arXiv subjects

H. Moradpour

Publications and source records attributed to H. Moradpour.

At least 37 records · Page 2Linked to original sources

arXiv2022

A note on inflation in dRGT massive gravity

Although the dRGT massive gravity successfully explains the late-time cosmic acceleration, it cannot justify inflation. On the other hand, and in the frameworks of General Relativity and modified gravity, the interests and attempts to describe dark energy and inflation by using Lagranginas, which may have pole, have recently been enhanced. Subsequently, we are going to show that this kind of Lagrangian may justify inflation in the framework of dRGT massive gravity. The study is done focusing on the power and exponential potentials, and the results show a plausible consistency with the Planck 2018 data and its combination with BK18 and BAO.

gr-qc↗

arXiv2022

Generalized uncertainty principle and quantum non-locality

The emergence of the generalized uncertainty principle and the existence of a non-zero minimal length are intertwined. On the other hand, the Heisenberg uncertainty principle forms the core of the EPR paradox. Subsequently, here, the implications of resorting to the generalized uncertainty principle (or equally, the minimal length) instead of the Heisenberg uncertainty principle on the quantum non-locality are investigated through focusing on the Franson experiment in which the uncertainty relation is the backbone of understanding and explaining the results.

quant-ph↗

arXiv2021

Quantum gravity and the square of Bell operators

The Bell's inequality is a strong criterion to distinguish classical and quantum mechanical aspects of reality. Its violation is the net effect of the existence of non-locality in systems, an advantage for quantum mechanics (QM) over classical physics. The quantum mechanical world is under the control of the Heisenberg uncertainty principle (HUP) that is generalized by quantum gravity (QG) scenarios, called generalized uncertainty principle (GUP). Here, the effects of GUP on the square of Bell operators of qubits and qutrits are studied. The achievements claim that the violation quality of the square of Bell inequalities may be a tool to get a better understanding of the quantum features of gravity. In this regard, it is obtained that the current accuracy of the Stern-Gerlach experiments implies upper bounds on the values of the GUP parameters.

quant-ph↗

arXiv2021

Minimal length implications on the Hartree-Fock theory

Hartree-Fock approximation suffers from two inabilities including i) the divergence of electron Fermi velocity , and ii) existence of bandwidth not confirmed experimentally. Here, we study the effects of minimal length on the ground state energy of the electron gas in the Hartree-Fock approximation. Our results indicate that considering some mathematical terms, similar to those of used for the minimal length correction to the Hamiltonian of system, can eliminate the weaknesses of Hartree-Fock approximation. These corrections, on the other hand, can be considered as relativistic corrections of electron in solids. Physically, it is obtained that electrons in metals can be employed to test the quantum gravity scenario, if the value of its parameter ($β$) lies within the range of 2 to 10, depending on the used metal. Indeed, the latter addresses an upper bound on $β$ which is comparable with previous works meaning that these types of systems may be employed in testing quantum gravity scenarios. To overcome the infinite Fermi velocity in Hartree-Fock method, the screening potential is used based on the Lindhard theory. We also found that considering the generalized Heisenberg uncertainly leads to some additional oscillating terms in the Friedel oscillations.

cond-mat.other↗

arXiv2021

Non-Singular Collapse Scenario From Matter-Curvature Coupling

In the present work we study spherically symmetric gravitational collapse of a homogeneous perfect fluid in the context of Generalized Rastall Theory (GRT). In this modified version of the original {Rastall Gravity (RG)}, the coupling parameter which is a representative of matter-curvature interaction is no longer a constant parameter. Such a dynamic coupling may play the role of dark energy which is responsible for the present accelerating expansion of the Universe. Assuming then a linear equation of state (EoS) for the fluid profiles, we seek for physically reasonable collapse scenarios in which the spacetime singularity that occurs in general relativity (GR) is replaced by a non-singular bounce. We therefore find that depending on model parameters, the collapse process which starts from regular initial data, will halt at a minimum value for the scale function and then turns into an expansion at a finite time. We further find that there exists a minimum value for the initial radius of collapsing object so that for radii smaller than this minimum radius, formation of apparent horizon can be avoided and hence the bounce can be visible to the observers within the Universe. We also compare our results to quantum corrected collapse scenarios and find that the mutual interaction between matter and geometry can play the role of quantum corrections to energy density.

physics.gen-ph↗

arXiv2021

The extended phase space thermodynamics of Planck-scale-corrected Reissner-Nordström-anti-de Sitter black hole

We analyze the effect of Planck-scale modified radiation equation of state on the Reissner-Nodström-anti-de Sitter black hole inspired by Kiselev's ansatz. Deformed thermodynamic quantities are found, phase transitions and black holes as heat engines are described for the Carnot and square cycles. Non-trivial differences between linear and quadratic Planck-scale corrections are discussed in detail.

