arXiv subjects

Hossein Ghaffarnejad

Publications and source records attributed to Hossein Ghaffarnejad.

At least 19 recordsLinked to original sources

arXiv2026

Brans Dicke scalar-tensor gravity and unimodular FRW cosmology with Coleman-Weinberg potential

As alternative gravity theories with respect to the general relativity, the Brans-Dicke (BD) scalar tensor model is well known for which the BD parameter is controller the best fit observational data and so its cosmological model is studied in the literature well. On the other side, the Coleman Weinberg (CW) self-interaction scalar field potential coming from radiation corrections of Feynman diagrams is used to describe the best fit cosmic inflation phase and reheating phase. This has a logarithmic term with respect to the Higgs potential and it is applicable even for massless bosons. From particle physics point of view, this is used usually to describe the Higgs mechanism and creation of massive Goldstone bosons. As a basic model to describe the cosmic inflation the cosmological constant is used to describe the de Sitter space where the cosmological constant play as dark energy density. Since, the cosmological constant is a hierarchy problem and it is added in the Einstein equation without to describe that where that is come originally? at a first time the Albert Einstein himself proposed a uni-modular frame for which the cosmological constant generate from a integral constant. This idea is very well because it resolve `fine tuning` problem of the cosmological constant parameter. Hence we use this idea for the BD theory in presence of the CW potential and obtained suitable solutions of the field equations for a flat Robertson-Walker space time by regarding the slow roll parameters of cosmic inflation and then we find the best fit correspondence between the theoretical predictions of the parameters of the solutions and the Planck2018, DESI2024, and ACT2025 observational data.

gr-qc↗

arXiv2024

Stabilization of spherically symmetric static Boson stars by Coleman-Weinberg self-interaction potential

Our motivation in this work is due to the great importance of relativistic boson stars as fierce competitors of black holes. From theoretical perspective, they described by complex Klein Gordon (KG) scalar fields interacting with curved background in general form. In this work we use the Einstein-Klein Gordon (EKG) scalar tensor gravity in presence of the Coleman-Weinberg (CW) self-interaction potential to study formation of a spherically symmetric boson star and situation of its stability. We choose the CW potential because of its important role in description of cosmic inflation.

gr-qc↗

arXiv2024

Thermodynamic phase transition of Anti-de Sitter Reissner-Nordström black holes with exotic Einstein-Maxwell gravities

To consider the inevitable cosmic magnetic effects instead of the unknown dark sector of matter/energy on the inflation phase of the expanding universe some authors have proposed several extended exotic Einstein-Maxwell gravities which are addressed in this work. Some of these exotic models include directional interaction terms between the electromagnetic vector field and the metric tensor field. We use one of them to investigate the physical effects of interaction terms on the thermodynamic behavior of the modified Reissner-Nordstrom (RN) black hole. We use the perturbation series method to find analytic solutions of the field equations because of the non-linearity of the field equations which cause they do not have analytic closed form solutions. We investigate possibility of the Hawking-Page and the small/large black hole phase transition and also, effects of the interaction part of the model on possibility of the coexistence of the several phases of the perturbed AdS RN black hole under consideration.

gr-qc↗

arXiv2024

Modified Gauge Invariant Einstein Maxwell Gravity and Stability of Spherical perfect fluid Stars with Magnetic Monopoles

As an alternative gravity model we consider an extended Einstein-Maxwell gravity containing a gauge invariance property. Extension is assumed to be addition of a directional coupling between spatial electromagnetic fields with the Ricci tensor. We will see importance of the additional term in making a compact stellar object and value of its radius. As an application of this model we substitute ansatz of magnetic field of a hypothetical magnetic monopole which has just time independent radial component and for matter part we assume a perfect fluid stress tensor. To obtain spherically symmetric internal metric of the perfect fluid stellar compact object we solve Tolman-Oppenheimer-Volkoff equation with a polytropic form of equation of state as $p(ρ)=aρ^2$. Using dynamical system approach we study stability of the solutions for which arrow diagrams show saddle (quasi stable) for $a<0$ (dark stars) and sink (stable) for $a>0$ (normal visible stars). We check also the energy conditions, speed of sound and Harrison-Zeldovich-Novikov static stability criterion for obtained solution and confirm that they make stable state.

gr-qc↗

arXiv2023

Particles creation from JNW quantum perturbed black holes by minimally coupled Klein Gordon scalar free fields

