and general anisotropic black brane in Horava-Lifshitz gravity. We calculate both the entropy and extremality bound by introducing a small constant correction to the action. Remarkably, we find that black brane violates the universal thermodynamic relations. In other words, a universal relation between corrections to entropy and extremality are not valid in the black brane structure.

gr-qc↗

arXiv2021

Inflation, Swampland, and Landscape

In this paper, we evaluate a new refined de Sitter (dS) conjecture perspective. This conjecture provides interesting conditions in studying various inflationary models. Therefore we challenge refined dS conjecture with a general method for analyzing the potentials with the inflectional point near the top (V''= 0). We find the compatible spaces for our inflationary model that live out of swampland according to the latest observational data, such as Planck 2018.

gr-qc↗

arXiv2021

Swampland Conjecture and Inflation Model from Brane Perspective

Over the past few decades, inflation models have been studied by researchers from different perspectives and conditions in order to introduce a model for the expanding universe. In this paper, we introduce a modified $f(R)$ gravitational model as ($R+γR^{p}$) in order to examine a new condition for inflation models. Given that our studies are related to a modified $f(R)$ gravitational model on the brane, therefore we will encounter modified cosmological parameters. So, we first introduce these modified cosmological parameters such as spectral index, a number of e-folds and etc. Then, we apply these conditions to our modified $f(R)$ gravitational model in order to adapt to the swampland criteria. Finally, we determine the range of each of these parameters by plotting some figures and with respect to observable data such as Planck 2018.

gr-qc↗

arXiv2021

Kiselev/CFT correspondence and black hole thermodynamics

In this paper, we study the thermodynamic properties of Kiselev black hole and its holographic dual. We obtain the thermodynamic product formula for the Kiselev black hole. We consider Kiselev Black hole surrounded by both dust and radiation. We find that the area (or entropy) product formula for both cases is mass-independent as well as the case in the Einstein gravity, which interpreted as universal quantity. Moreover, we calculate the black hole entropy bound for both the inner and outer horizons. Furthermore, we show that the central charges of the left and right moving sectors are not the same via universal thermodynamic relations. Such universal relations lead us to calculate the central charge of conformal field theory (CFT). Finally, we see that the left and right of central charge of CFT are same.

hep-th↗

arXiv2021

Geometrical thermodynamics and P-V criticality of charged accelerating AdS black holes

The unusual asymptotic structure of the accelerating black holes led to ambiguity in their geometric characteristics and thermodynamic behavior. Motivated by the interesting properties of such black holes and the significant role of electric charge and string tension on their structure, we study the thermodynamic behavior of these black holes by two methods and examine the changes of free parameters on the thermal behavior of the black holes. First, we investigate phase transition and thermal stability of the system through the use of heat capacity in the non-extended phase space. We examine the effects of electric charge, string tension and the cosmological constant on the phase transition and stability of the system. We also find that to have a phase transition, we have to apply some constraints on the free parameters. Then, we employ the geometrical thermodynamic (GT) method to study phase transition and compare the obtained results with those of the heat capacity. Next, we work in the extended phase space by considering the cosmological constant as a dynamical pressure and evaluate the existence of van der Waals like phase transition. We obtain critical quantities and study the effective role of electric charge and string tension on these quantities. Finally, we make use of the GT method in the extended phase space and find that the results of the GT method, heat capacity and $P-V$ diagram lead to a consistent conclusion.

hep-th↗

arXiv2021

Constant-roll inflation from a fermionic field

We study the inflationary period driven by a fermionic field which is non-minimally coupled to gravity in the context of the constant-roll approach. We consider the model for a specific form of coupling and perform the corresponding inflationary analysis. By comparing the result with the Planck observations coming from CMB anisotropies, we find the observational constraints on the parameters space of the model and also the predictions the model. We find that the values of $r$ and $n_{s}$ for $-1.5<β\leq-0.9$ are in good agreement with the observations when $|ξ|=0.1$ and $N=60$.

gr-qc↗

arXiv2021

Energy spectrum of massive Dirac particles in gapped graphene with Morse potential

In this paper, we study the massive Dirac equation with the presence of the Morse potential in polar coordinate. The Dirac Hamiltonian is written as two second-order differential equations in terms of two spinor wavefunctions. Since the motion of electrons in graphene is propagated like relativistic fermionic quasi-particles, then one is considered only with pseudospin symmetry for aligned spin and unaligned spin by arbitrary $k$. Next, we use the confluent Heun's function for calculating the wavefunctions and the eigenvalues. Then, the corresponding energy spectrum obtains in terms of $N$ and $k$. Afterward, we plot the graphs of the energy spectrum and the wavefunctions in terms of $k$ and $r$, respectively. Moreover, we investigate the graphene band structure by a linear dispersion relation which creates an energy gap in the Dirac points called gapped graphene. Finally, we plot the graph of the valence and conduction bands in terms of wavevectors.

