arXiv subjects

H. Ghaffarnejad

Publications and source records attributed to H. Ghaffarnejad.

7 recordsLinked to original sources

arXiv2020

Navarro-Frenk-White dark matter profile and the dark halos around disk systems

The $Λ$ cold dark matter ($Λ$CDM) scenario well describes the Universe at large scales, but shows some serious difficulties at small scales: the inner dark matter (DM) density profiles of spiral galaxies generally appear to be cored, without the $r^{-1}$ predicted by N-body simulations in the above scenario. In a more physical context, the baryons in the galaxy might backreact and erase the original cusp through supernova explosions. Before that this effect be investigated, it is important to determine how wide and frequent the discrepancy between observed and N-body predicted profiles is and what its features are. We used more than 3200 quite extended rotation curves (RCs) of good quality and high resolution of disk systems. The curves cover all magnitude ranges. These RCs were condensed into 26 coadded RCs, each of them built with individual RCs of galaxies of similar luminosity and morphology. We performed mass models of these 26 RCs using the Navarro-Frenk-White (NFW) profile for the contribution of the DM halo to the circular velocity and the exponential Freeman disk for that of the stellar disk. The fits are generally poor in all the 26 cases: in several cases, we find $χ^2_{red}>2$. Moreover, the best-fitting values of three parameters of the model ($c$, $M_D$, and $M_{vir}$) combined with those of their 1$σ$ uncertainty clearly contradict well-known expectations of the $Λ$CDM scenario. We also tested the scaling relations that exist in spirals with the fitting outcome: the modeling does not account for these scaling relations. Therefore, NFW halo density law cannot account for the kinematics of the whole family of disk galaxies. It is therefore mandatory for the $ΛCDM$ scenario in any disk galaxy of any luminosity to transform initial cusps into the observed cores.

astro-ph.CO↗

arXiv2018

Holographic entanglement entropy for small subregions and thermalization of Born-Infeld AdS black holes

AApplying the Born-Infeld Anti de Sitter charged black hole metric we calculate holographic entanglement entropy (HEE) by regarding the proposal of Ryu and Takanayagi. To do so we assume that time dependence of the black hole mass and charge to be as step function. Our work is restricted to small subregions where a collapsing null shell dose not penetrate the black holes horizon. To calculate time dependent HEE we use perturbation method for small subregions where turning point is much smaller than local equilibrium point of black hole. We choose two shape functions for entangled regions on the boundary which are the strip and the ball regions. There is a saturation time at which the null shell grazes the turning point and the HEE reaches to its maximum value. In general, this work satisfies result of the works presented by Camelio et al and Zeng et al. We must point out that they used equal time two-point correlation functions and Wilson loops instead of the entanglement entropy (EE) as non-local observable to study this thermalization by applying the numerical method.

hep-th↗

arXiv2018

Quintessence Reissner Nordström Anti de Sitter Black Holes and Joule Thomson effect

In this work we investigate corrections of the quintessence regime of the dark energy on the Joule-Thomson (JT) effect of the Reissner Nordström anti de Sitter (RNAdS) black hole. The quintessence dark energy has equation of state as $p_q=ωρ_q$ in which $-1<ω<-\frac{1}{3}.$ Our calculations are restricted to ansatz: $ω=-1$ (the cosmological constant regime) and $ω=-\frac{2}{3}$ (quintessence dark energy). To study the JT expansion of the AdS gas under the constant black hole mass, we calculate inversion temperature $T_i$ of the quintessence RNAdS black hole where its cooling phase is changed to heating phase at a particular (inverse) pressure $P_i.$ Position of the inverse point $\{T_i,P_i\}$ is determined by crossing the inverse curves with the corresponding Gibbons-Hawking temperature on the T-P plan. We determine position of the inverse point verse different numerical values of the mass $M$ and the charge $Q$ of the quintessence AdS RN black hole. The cooling-heating phase transition (JT effect) is happened for $M>Q$ in which the causal singularity is still covered by the horizon. Our calculations show sensitivity of the inverse point $\{T_i,P_i\}$ position on the T-P plan to existence of the quintessence dark energy just for large numerical values of the AdS RN black holes charge $Q$. In other words the quintessence dark energy dose not affects on position of the inverse point when the AdS RN black hole takes on small charges.