gr-qc↗

arXiv2021

Hubble tension bounds the GUP and EUP parameters

In recent years the discrepancy in the value of the Hubble parameter has been growing. Recently, there are works supporting the proposal that the uncertainty principles can solve the Hubble tension. Motivated by this proposal, we work with an isotropic and homogeneous FRW universe, obtain its Hamiltonian equations, and thus, the Hubble parameter through the first Friedmann equation. In the context of GUP and EUP models, the Hubble parameter is modified. Since the fingerprints of quantum gravity are imprinted on the CMB, we consider the GUP/EUP-modified Hubble parameter in the first Friedmann equation to be the one measured by the Planck collaboration which uses the CMB data. The unmodified Hubble parameter in the first Friedmann equation is considered to be the one measured by the HST group which uses the SNeIa data. Therefore, upper bounds for the dimensionless parameters of GUP and EUP are obtained.

gr-qc↗

arXiv2021

Kaniadakis Holographic Dark Energy in non-flat Universe

In present research, we construct Kaniadakis holographic dark energy (KHDE) model within a non-flat Universe by considering the Friedmann-Robertson-Walker (FRW) metric with open and closed spatial geometries. We therefore investigate cosmic evolution by employing the density parameter of the dark energy (DE), the equation of state (EoS) parameter and the deceleration parameter (DP). The transition from decelerated to accelerated expanding phase for the KHDE Universe is explained through dynamical behavior of DP. With the classification of matter and DE dominated epochs, we find that the Universe thermal history can be defined through the KHDE scenario, and moreover, a phantom regime is experienceable. The model parameters are constrained by applying the newest $30$ data cases of $H(z)$ measurements, over the redshift span $0.07 \leq z \leq 2.36$, and the distance modulus measurement of the recent Union $2.1$ data set of type Ia supernovae. The classical stability of KHDE model has also been addressed.

physics.gen-ph↗

arXiv2021

Tsallis uncertainty

It has been recently shown that the Bekenstein entropy bound is not respected by the systems satisfying modified forms of Heisenberg uncertainty principle (HUP) including the generalized and extended uncertainty principles, or even their combinations. On the other, the use of generalized entropies, which differ from Bekenstein entropy, in describing gravity and related topics signals us to different equipartition expressions compared to the usual one. In that way, The mathematical form of an equipartition theorem can be related to the algebraic expression of a particular entropy, different from the standard Bekenstein entropy, initially chosen to describe the black hole event horizon, see E. M. C. Abreu et al., MPLA 32, 2050266 (2020). Motivated by these works, we address three new uncertainty principles leading to recently introduced generalized entropies. In addition, the corresponding energy-time uncertainty relations and Unruh temperatures are also calculated. As a result, it seems that systems described by generalized entropies, such as those of Tsallis, do not necessarily meet HUP and may satisfy modified forms of HUP.

gr-qc↗

arXiv2020

Structure Formation in Generalized Rastall Gravity

Recently a modified version of Rastall theory of gravity has been introduced in which a varying coupling parameter could act as dark energy (DE) and thus, it can be held responsible for the current accelerated expansion of the Universe. Motivated by this modification, we study here the evolution of linear and non-linear perturbations in the matter content of the Universe, utilizing spherically symmetric top-hat collapse scenario. The exact solutions we obtain in linear regime show that as the Universe evolves, matter density perturbations grow and reach a maximum value at a certain redshift after which these perturbations start decreasing towards a finite positive value at the present time. Depending on model parameters, exact oscillatory solutions can be also found representing that matter perturbations could experience either overdense and underdense regions during the dynamical evolution of the Universe. Numerical solutions in non-linear regime show that the amplitude of perturbations grow much faster than the linear one and diverges at a critical redshift. However, the formation of collapsed structures is delayed as compared to $Λ${\rm CDM} model. It is found that the running mutual interaction between matter and geometry, encoded in the variable Rastall coupling parameter, could drastically affect the dynamics of matter perturbations and their growth rate during the evolution of the Universe.

physics.gen-ph↗

arXiv2020

Holographic dark energy in Rastall theory

Bearing holographic dark energy hypothesis in mind, the ability of vacuum energy in describing the current accelerated universe is studied in the framework of Rastall theory. Here, in addition to the ordinary approach in which it is expected that this energy plays the role of dark energy, we also address a new approach where the sum of this energy and Rastall term is responsible for the current accelerated universe. We also investigate the cosmological outcomes of using Tsallis entropy in quantifying the energy of fields in vacuum for both mentioned approaches. The implications of considering an interaction between the various segments of cosmic fluid have been addressed in each studied cases. The normalized Hubble parameter for the models have also been plotted and compared that with the H(z) data consisting of 41 data points in the redshift range of 0.07< z<2.36.

gr-qc↗

arXiv2020

Effects of minimal length on Berry phase and spin-orbit interactions

The effect of Generalized Uncertainty Principle (GUP) on Berry phase is studied using the perturbation approach and up to the first order of approximation. Thereinafter, the obtained results are extended to a quantum ring in which two types of spin-orbit interactions, including Rashba and Dresselhaus interactions, can be felt by electrons. Comparing the final results with the accuracy of Berry phase detectors, one can find an upper bound on GUP parameter as $β_{0}<10^{46}$ and $β_{0}<10^{51}$ from Rashba and Dresselhaus interactions, respectively, in agreement with previous results.

