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H. Moradpour

Publications and source records attributed to H. Moradpour.

At least 55 records · Page 3Linked to original sources
arXiv2019

Black hole thermodynamics in the Sharma-Mittal generalized entropy formalism

Using the Sharma-Mittal entropy, we study some properties of the Schwarzschild and Schwarzschild-de Sitter black holes. The results are compared with those obtained by attributing the Bekenstein entropy bound to the mentioned black holes. Our main results show that while the Schwarzschild black hole is always stable in the micro-canonical ensemble, it can be stable in the canonical ensemble if its mass is bigger than the mass of the coldest Schwarzschild black hole. A semi-classical analysis has also been used to find an approximate relation between the entropy free parameters. Throughout the paper, we use units $c=G=\hbar=k_B=1$, where $k_B$ denotes the Boltzmann constant.

physics.gen-ph↗
arXiv2019

The extended uncertainty principle inspires the Rényi entropy

We use the extended uncertainty principle (EUP) in order to obtain the Rényi entropy for a black hole (BH). The result implies that the non-extensivity parameter, appeared in the Rényi entropy formalism, may be evaluated from the considerations which lead to EUP. It is also shown that, for excited BHs, the Rényi entropy is a function of the BH principal quantum number, i.e. the BH quantum excited state. Temperature and heat capacity of the excited BHs are also investigated addressing two phases while only one of them can be stable. At this situation, whereas entropy is vanished, temperature may take a non-zero positive minimum value, depending on the value of the non-extensivity parameter. The evaporation time of excited BH has also been studied.

gr-qc↗
arXiv2019

Gravitational Collapse in Rastall Gravity

We study spherically symmetric gravitational collapse of a homogeneous perfect fluid in Rastall gravity. Considering a linear equation of state (EoS) for the fluid profiles, we examine the conditions under which the collapse scenario could end in a spacetime singularity. Depending on the model parameters, the singularity could be either naked or covered by a horizon. We find that a non-vanishing Rastall parameter could affect the formation of apparent horizon so that, naked singularities may form for those values of EoS parameter for which a homogeneous perfect fluid collapse in general relativity (GR) terminates at the black hole formation. Hence the visibility of the resulting singularity depends on the Rastall parameter. The solutions we obtain respect the weak energy condition (WEC) which is crucial for physical validity of the model.

gr-qc↗
arXiv2019

A note on cosmological features of modified Newtonian potentials

Considering some modified Newtonian potentials and the Hubble law in writing the total energy of a test mass located at the edge of a flat Friedmann-Robertson-Walker universe, we obtain several modified Friedmann equations. Interestingly enough, our study shows that the employed potentials, while some of them have some successes in modelling the spiral galaxies rotation curves, may also address an accelerated universe. This fact indicates that dark energy and dark matter may have some common origins and aspects.

physics.gen-ph↗
arXiv2019

Inflation in the Rényi cosmology

The description of the accelerated phases for the current and primordial cosmos has been obtained in the framework of Rényi cosmology. The results show that corrections to the Friedmann equations, originated from differences between Rényi and Bekenstein entropies, can describe both the current accelerated and inflationary eras. The effects of these corrections can be appointed to a hypothetical fluid which is classically unstable. Although inflation can be obtained in Rényi cosmology as the vacuum solution, the values found for the parameters of the hypothetical fluid which supports the current accelerated and primary inflationary cosmos differ from each other.

gr-qc↗
arXiv2019

Tsallis Agegraphic Dark Energy Model

Using the nonextensive Tsallis entropy and the holographic hypothesis, we propose a new dark energy (DE) model with time scale as infrared (IR) cutoff. Considering the age of the Universe as well as the conformal time as IR cutoffs, we investigate the cosmological consequences of the proposed DE models and study the evolution of the Universe filled by a pressureless matter and the obtained DE candidates. We find that although these models can describe the late time acceleration and the density, deceleration and the equation of state parameters show satisfactory behavior by themselves, however, these models are classically unstable unless the interaction between the two dark sectors of the Universe is taken into account. In addition, the results of the existence of a mutual interaction between the cosmos sectors are also addressed. We find out that the interacting models are stable at the classical level which is in contrast to the original interacting agegraphic dark energy models which are classically unstable [1].

