arXiv subjects

N. Riazi

Publications and source records attributed to N. Riazi.

At least 19 recordsLinked to original sources

arXiv2022

Quantum Cosmology in Thick Brane

One of the interesting open problems in the cosmological framework is applying quantum physics to the whole universe, consistently. Although different conceptual aspects of quantum cosmology are still very much alive, till now, all the attempts of obtaining a unique and well-defined version of quantum cosmology have been unfeasible. Motivated by what was mentioned above, in this paper, the quantum tunneling of thick brane is investigated by deriving the Wheeler-DeWitt equation and corresponding solutions. Besides, the cosmological analysis of the brane is studied based on two special cases of the scalar field. Finally, it is found that the constant scalar potential arising from a time-dependent scalar field is supported by the so-called slow-roll inflation.

gr-qc↗

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

The relation between the radio emission of the core and host galaxy properties in Fanaroff-Riley type II radio galaxies

We study the radio power of the core and its relation to the optical properties of the host galaxy in samples of high excitation (HERG) and low excitation (LERG) Fanaroff-Riley type II (FRII) radio galaxies. The radio galaxy sample is divided into two groups of core/non-core FRII, based on the existence of strong, weak or lack of single radio core component. We show that FRII LERGs with radio emission of the core have significantly higher [O III] line luminosities compared to the non-core LERG FRIIs. There is no significant difference between the hosts of the core and non-core FRIIs of LERG type in galaxy sizes, concentration indices, star formation rates, 4000-Å break strengths, colours, black hole masses and black hole to stellar masses. We show that the results are not biased by the stellar masses, redshifts and angular sizes of the radio galaxies. We argue that the detection of higher [O III] luminosities in the core FRIIs may indicate the presence of higher amounts of gas, very close to the AGN nucleus in the core FRIIs compared to the non-core FRIIs or may result from the interaction of the radio jets with this gas. The core and non-core FRIIs of the HERG type show no significant differences perhaps due to our small sample size. The effect of relativistic beaming on the radio luminosities and the contribution of restating AGN activity have also been considered.

astro-ph.GA↗

arXiv2021

HD207897 b: A dense sub-Neptune transiting a nearby and bright K-type star

We present the discovery and characterization of a transiting sub-Neptune orbiting with a 16.20 day period around a nearby (28 pc) and bright(V=8.37) K0V star HD207897 (TOI-1611). This discovery is based on photometric measurements from the Transiting Exoplanet Survey Satellite(TESS) mission and radial velocity (RV) observations from the SOPHIE, Automated Planet Finder (APF) and HIRES high precision spectrographs. We used EXOFASTv2 for simultaneously modeling the parameters of the planet and its host star, combining photometric and RV data to determine the planetary system parameters. We show that the planet has a radius of 2.50+/-0.08 RE and a mass of either 14.4+/-1.6 ME or 15.9+/-1.6 ME with nearly equal probability; the two solutions correspond to two possibilities for the stellar activity period. Hence, the density is either 5.1+/-0.7 g cm^-3 or 5.5^{+0.8}_{-0.7} g cm^-3, making it one of the relatively rare dense sub-Neptunes. The existence of such a dense planet at only 0.12 AU from its host star is unusual in the currently observed sub-Neptune (2<RE<4) population. The most likely scenario is that this planet has migrated to its current position.

astro-ph.EP↗

arXiv2020

Z2-Symmetric Thick Brane with Specific Warp Function

In this work, we investigate a new warped five-dimensional, Z2-symmetric thick brane solution in the presence of a real scalar field. We examine the different geometric aspects of the model. We discuss the stability of the solution under gravitational fluctuations and study both the graviton ground state and the continuum of Kaluza-Klein modes to find a correction to Newtons law. Then, we study the quantum version of the model by deriving the Wheeler-DeWitt equation and looking for the corresponding solution. Finally, the cosmology of the brane is studied.

gr-qc↗

arXiv2020

Analytically Approximation Solution to Higher Derivative Gravity

We obtain analytical approximate black hole solutions for higher derivative gravity in the presence of Maxwell electromagnetic source. We construct near horizon and asymptotic solutions and then use these to obtain an approximate analytic solution using a continued fraction method to get a complete solution. We compute the thermodynamic quantities and check the first law and Smarr formula. Finally, we investigate the null and time-like geodesics of this black hole.