hep-th↗

arXiv2020

Heat capacity of Holographic screen inspires MOND theory

It is argued that Planck mass may be considered as a candidate for the mass content of degrees of freedom of holographic screen. In addition, employing the Verlinde hypothesis on emergent gravity and considering holographic screen degrees of freedom as a $q$-deformed fermionic system, it is obtained that the heat capacity per degree of freedom inspires the MOND interpolating function. Moreover, the MOND acceleration is achieved as a function of Planck acceleration. Both ultra-relativistic and non-relativistic statistics are studied.

hep-th↗

arXiv2020

Thin-shell wormholes in Rastall gravity

We constructed thin-shell wormholes using the well-known "cut and paste" technique for static black holes sourced by an anisotropic fluid within the context of the Rastall gravity. Using the generalized Lanczos equations we explored the energy conditions at the wormhole's throat along with the traversability condition. We determined the stability regions when the wormhole configurations are surrounded by different bulk sources and obtained that the stability regions are considerably modified due to the non-trivial dependence of the Rastall parameter in the effective potential energy.

gr-qc↗

arXiv2020

Generalized entropies and corresponding holographic dark energy models

Using Tsallis statistics and its relation with Boltzmann entropy, the Tsallis entropy content of black holes is achieved, a result in full agreement with a recent study (Phys. Lett. B 794, 24 (2019)). In addition, employing Kaniadakis statistics and its relation with that of Tsallis, the Kaniadakis entropy of black holes is obtained. The Sharma-Mittal and Rényi entropy contents of black holes are also addressed by employing their relations with Tsallis entropy. Thereinafter, relying on the holographic dark energy hypothesis and the obtained entropies, two new holographic dark energy models are introduced and their implications on the dynamics of a flat FRW universe are studied when there is also a pressureless fluid in background. In our setup, the apparent horizon is considered as the IR cutoff, and there is not any mutual interaction between the cosmic fluids. The results indicate that the obtained cosmological models have $i$) notable powers to describe the cosmic evolution from the matter-dominated era to the current accelerating universe, and $ii$) suitable predictions for the universe age.

gr-qc↗

arXiv2019

Generalized Misner-Sharp energy in the generalized Rastall theory

Employing the unified first law of thermodynamics and the field equations of the generalized Rastall theory, we get the generalized Misner-Sharp mass of spacetimes for which $g_{tt}=-g^{rr}=-f(r)$. The obtained result differs from those of the Einstein and Rastall theories. Moreover, using the first law of thermodynamics, the obtained generalized Misner-Sharp mass and the field equations, the entropy of the static spherically symmetric horizons is also addressed in the framework of the generalized Rastall theory. In addition, by generalizing the study to the flat FRW universe, the apparent horizon entropy is also calculated. Considering the effects of applying the Newtonian limit to the field equations on the coupling coefficients of the generalized Rastall theory, our study indicates $i$) the obtained entropy-area relation is the same as that of the Rastall theory, and $ii$) the Bekenstein entropy is recovered when the generalized Rastall theory reduces to the Einstein theory. The validity of the second law of thermodynamics is also investigated in the flat FRW universe.

physics.gen-ph↗

arXiv2019

Black hole solutions and Euler equation in Rastall and generalized Rastall theories of gravity

Focusing on the special case of generalized Rastall theory, as a subclass of the non-minimal curvature-matter coupling theories in which the field equations are mathematically similar to the Einstein field equations in the presence of cosmological constant, we find two classes of black hole (BH) solutions including $i$) conformally flat solutions and $ii$) non-singular BHs. Accepting the mass function definition and by using the entropy contents of the solutions along with thermodynamic definitions of temperature and pressure, we study the validity of Euler equation on the corresponding horizons. Our results show that the thermodynamic pressure, meeting the Euler equation, is not always equal to the pressure components appeared in the gravitational field equations and satisfies the first law of thermodynamics, a result which in fact depends on the presumed energy definition. The requirements of having solutions with equal thermodynamic and Hawking temperatures are also studied. Additionally, we study the conformally flat BHs in the Rastall framework. The consequences of employing generalized Misner-Sharp mass in studying the validity of the Euler equation are also addressed.

physics.gen-ph↗

arXiv2019

The generalized and extended uncertainty principles and their implications on the Jeans mass

The generalized and extended uncertainty principles affect the Newtonian gravity and also the geometry of the thermodynamic phase space. Under the influence of the latter, the energy-temperature relation of ideal gas may change. Moreover, it seems that the Newtonian gravity is modified in the framework of the Rényi entropy formalism motivated by both the long-range nature of gravity, and the extended uncertainty principle. Here, the consequences of employing the generalized and extended uncertainty principles, instead of the Heisenberg uncertainty principle, on the Jeans mass are studied. The results of working in the Rényi entropy formalism are also addressed. It is shown that unlike the extended uncertainty principle and the Rényi entropy formalism which lead to the same increase in the Jeans mass, the generalized uncertainty principle can decrease it. The latter means that a cloud with mass smaller than the standard Jeans mass, obtained in the framework of the Newtonian gravity, may also undergo the gravitational collapse process.

gr-qc↗