In this work, we choose a minimal coupling interaction between massive Klein Gordon (KG) quantum scalar free fields and Janis-Newman-Winicour (JNW) spherically symmetric static black hole, to produce its Hawking temperature and luminosity. This is done by calculating asymptotic wave solutions at near and far from the black hole horizon. They are orthogonal mode solutions of local Hilbert spaces. By using these mode solutions, we calculated Bogolubov coefficients and then, we investigated number density matrix of created particles. Mathematical calculations show that this is not exactly similar to the Planck`s black body radiation energy density distribution but, it is "gray" body radiation distribution depended to the emitted Hawking particles frequency. Their difference is a non-vanishing absorptivity factor of backscattered particles after to form horizon of a collapsing body. Our motivation is determination of position of Hawking created pairs in which, two different proposals are proposed, so called as "fairwall" and "quantum atmosphere".

hep-th↗

arXiv2023

Central AdS Generalized Ay{ó}n-Beato-Garc{\'ı}a Black Holes and Joule-Thomson Expansion

In this paper, the Joule-Thomson (JT) adiabatic expansion is investigated for generalized Ayon-Beato-Garcia (ABG) regular black hole. It has a magnetic charge which makes central region of the black hole metric to be AdS spacetime and so become non singular. Thus we not need to use an additional cosmological parameter coming from ADS/CFT correspondence for production of pressure coordinate in the black hole equation of state. Form this point of view our work is a new approach versus to conventional methods which are addressed in the references of this paper. However we will see important behavior of parameters of this modified ABG black hole on possibility of being of JT adiabatic expansion. This black hole is characterized by five parameters including mass $m$, magnetic charge $q$, and three other parameters related to form of used nonlinear electromagnetic interaction fields. Inversion points are in fact a particular T-P curve at constant enthalpy and it separates cooling and heating phase of the modified AdS ABG black hole.

gr-qc↗

arXiv2023

On the stability of electrostatics stars with modified non-gauge invariant Einstein-Maxwell gravity

We use a modified Einstein-Maxwell gravity to study stability of an electrostatic spherical star. Correction terms in this model are scalers which are made from contraction of Ricci tensor and electromagnetic vector potential. Our motivation to use this kind of exotic EM gravity is inevitable influence of cosmic magnetic field in inflation of the universe which is observed now but its intensity suppresses in the usual gauge invariant EM gravity. In this work we use dynamical systems approach to obtain stability conditions of such a star and investigation of affects of interaction parts of the model on the stability.

gr-qc↗

arXiv2023

Canonical quantization of modified non-gauge invariant Einstein-Maxwell gravity and stability of spherically symmetric electrostatic stars

We consider a non-minimally coupled Einstein-Maxwell gravity with no $U(1)$ symmetry property to study stability of an electrostatic star via canonical quantization approach and obtain that the stability is free of gauge field effects. By calculating the Hamiltonian density of the stellar system we show that the corresponding Wheeler-DeWitt wave functional is similar to a simple harmonic quantum Oscillator for which a non zero ADM mass of the system causes a quantization condition on the metric fields. Probability wave packets are described by the Hermit polynomials. Our mathematical calculations show that in this approach of quantum gravity the metric fields are regular for all values of the electric potential and so the quantized spacetime has not both of event and apparent horizons. The most probability of the quantized line element is for ground state of the system. To check validation of the model we use Bohr`s correspondence principal and generate directly semi classical approach of the quantized metric states at large quantum numbers where they reach to Schwarzschild like metric according to the Birkhoff's theorem. Also we check that the generated semi classical solutions are satisfied exact classical metric solutions which are obtained from Euler Lagrange equations. We show that `charge to mass ratio` of the electrostatic star is a constant defined by the coupling constant of the model and it is in accord to other alternative approaches.

gr-qc↗

arXiv2023

Thermodynamic Phase Transition of Generalized Ayon-Beato Garcia Black Holes

In this work we study thermodynamics of generalized Ayon-Beato and Garcia (ABG) black hole metric which contains three parameters named as mass $m$, magnetic charge $q$ and dimensionless coupling constant of nonlinear electrodynamics interacting field $γ$. We showed that central regions of this black hole behaves as dS(AdS) vacuum space by setting $q<2m (q>2m)$ and in the case $q=2m$ reaches to a flat Minkowski space. In the large distances this black hole behaves as a Reissner-Nordstrom BH. However important role of the charge q is appeared in produce of a formal variable cosmological parameter which will support pressure coordinate in the thermodynamic perspective of this black hole in our setup. We should be point that this formal variable cosmological parameter is different with cosmological constant which comes from AdS/CFT correspondence and it is effective at large distances as AdS space pressure. In our setup the assumed pressure is originated from internal material of the black hole say q and m here. By calculating the Hawking temperature of this black hole we obtain equation of state. Then we plotted isothermal P-v curves and heat capacity at constant pressure. They show that the system participates in the small to large phase transition of the black hole or the Hawking-Page phase transition which is similar to the Van der Waals phase transition in the ordinary thermodynamics systems . In fact in the Hawking-Page phase transition disequilibrium evaporating generalized ABG black hole reaches to a vacuum AdS space finally.

