arXiv subjects

Nematollah Riazi

Publications and source records attributed to Nematollah Riazi.

At least 19 recordsLinked to original sources

arXiv2024

Galactic black hole immersed in a dark halo with its surrounding thin accretion disk

By considering the analytic, static and spherically symmetric solution for the Schwarzschild black holes immersed in dark matter fluid with non-zero tangential pressure \cite{Jusufi:2022jxu} and Hernquist-type density profiles \cite{Cardoso}, we compute the luminosity of accretion disk. We study the circular motion of test particles in accretion disk and calculate the radius of the innermost stable circular orbits. Using the steady-state Novikov-Thorne model we also compute the observational characteristics of such black hole's accretion disk and compare our results with the usual Schwarzschild black hole in the absence of dark matter fluid. We find that the tangential pressure plays a significant role in decreasing the size of the innermost stable circular orbits and thus increases the luminosity of black hole's accretion disk.

gr-qc↗

arXiv2024

Dynamics of kink train solutions in deformed Multiple sine-Gordon models

This paper examines the effects of a thin layer of inhomogeneity on periodic solutions of the Multiple-sine-Gordon (MsG) model. We investigate the dynamics of the perturbed Double-sine-Gordon (DsG) system as a significant and more practical case of such configurations. The thin barrier acts as a potential well (potential barrier) and causes critical deformations in kink train solutions and some basic properties of the periodic solutions, such as the type of sub-kinks, their amplitude, energy and wavelength. Stability of the initial kink chain during the interaction with medium defects is analyzed using their phase diagram. Sudden changes in the profile of kink trains due to the disruption of their amplitude and wavelength are considered. The time evolution of moving kink chain solutions while interacting with medium fractures is also studied.

nlin.PS↗

arXiv2024

Thin accretion disk images of rotating hairy Horndeski black holes

By considering the steady-state Novikov-Thorne model, we study thin accretion disk processes for rotating hairy black holes in the framework of the Horndeski gravity. We obtain the electromagnetic properties of accretion disk around such black holes and investigate the effects of the hair parameter $h$ on them. We find that by decreasing the hair parameter from the Kerr limit, $h\rightarrow0$, the radius of the innermost stable circular orbit decreases which makes thin accretion disks around rotating hairy black holes in Horndeski gravity more efficient than that for the Kerr black hole in general relativity. Furthermore, using the numerical ray-tracing method, we plot thin accretion disk images around these black holes and investigate the effects of hair parameter on the central shadow area of accretion disk.

gr-qc↗

arXiv2023

Shadows and photon rings of a spherically accreting Kehagias-Sfetsos black hole

By considering Kehagias-Sfetsos black hole in the framework of the Hořava-Lifshitz gravity, we study the optical appearance of such black holes surrounded by spherical accretion flow. For the static/infalling spherical accretion flow, we compute the observed specific intensity as a function of impact parameter. We also investigate the effect of the Hořava parameter and accreting matter on the luminosity of shadows and photon rings. It is found that an increase in the Hořava parameter decreases the shadow size, while the shadows and photon rings luminosities increase. Moreover, we constrain the Hořava parameter from the observational data reported by the Event Horizon Telescope for M87* and Sgr A*.

gr-qc↗

arXiv2023

Fluid Dynamical Description of Sine-Gordon Solitons

We present a fluid dynamical description of a relativistic scalar field in $1+1$ dimensions and apply the general results to the special case of Sine-Gordon solitons. The results which include the local quantities pressure, density and fluid velocity are compared to the standard quantities attributed to the solitons.

hep-th↗

arXiv2022

Faster-Than-Light Solitons in 1 + 1 Dimensions

The existence of a faster-than-light particle is in direct opposition to Einstein's relativity and the principle of causality. However, we show that the theory of classical relativistic fields is not inherently inconsistent with the existence of the faster-than-light particle-like soliton solutions in $1+1$ dimensions. We introduce two extended KG models ($k$-fields) that lead to a zero-energy and a nonzero-energy stable particle-like solution with faster-than light speeds, respectively.