gr-qc↗

arXiv2020

Dynamical and Thermal Stabilities of Nonlinearly Charged AdS Black Holes

In this paper, we study an extended phase space thermodynamics of a nonlinearly charged AdS black hole. We examine both the local and global stabilities, and possible phase transition of the black hole solutions. Finally, we compute quasi-normal modes via scalar perturbations and compare the obtained results with those of RN-AdS black hole.

gr-qc↗

arXiv2020

Dipole and String Solutions of the Kaluza-Klein Theory

We consider a Kaluza-Klein string solution of five-dimensional spacetime. We study its physical properties as appearing in (3+1) spacetime. This metric embraces several different aspects of solutions. The two most notable examples are the dipole and the boosted dipole solutions. In general, the 4D metric is a rotating stationary solution with both electric and magnetic fields. The extension to the Kaluza-Klein dipole soliton solution is briefly discussed.

gr-qc↗

arXiv2019

Phantom Origin for f(R) Inflation

We argue that for $f(R)=R+αR^n$ model of the early universe, the de Sitter point appears as partial attractor for phantom paths. The phantom trajectories then bend their way to the oscillatory period passing near the de Sitter saddle point. Surprisingly, this vicinity is minimized for $1<n\le 3$ where a trajectory could cross the phantom border tangential to the de Sitter point. The only integers in this interval are $2$ and $3$ where $n=2$ reproduces also the most acceptable $f(R)$ model of inflation with strong motivations from quantum theory and also avoiding the vacuum meta-stability. Therefore, one may ask about a possible phantom pre-inflationary scenario. Combining with the increasing chance of falling in a phantom era in our time, proposing a singularity-free cyclic universe seems appealing, too. We use the dynamical system approach throughout this work which allows us to provide numerical support for the analytical discussions.

gr-qc↗

arXiv2018

$AdS_{4}$ dyonic black holes in gravity's rainbow

In this paper, we investigate thermodynamical structure of dyonic black holes in the presence of gravity's rainbow. We confirm that for super magnetized and highly pressurized scenarios, the number of black holes' phases is reduced to a single phase. In addition, due to specific coupling of rainbow functions, it is possible to track the effects of temporal and spatial parts of our setup on thermodynamical quantities/behaviors including equilibrium point, existence of multiple phases, possible phase transitions and conditions for having a uniform stable structure.

gr-qc↗

arXiv2018

Gauss-Bonnet Dyonic Black Holes: geometry, thermodynamics and test particles' trajectories

In this paper, we investigate a class of $5$-dimensional black holes in the presence of Gauss-Bonnet gravity with dyonic charges. At first step, thermodynamical quantities of the black holes and their behaviors are explored for different limits. Thermal stability and the possibility of the van der Waals like phase transition are addressed and the effects of different parameters on them are investigated. The second part is devoted to simulation of the trajectory of particles around these black holes and investigation of the angular frequency of particles' motion. The main goal is understanding the effects of higher curvature gravity (Gauss-Bonnet gravity) and magnetic charge on the structure of black holes and the geodesic paths of particles moving around these black holes.

gr-qc↗

arXiv2018

Gravitational Lensing by Polytropic Wormholes

We obtain static multi-polytropic wormhole solutions in the framework of GR gravity. The resulting metric is asymptotically Minkowskian, and locally that of a wormhole. We also examine gravitational lensing by the wormhole, and calculate the deflection angle for weak and strong field limits. We investigate microlensing in the weak field limit and obtain corresponding light curves for both galactic and extragalactic situations. We discuss the multi-polytropic equation of state for the energy-momentum tensor which supports this geometry and finally, we check for the satisfaction of the weak energy condition.

gr-qc↗

arXiv2017

Holographical aspects of dyonic black holes: Massive gravity generalization

The content of this paper includes studying holographical and thermodynamical aspects of dyonic black holes in the presence of massive gravity. For the first part of paper, thermodynamical properties of the bulk which includes black holes are studied and the main focus is on critical behavior. It will be shown that the existence of massive gravitons introduces remnant for temperature after evaporation of black holes, van der Waals phase transition for non-spherical black holes and etc. The consistency of different thermodynamical approaches toward critical behavior of the black holes is presented and the physical properties near the region of thermal instability are given. Next part of the paper studies holographical aspects of the boundary theory. Magnetization and susceptibility of the boundary are extracted and the conditions for having diamagnetic and paramagnetic behaviors are investigated. It will be shown that generalization to massive gravity results into the existence of diamagnetic/paramagnetic phases in phase structure of the hyperbolic and horizon flat of boundary conformal field theory.