hep-th↗

arXiv2022

Bianchi I cosmology and scalar vector tensor Brans Dicke gravity

We consider Brans Dicke scalar vector tensor gravity to study an inflationary scenario of the accelerating expansion of the universe for which the anisotropy property occurs. We study both primordial inflation and late inflationary period of the universe. To do so we, use Bianchi I line element where spatial part has cylindrical symmetry along $x$ direction in the local Cartesian coordinates. To seek stabilization of our obtained metric solution, we apply dynamical system approach to obtain critical manifolds in phase space and determine which of them confirms the inflation with stable nature in presence of the anisotropy property of spacetime. We solve dynamical equations for different directions of the timelike dynamical vector field. We obtain several critical manifolds whose nature of stable (sink) or quasi stable (saddle) are dependent on the direction of the used vector field. At last, we should point that observational constraint on the Brans Dicke parameter $ω>40000$ which satisfied by the well known Brans Dicke scalar tensor gravity is not valid for our modified scalar vector Brans Dicke gravity because of presence of timelike dynamical vector field.

gr-qc↗

arXiv2022

Thermodynamic phase transition and Joule Thomson adiabatic expansion for dS/AdS Bardeen Black Holes with consistent 4D Gauss-Bonnet gravity

Instead of the work \cite{1} which in according to the Lovelock theorem it could not applicable for all types of 4D curved spacetimes of Einstein Gauss Bonnet (EGB) gravity, authors of the work \cite{2} applied break of diffeomorphism property to present a consistent EGB gravity theory. In this work we use the latter model by adding a nonlinear electromagnetic field lagrangian density to study affects of GB coupling constant into the thermodynamic phase transition and Joule Thomson (JT) adiabatic expansion of a 4D $dS/AdS$ GB Bardeen black hole.

hep-th↗

arXiv2022

Magnetic charge effects on thermodynamic phase transition of modified Anti de Sitter Ayón-Beato-García Black Holes with five parameters

In this letter we choose generalized Ayon Beato Gracia (ABG) magnetic charged black hole with five parameters to investigate possibility of thermodynamic phase transition and coexistence of different gass/liquid/solid phases of this black hole. In fact this work is an extension of our recently work where ABG black hole with three parameters was used to seek the phase transition. In this work we obtain other physical values on the parameters with respect to our previous work where the phase transition is happened together with coexistence point of different phases in the phase space.

gr-qc↗

arXiv2022

Thermodynamics of 4D dS/AdS Gauss-Bonnet black holes from consistent gravity theory in presence of cloud of strings

By looking at the Lovelock theorem one can infer that the gravity model given by \cite{GL} could not applicable for all type of 4D Einstein Gauss Bonnet (GB) curved spacetimes. Because in 4D space time the Gauss-Bonnet invariant is a total derivative, and hence it does not contribute to the gravitational dynamics. Hence, authors in the work \cite{AL} presented an alternative consistent EGB gravity model instead of \cite{GL} by applying break of diffeomorphism property. In this work we use it to produce a dS/AdS black hole metric and then investigate its thermodynamic behavior in presence of cloud of Numbo Goto strings.

gr-qc↗

arXiv2021

Physics-informed deep learning for three dimensional black holes

According to AdS/DL (Anti de Sitter/ Deep Learning) correspondence given by \cite{Has}, in this paper with a data-driven approach and leveraging holography principle we have designed an artificial neural network architecture to produce metric field of planar BTZ and quintessence black holes. Data has been collected by choosing minimally coupled massive scalar field with quantum fluctuations and try to process two emergent and ground-truth metrics versus the holographic parameter which plays role of depth of the neural network. Loss or error function which shows rate of deviation of these two metrics in presence of penalty regularization term reaches to its minimum value when values of the learning rate approach to the observed steepest gradient point. Values of the regularization or penalty term of the quantum scalar field has critical role to matching this two mentioned metric. Also we design an algorithm which helps us to find optimum value for learning parameter and at last we understand that loss function convergence heavily depends on the number of epochs and learning rate.

physics.gen-ph↗

arXiv2021

Wave optics and weak gravitational lensing of lights from spherically symmetric static scalar vector Brans Dicke black holes in presence of the cosmological constant