physics.class-ph↗

arXiv2021

A new nonlinear electrodynamics and electrically charged regular black holes

A regular static, spherically symmetric electrically charged black hole solution of general relativity coupled to a new theory for nonlinear electrodynamics is presented. This theory has the interesting feature that, at far distances from the black hole, in the weak field limit, the theory reduces to Maxwell Lagrangian with Heisenberg-Euler correction term of quantum electrodynamics. The singular center of the black hole is replaced by flat, de Sitter, or anti de Sitter space, if the spacetime in which the black hole is embedded is asymptotically flat, de Sitter, or anti de Sitter, respectively. Requiring the correspondence to Heisenberg-Euler Lagrangian at large distances, in the weak field limit, we find that (\textit{i}) a minimum mass is required for the formation of an event horizon for the regular static, spherically symmetric solution of the theory, and, (\textit{ii}) the mass of the solution must be quantized. We also study the basic thermodynamic properties of the black hole solution and show that they are qualitatively similar to those of Reissner-Nordström black hole.

hep-th↗

arXiv2021

Novel thick brane solutions with $U(1)$ symmetry breaking

In this work, using two scalar fields ($ϕ$, $ψ$) coupled to 4+1 dimensional gravity, we construct novel topological brane solutions through an explicit $U(1)$ symmetry breaking term. The potential of this model is constructed so that two distinct degenerate vacua in the $ϕ$ field exist, in analogy to the $ϕ^{4}$ potential. Therefore, brane solutions appear due to the vacuum structure of the $ϕ$ field. However, the topology and vacuum structure in the $ψ$ direction depends on the symmetry breaking parameter $β^{2}$, which leads to different types of branes. As a result, one can interpret the present model as a combination of a $ϕ^{4}$ brane with an auxiliary field, which leads to deviations from the $ϕ^{4}$ system with the brane achieving a richer internal structure. Furthermore, we analyse in detail the behaviour of the superpotentials, the warp factors, the Ricci and Kretschmann scalars and the Einstein tensor components. In addition to this, we explore the stability of the brane in terms of the free parameters of the model. The analysis presented here complements previous work and is sufficiently novel to be interesting.

gr-qc↗

arXiv2020

Evolving wormhole in the brane-world scenario

In this paper, evolving wormholes in the context of brane-world scenario are investigated. We have studied the possible dynamic solutions with different forms of Ricci scalar. The possibility of existence of dynamic traversable wormholes, without resorting to an exotic matter, has been studied. By using the fact that the Einstein field equations are modified in 3+1 dimensions due to the brane corrections, we investigate the exact solutions which satisfy null energy condition. Asymptotic flatness is an important property of these solutions. We discuss some physical and mathematical properties of the solutions.

gr-qc↗

arXiv2019

The Affective Factors on the Uncertainty in the Collisions of the Soliton Solutions of the Double Field sine-Gordon System

Inspired by the well known sine-Gordon equation, we present a symmetric coupled system of two real scalar fields in $1+1$ dimensions. There are three different topological soliton solutions which be labelled according to their topological charges. These solitons can absorb some localized non-dispersive wave packets in collision processes. It will be shown numerically, during collisions between solitons, there will be an uncertainty which originates from the amount of the maximum amplitude and arbitrary initial phases of the trapped wave packets.

nlin.PS↗

arXiv2018

Boosted Kaluza- Klein magnetic monopole

We consider a Kaluza-Klein vacuum solution which is closely related to the Gross-Perry-Sorkin (GPS) magnetic monopole. The solution can be obtained from the Euclidean Taub-NUT solution with an extra compact fifth spatial dimension within the formalism of Kaluza-Klein reduction. We study its physical properties as appearing in $(3+1)$ spacetime dimensions, which turns out to be a static magnetic monopole. We then boost the GPS magnetic monopole along the extra dimension, and perform the Kaluza-Klein reduction. The resulting four-dimensional spacetime is a rotating stationary system, with both electric and magnetic fields. In fact, after the boost the magnetic monopole turns into a string connected to a dyon.

gr-qc↗

arXiv2018

Boosted Cylindrical Magnetized Kaluza-Klein Wormhole

In this work, we consider a vacuum solution of Kaluza-Klein theory with cylindrical symmetry. We investigate the physical properties of the solution as viewed in four dimensional spacetime, which turns out to be a stationary, cylindrical wormhole supported by a scalar field and a magnetic field oriented along the wormhole.We then apply a boost to the five dimensional solution along the extra dimension, and perform the Kaluza-Klein reduction. As a result, we show that the new solution is still a wormhole with a radial electric field and a magnetic field stretched along the wormhole throat.

gr-qc↗

arXiv2017

Phase transition and thermodynamic stability in extended phase space and charged Hořava-Lifshitz black holes