quant-ph↗

arXiv2021

Anisotropic constant-roll inflation with noncommutative model and swampland conjectures

In this paper, we study a constant-roll inflationary model in the presence of a noncommutative parameter with a homogeneous scalar field minimally coupled to gravity. The specific noncommutative inflation conditions proposed new consequences. On the other hand, we use anisotropic conditions and find new anisotropic constant-roll solutions with respect to noncommutative parameter. Also, we will plot some figures with respect to the specific values of the corresponding parameter and the swampland criteria which is raised from the exact potential obtained from the constant-roll condition. Finally, different figures lead us to analyze the corresponding results and also show the effect of above mentioned parameter on the inflationary model.

hep-th↗

arXiv2020

Thermal width and entropic force of a moving quarkonia with charged deformed AdS background

The entropic force and the imaginary part of potential experienced by a moving quarkonia in a plasma, are computed in a charged deformed AdS background by holography approaches. In that case, the axis of the moving quarkonia has an arbitrary angle with wind. We show that the charge in deformed AdS background deeply effect on the increasing entropic force and decreasing thermal width. So, the dissociation length of the meson becomes short when charge is increasing. In addition, the entropic force and thermal width are increased by the deformation of the space. Therefore the deformation parameter in such background leads us to find process with arbitrary dissociation length of quark- anti quark.

hep-th↗

arXiv2019

Logarithmic corrected $f(R)$ gravitational model and swampland conjectures

In this paper, we use corrected $f(R)$ gravitational model which is polynomial function with a logarithmic term. In that case, we employ the slow-roll condition and obtain the number of cosmological parameter. This help us to verify the swampland conjectures which is guarantee the validation of low - energy quantum field theory. The obtained results shown that the corresponding model is consistent with the swampland conjectures. Also the upper and lower limit of the parameter \textcolor{red}{$n$} are \textcolor{blue}{0.15} and \textcolor{blue}{0.0033}. Finally, by using scalar spectrum index $ n_{s} $ and tensor to scalar ratio $ r $ relations and compared with Planck 2018 empirical data, we obtain the coefficients $α$,$β$ and $γ$. Also, the corresponding results are creaked by several figures, literature and also plank 2018 data.

hep-th↗

arXiv2019

Phase transition of modified Horndeski gravity with new method

In this paper, we investigate the critical points of the $ P-V $ diagram and the phase transitions of the Horndenski black holes. In fact, the usual Horndeski black holes do not have $ P-V $ critical points, hence do not show any phase transitions. However, we successfully modify the Horndeski black hole solution to obtain such a phase transition behavior. This modified black holes solution is satisfied by the equation of state of liquid-gas phase transition. Also, we study the thermodynamics of our modified Horndeski black hole by applying a new method based on the equation of state that originated from the slope of temperature versus entropy. This new prescription provides us a simple powerful way to study the critical behavior and the phase transition of the black holes and concludes the novel results. The analytical interpretation of possible phase transition points leads us to set some nonphysical range on the horizon radius for the black hole.

gr-qc↗

arXiv2019

Charged accelerating AdS black hole of $f(R)$ gravity and the Joule-Thomson expansion

In this paper, the thermodynamical properties and the phase transitions of the charged accelerating anti-de Sitter (AdS) black holes are investigated in the framework of the $f(R)$ gravity. By studying the conditions for the phase transitions, it has been shown that the $P-V$ criticality and the van der Waals like phase transitions can be achieved for $ T \approx T_{c} $. The Joule-Thomson expansion effects are also examined for the charged accelerating AdS black holes of the $f(R)$ gravity. Here, we derive the inversion temperatures as well as the inversion curves. Then, we determine the position of the reverse point for different values of mass $M$ and parameter $b$ for the corresponding black hole. At this point, the Joule-Thompson coefficient is zero. So, in such case, we can say that such point is very important for the finding of cooling - heating regions. Finally, we calculate the ratio of minimum inversion temperature and critical temperature for such black hole.

gr-qc↗

arXiv2019

The $QCD$ parameters are affected by power-law Maxwell field

In this paper, we introduce Einstein power-law Maxwell-Scalar gravity with Liouvill potential in n + 1 dimension. The corresponding metric background or solution of above mentioned action lead us to achieve some thermodynamics quantities of black hole. On the other hand, we take information from $AdS/CFT$ correspondence and investigate some parameters in $QCD$ as jet quenching and entropic force. We note that two obtained parameters give some important information about $Q\bar{Q}$ moving in $QGP$ media and dissociation length of quark-antiquark. Finally, we have several figures for the jet quenching parameter and entropic force. The corresponding figures show how two parameters are affected by power law Maxwell field with different of $p$.

hep-th↗