This paper has three parts. In first step we use modified scalar tensor vector Brans Dicke gravity \cite{GH0} to obtain metric solution of a spherically symmetric static black hole via perturbation method which asymptotically behaves as modified Schwarzschild de Sitter black hole in weak field limits. Corrections on the line element with respect to the point mass Schwarzschild de Sitter black hole are two parts: power law function for black hole mass instead of point mass and additional logarithmic metric potential which they are produced from effects of interacting timelike dynamical vector fields. Second part of the paper is dedicated to review Fresnel-Kirchhoff diffraction theory for massless scalar field instead of the vector electromagnetic waves. In third part of the work we consider the obtained black hole metric to be lens and we study weak gravitational lensing of moving light originated from point star. Production of stationary images is investigated via both interference of waves and geometric optics approaches. At last we compare results of these approaches and give some outlooks.

physics.gen-ph↗

arXiv2021

Quantum State Dependence of Thermodynamic Phase Transition in 4D AdS Gauss-Bonnet Quantum Black Holes Surrounded With Cloud of Strings

According to the Lovelock theorem where the model \cite{Glav} could not applicable for Einstein Gauss Bonnet (EGB) gravity in all 4D curved spacetimes, authors of the reference \cite{ali} presented an effective model by applying break of diffeomorphism property. Hence we use the latter model instead of former for study of thermodynamic behavior of a 4D AdS EGB spherically symmetric static black hole which surrounded with a cloud of string. In short our work is extension of the works given by \cite{Veer,Heg} but not by using \cite{Glav} but by applying \cite{ali}. Our metric solutions are obtained versus the Hermite polynomials (quantum harmonic Oscillator) for which eigen values come from single scale defined by multiplication of the coupling constants of the model: Namely the regularized GB parameter, AdS radius, the black hole ADM mass and the string tension. Hence we claim the obtained metric solution is in fact behavior of quantized black hole. Because the GB term is originated from renormalization of quantum matter fields. Also we should pointed that this kind of quantization is different with the canonical quantization (Wheeler De Witt). Our study shows that all phase transitions of this quantum black hole are dependent to the Hermite quantum numbers.

hep-th↗

arXiv2020

Thermodynamics phase transition of Anti de Sitter Schwarzschild scalar-tensor-vector-Black Holes

Instead of scalar tensor gravity models which is applicable for description of cosmic inflation with unknown dark sector of matter/energy, at presentense there are presented different alternative scalar tensor vector gravities where meaningful dynamical vector fields can support cosmic inflation well without to use dark matter/energy concept. One of these gravity models was presented by Moffat which its modified Schwarzschild black hole solution is used to study thermodynamic phase transition in presence of the AdS space pressure in this article. To do so we obtained an equation of state which asymptotically reaches to equation of state of ideal gas for large black holes but for small scale black holes we obtained a critical point at phase space where the black hole can be exhibit with a phase transition at processes of isotherm and isobaric. By looking at diagrams of the Gibbs free energy and the heat capacity at constant pressure which are plotted versus the temperature and the specific volume one can see an inflection point which means that the phase transition is type of second order. In fact there is small to large phase transition for the black hole which is equivalent to the Van der Waals liquid-gas phase transition in ordinary thermodynamic systems. The phase transition is happened below the critical point in phase space when the gravitational charge of the black hole is equal to its mass.

gr-qc↗

arXiv2020

Bianchi I metric solutions with nonminimally coupled Einstein-Maxwell gravity theory

By using Bianchi I type of homogenous and anisotropic background metric having cylindrical symmetry in $x$ direction of a local cartesian coordinates system, we solve metric field equations for a non-minimally coupled Einstein-Maxwell gravity. To do so we choose long wavelength EM waves where spatial dependence of the waves are negligible at the expansion duration. Motivation of this work is study directional effects of the EM vector potential on anisotropy trajectory of the space time expansion scale factor. This problem is checked just for de Sitter inflationary epoch because of its importance in the expansion of the universe. By applying the dynamical system approach we investigate stability nature of the cosmic system in two different cases: (a) direction of the EM vector potential is parallel to $x$ direction of the spacetime and (b) it is perpendicular to $x$ direction. We obtained at stable critical points for some of obtained inflationary solutions the EM waves behave as dark energy in cases $a$ and $b$ where the anisotropy is negligible by expanding of the universe while there are some stable critical points with inflationary stable solutions in case $b$ for which the EM waves behave as baryonic visible matter with non-vanishing anisotropy.

gr-qc↗