For charged black holes in Horava-Lifshitz gravity, a second order phase transition takes place in extended phase space where the cosmological constant is taken as thermodynamic pressure. We relate the second order nature of phase transition to the fact that the phase transition occurs at a sharp temperature and not over a temperature interval. Once we know the continuity of the first derivatives of the Gibbs free energy, we show that all the Ehrenfest equations are readily satisfied. We study the effect of the perturbation of the cosmological constant as well as the perturbation of the electric charge on thermodynamic stability of Horava-Lifshitz black hole. We also use thermodynamic geometry to study phase transition in extended phase space. We investigate the behavior of scalar curvature of Weinhold, Ruppeiner, and Quevedo metric in extended phase space of charged Horava-Lifshitz black holes. It is checked if these curvatures could reproduce the result of specific heat for the phase transition.

hep-th↗

arXiv2017

Evolution of spherical domain walls in solitonic symmetron models

In this work, inspired by the symmetron model, we analyse the evolution of spherical domain walls by considering specific potentials that ensure symmetry breaking and the occurrence of degenerate vacua that are necessary for the formation of domain walls. By considering a simple analytical model of spherical domain wall collapse in vacuum, it is shown that this model fits the more accurate numerical results very well until full collapse, after which oscillations and scalar radiation take place. Furthermore, we explore the effect of a central non-relativistic matter lump on the evolution of a spherical domain wall and show that the central lump can prevent the full collapse and annihilation of the domain wall bubble, due to the repulsion between the domain wall and matter over-density within the adopted symmetron inspired model.

gr-qc↗

arXiv2016

Dark Matter From f(R,T) Gravity

We consider f(R,T) modified theory of gravity, in which the gravitational Lagrangian is given by an arbitrary function of the Ricci scalar and the trace of the energy-momentum tensor of the matter, in order to investigate the dark-matter effects on the galaxy scale. We obtain the metric components for a spherically symmetric and static spacetime in the vicinity of general relativity solutions. However, we concentrate on a specific model of the theory where the matter is minimally coupled to the geometry, and derive the metric components in the galactic halo. Then, we fix the components by the rotational velocities of the galaxies for the model, and show that the mass corresponding to the interaction term (which appears in the Einstein modified field equation) leads to a flat rotation curve in the halo of galaxies. In addition, for the proposed model, the light-deflection angle has been derived and drawn using some observed data.

gr-qc↗

arXiv2016

Vacuum $ f(R)$ thick brane solution with a Gaussian warp function

This work deals with $f(R)$ modified gravity in five dimensional space-time. The Gaussian thick brane is shown to be an exact solution in the frame work of $f(R)$ gravity in five dimensions with a bulk cosmological constant. Response of the brane to gravitational fluctuations and concordance with the Starobinsky model is addressed. It is shown that the matter which supports the Starobinsky $f(R)$ solution with the background geometry being flat FLRW with a Gaussian warp function, behaves like a radiation dominated era of universe, gradually changing to a dark energy dominated era.

gr-qc↗

arXiv2016

Soliton models for thick branes

In this work, we present new soliton solutions for thick branes in $4+1$ dimensions. In particular, we consider brane models based on the sine-Gordon ($SG$), $φ^{4}$ and $φ^{6}$ scalar fields, which have broken $Z_{2}$ symmetry in some cases, and are responsible for supporting and stabilizing the thick branes. The origin of the symmetry breaking in these models resides in the fact that the modified scalar field potential may have non-degenerate vacuua. These vacuua determine the cosmological constant on both sides of the brane. We also study the geodesic equations along the fifth dimension, in order to explore the particle motion in the neighbourhood of the brane. Furthermore, we examine the stability of the thick branes, by determining the sign of the $w^2$ term in the expansion of the potential for the resulting Schrodinger-like equation, where $w$ is the 5-dimensional coordinate. It turns out that the $φ^4$ brane is stable, while there are unstable modes for certain ranges of the model parameters in the SG and $φ^6$ branes.

gr-qc↗

arXiv2016

From Taub-NUT to Kaluza-Klein magnetic monopole

We present a Kaluza-Klien vacuum solution which closely resembles the Taub-NUT magnetic monopole and we investigate its physical properties as viewed from four space-time dimensions. We show that the Taub-NUT Kaluza-Klein vacuum solution in five dimensions is a static magnetic monopole. We find that the four dimensional matter properties do not obey the equation of state of radiation and there is no event horizon. A comparison with the available magnetic monopole solutions and the issue of vanishing and negative mass are discussed.

gr-qc↗