hep-th↗

arXiv2017

Higher dimensional dyonic black holes

The paper at hand presents a novel class of dyonic black holes in higher dimensions through a new proposal for the electromagnetic field tensor. The black hole solutions are extracted analytically and their geometrical/physical properties are studied. In addition, the details regarding thermodynamical structure and phase transition behavior of the solutions for $4$ different cases are investigated: i) general case, ii) constant electric field, iii) constant magnetic field, and iv) constant electric and magnetic fields. It will be shown that depending on the picture under consideration, the thermodynamical properties are modified. To have better picture regarding the phase transitions, the concept of the extended phase space is employed. It will be shown that in the absence of the electric field, magnetic black holes present van der Waals like phase transition. Furthermore, it will be highlighted that for super magnetized black holes, no phase transition exists.

gr-qc↗

arXiv2016

Thermodynamic Stability of Wormholes

In the context of GR, we study the thermodynamic stability of evolving Lorentzian wormholes at the apparent horizon. The average pressure of the anisotrropic components is considered as the pressure of the wormhole. According to the requirements of stable equilibrium in conventional thermodynamics, we calculate the heat capacity at constant pressure and Gibbs free energy and analyze the local and global thermodynamic stability of the wormhole.

gr-qc↗

arXiv2016

$Z_2$ Massive Axions, Domain Walls and Inflation

We have analyzed a $U(1)$ model which is broken explicitly to a $Z_2$ model. The model results in generating two stable domain walls, in contrast with the more common $N_{DW}=1$ version which is prevalently used to explain axion invisibility for $U_{PQ}(1)$ model. We have been meticulous to take into account any possible relation with previous studies, even if they apparently belong to different lines of research. Then we have scrutinized the domain wall properties of the model,proposing a rigorous approximate solution which fully satisfies boundary conditions and the static virial theorem simultaneously. Invoking the mentioned approximation, we have been able to obtain an analytical insight about the effect of parameters on the domain wall features, particularly on their surface energy density which is of great importance in cosmological studies when one tries to avoid domain wall energy domination problem. Next, we have mainly focused on the likely inflationary scenarios of the model, including saddle point inflation, again insisting on analytical discussions to be able to follow the role of parameters. We have tried to categorize any inflationary scenario into the known categories to take advantage of the previous detailed studies under the inflationary topic over the decades. We have concluded that any successful inflationary scenario requires large fields definition for the model. Calculations are mainly done analytically and numerical results are used as supportive material.

gr-qc↗

arXiv2015

Kaluza-Klein magnetic monopole a la Taub-NUT

In this paper, we consider a new vacuum solution of the Kaluza-Klien theory which resembles the Taub-NUT metric in five dimensions and we investigate its physical properties as projected into four dimensions. We show that the Taub-NUT Kaluza-Klein vacuum solution in five dimensions is a static magnetic monopole in four dimensions. We find that the four dimensional matter properties do not obey the equation of state of radiation and there is no event horizon.

gr-qc↗

arXiv2015

Phantom Wormhole Solutions in a Generic Cosmological Constant Background

There are a number of reasons to study wormholes with generic cosmological constant $Λ$. Recent observations indicate that present accelerating expansion of the universe demands $Λ>0$. On the other hand, some extended theories of gravitation such as supergravity and superstring theories posses vacuum states with $Λ<0$. Even within the framework of general relativity, a negative cosmological constant permits black holes with horizons topologically different from the usual spherical ones. These solutions are convertible to wormhole solutions by adding some exotic matter. In this paper, the asymptotically flat wormhole solutions in a generic cosmological constant background are studied. By constructing a specific class of shape functions, mass function, energy density and pressure profiles which support such a geometry are obtained. It is shown that for having such a geometry, the wormhole throat $r_0$, the cosmological constant $Λ$ and the equation of state parameter $ω$ should satisfy two specific conditions. The possibility of setting different values for the parameters of the model helps us to find exact solutions for the metric functions, mass functions and energy-momentum profiles. At last, the volume integral quantifier, which provides useful information about the total amount of energy condition violating matter is discussed briefly.

gr-qc↗