physics.gen-ph↗
arXiv2019

$4$-index theory of gravity and its relation with the violation of the energy-momentum conservation law

Recently, a $4$-index generalization of the Einstein theory is proposed by Moulin (Eur. Phys. J. C 77, 878 (2017)). Using this method, we find the most general $2$-index field equations derivable from the Einstein-Hilbert action. The application of Newtonian limit, the role of gravitational coupling constant and the effects of the properties of ordinary energy-momentum tensor in obtaining a $4$-index gravity theory have been studied. We also address the results of building Weyl free $4$-index gravity theory. Our study displays that both the Einstein and Rastall theories can be obtained as the subclasses of a $4$-index gravity theory which shows the power of $4$-index method in unifying various gravitational theories. It is also obtained that the violation of the energy-momentum conservation law may be allowed in $4$-index gravity theory, and moreover, the contraction of $4$-index theory generally admits a non-minimal coupling between geometry and matter field in the Rastall way. This study also shows that, unlike the Einstein case, the gravitational coupling constant of $4$-index Rastall theory generally differs from that of the ordinary $2$-index Rastall theory.

physics.gen-ph↗
arXiv2019

Effects of Anisotropy on the Sign-Changeable Interacting Tsallis Holographic Dark Energy

A spatially homogeneous and anisotropic Bianchi type I universe is considered while it is filled by pressureless dark matter (DM) and Tsallis holographic dark energy (DE) interacting with each other throughout a sign-changeable mutual interaction. Various infra-red (IR) cutoffs are studied, and it has been obtained that while the current universe can classically be stable for some cases, all models display classical instability by themselves at the future ($z\rightarrow-1$). Moreover, we find out that some models can cross the phantom line. In order to have a more comprehensive study, the statefinder diagnostic and the $ω_D-ω^{\prime}_{D}$ plane are also investigated showing that the model parameters significantly affect the evolution trajectories in the $r-s$ and $ω_D-ω^{\prime}_{D}$ planes.

physics.gen-ph↗
arXiv2018

Cosmological Implications of the Generalized Entropy Based Holographic Dark Energy Models in Dynamical Chern-Simons Modified Gravity

Recently, Tsallis, Rényi and Sharma-Mitall and entropies have widely been used to study the gravitational and cosmological setups. We consider a flat FRW universe with linear interaction between dark energy and dark matter. We discuss the dark energy models using Tsallis, Rényi and Sharma-Mitall entropies in the framework of Chern-Simons modified gravity. We explore various cosmological parameters (equation of state parameter, squared sound of speed ) and cosmological plane ($ω_{d}-ω_{d}'$, where $ω_{d}'$, is the evolutionary equation of state parameter). It is observed that the equation of state parameter gives quintessence-like nature of the universe in most of the cases. Also, the squared speed of sound shows stability of the models for Tsallis, Rényi dark energy model while unstable behavior for Sharma-Mitall dark energy model. The $ω_{d}-ω_{d}'$ plane represents the thawing region for all dark energy models.

physics.gen-ph↗
arXiv2018

A Note on Tsallis Holographic Dark Energy

We explore the effects of considering various infrared (IR) cutoffs, including the particle horizon, Ricci horizon and Granda-Oliveros (GO) cutoffs, on the properties of Tsallis holographic dark energy (THDE) model, proposed inspired by Tsallis generalized entropy formalism \cite{THDE}. Interestingly enough, we find that for the particle horizon as IR cutoff, the obtained THDE model can describe the accelerated universe. This is in contrast to the usual HDE model which cannot lead to an accelerated universe, if one consider the particle horizon as IR cutoff. We also investigate the cosmological consequences of THDE under the assumption of a mutual interaction between the dark sectors of the Universe. It is shown that the evolution history of the Universe can be described by these IR cutoffs and thus the current cosmic acceleration can also been realized. The sound instability of THDE models for each cutoff are also explored, separately.

gr-qc↗
arXiv2018

Tsallis holographic dark energy in the brane cosmology

We study some cosmological features of Tsallis holographic dark energy (THDE) in Cyclic, DGP and RS II braneworlds. In our setup, a flat FRW universe is considered filled by a pressureless source and THDE with the Hubble radius as the IR cutoff, while there is no interaction between them. Our result shows that although suitable behavior can be obtained for the system parameters such as the deceleration parameter, the models are not always stable during the cosmic evolution at the classical level.

physics.gen-ph↗
arXiv2018

Thermodynamic approach to holographic dark energy and the Rényi entropy

Using the first law of thermodynamics, we propose a relation between the system entropy ($S$) and its IR ($L$) and UV ($Λ$) cutoffs. In addition, applying this relation to the apparent horizon of flat FRW universe, whose entropy meets the Rényi entropy, a new holographic dark energy model is addressed. Thereinafter, the evolution of the flat FRW universe, filled by a pressureless source and the obtained dark energy candidate, is studied. In our model, there is no mutual interaction between the cosmos sectors. We find out that the obtained model is theoretically powerful to explain the current accelerated phase of the universe. This result emphasizes that the generalized entropy formalism is suitable for describing systems including the long-range interactions such as gravity.

physics.gen-ph↗
arXiv2018

Tsallis holographic dark energy in the Brans-Dicke cosmology

Using the Tsallis generalized entropy, holographic hypothesis and also considering the Hubble horizon as the IR cutoff, we build a holographic model for dark energy and study its cosmological consequences in the Brans-Dicke framework. At first, we focus on a non-interacting universe, and thereinafter, we study the results of considering a sign-changeable interaction between the dark sectors of the cosmos. Our investigations show that, compared with the flat case, the power and freedom of the model in describing the cosmic evolution is significantly increased in the presence of the curvature. The stability analysis also indicates that, independent of the universe curvature, both the interacting and non-interacting cases are classically unstable. In fact, both the classical stability criterion and an acceptable behavior for the cosmos quantities, including the deceleration and density parameters as well as the equation of state, are not simultaneously obtainable.

gr-qc↗
arXiv2018

Energy definition and dark energy: a thermodynamic analysis

Accepting the Komar mass definition of a source with energy-momentum tensor $T_{μν}$, and using the thermodynamic pressure definition, we find a relaxed energy-momentum conservation law. Thereinafter, we study some cosmological consequences of the obtained energy-momentum conservation law. It has been found out that the dark sectors of cosmos are unifiable into one cosmic fluid in our setup. While this cosmic fluid impels the universe to enter an accelerated expansion phase, it may even show a baryonic behavior by itself during the cosmos evolution. Indeed, in this manner, while $T_{μν}$ behaves baryonically, some parts of it, namely $T_{μν}(e)$ which is satisfying the ordinary energy-momentum conservation law, are responsible for the current accelerated expansion.

physics.gen-ph↗
arXiv2018

Implications of the Generalized Entropy Formalisms on the Newtonian Gravity and Dynamics

Employing the Verlinde's hypothesis, and considering two well-known generalized entropy formalisms, two modifications to the Newtonian gravity are derived. In addition, it has been shown that the generalized entropy measures may also provide theoretical basis for the Modified Newtonian Dynamics (MOND) theory and generate its modified forms. Since these entropy measures are also successful in describing the current accelerated universe, our results indicate that the origin of dark sectors of cosmos may be unified to meeting the generalized entropy measures instead of the Boltzmann-Gibbs entropy by the gravitational systems due to the long-range nature of gravity.

physics.gen-ph↗
arXiv2018

Implications of maximum acceleration on dynamics

Considering the corrected Unruh temperature as well as the entropic force perspective of gravity, we are able to derive the modification of the Newton's law of gravity. In addition, we also investigate the effects of the highest achievable acceleration correction to the Unruh temperature on the Poisson equation. Moreover, we address modifications to the Newtonian cosmology as well as the Friedmann equations corresponding to the upper bound for acceleration.

gr-qc↗
arXiv2018

Tsallis Holographic Dark Energy

Employing the modified entropy-area relation suggested by Tsallis and Cirto \cite{non3}, and the holographic hypothesis, a new holographic dark energy (HDE) model is proposed. Considering a flat Friedmann-Robertson-Walker (FRW) universe in which there is no interaction between the cosmos sectors, the cosmic implications of the proposed HDE are investigated. Interestingly enough, we find that the identification of IR-cutoff with the Hubble radius, can lead to the late time accelerated Universe even in the absence of interaction between two dark sectors of the Universe. This is in contrast to the standard HDE model with Hubble cutoff, which does not imply the accelerated expansion, unless the interaction is taken into account.

gr-qc↗