gr-qc↗

arXiv2017

Gravitational lensing of charged Ayon-Beato-Garcia black holes and non-linear effects of Maxwell fields

Non-singular Ayon-Beato-Garcia (ABG) spherically symmetric static black hole (BH) with charge to mass ratio $q$ is metric solution of Born-Infeld nonlinear Maxwell-Einstein theory. Central region of the BH behaves as (anti-) de Sitter for $(|q|>1)~|q|<1 .$ In case of $|q|=1$ the BH central region behaves as Minkowski flat metric. Nonlinear Electromagnetic (NEM) fields counterpart causes to deviate light geodesics and so light rays will forced to move on effective metric. In this paper we study weak and strong gravitational lensing of light rays by seeking affects of NEM fields counterpart on image locations and corresponding magnification. We set our calculations to experimentally observed Sgr A$^*$ BH. In short we obtained: For large distances the NEM counterpart is negligible reaching to linear Maxwell fields. The NEM makes enlarge the BH photon sphere radius as linearly by raising $|q|>1$ but deceases by raising $|q|\leq1.$ Sign of deflection angle of bending light rays is changed in presence of NEM effects with respect to ones obtained in absence of NEM fields. Absolute value of deflection angle raises by increasing $|q|\to1.$ Weak image locations decreases (increases) by raising $0<|q|<1$ in presence (absence) of NEM fields. By raising the closest distance of the bending light rays weak image locations changes from left (right) to right (left) in absence (presence) of NEM fields. Einstein rings radius and corresponding Magnification centroid become larger (smaller) in presence (absence) of NEM fields in case of weak lensing. Angular separation $s$ between the innermost and outermost relativistic images increases (decreases) by increasing $0<|q|<1$ in absence (presence) of NEM fields. Corresponding magnification $r$ decreases (increases) by raising $0<|q|<1$ in absence (presence) of NEM fields. $s (r)$ raises (decreases) by increasing $|q|>>1.$

physics.gen-ph↗

arXiv2016

Dynamical system approach to scalar-vector-tensor cosmology

Using scalar-vector-tensor Brans Dicke (VBD) gravity [3] in presence of self interaction BD potential $V(ϕ)$ and perfect fluid matter field action we solve corresponding field equations via dynamical system approach for flat Friedmann Robertson Walker metric (FRW). We obtained 3 type critical points for $ΛCDM$ vacuum de Sitter era where stability of our solutions are depended to choose particular values of BD parameter $ω.$ One of these fixed points is supported by a constant potential which is stable for $ω<0$ and behaves as saddle (quasi stable) for $ω\geq0.$ Two other ones are supported by a linear potential $V(ϕ)\simϕ$ which one of them is stable for $ω=0.27647.$ For a fixed value of $ω$ there is at least 2 out of 3 critical points reaching to a unique critical point. Namely for $ω=-0.16856(-0.56038)$ the second (third) critical point become unique with the first critical point. In dust and radiation eras we obtained 1 critical point which never become unique fixed point. In the latter case coordinates of fixed points are also depended to $ω.$ To determine stability of our solutions we calculate eigenvalues of Jacobi matrix of 4D phase space dynamical field equations for de Sitter, dust and radiation eras. We should be point also potentials which support dust and radiation eras must be similar to $V(ϕ)\simϕ^{-\frac{1}{2}}$ and $V(ϕ)\simϕ^{-1}$ respectively. In short our study predicts that radiation and dust eras of our VBD-FRW cosmology transmit to stable de Sitter state via non-constant potential (effective variable cosmological parameter) by choosing $ω=0.27647$.

physics.gen-ph↗

arXiv2016

Weak Gravitational Lensing of quantum perturbed Lukewarm Black Holes and cosmological constant effect

Aim of the paper is study weak gravitational lensing of quantum (perturbed) (QLBHL) and classical (CLBHL) Lukewarm black hole in presence of cosmological parameter $Λ$. We apply numerical method to evaluate deflection angle of bending light rays, images locations $θ$ of sample source $β=-\fracπ{4},$ and corresponding magnifications $μ.$ There is not obtained real values for Einstein rings locations $θ_E(β=0)$ for CLBHL but we calculate them for QLBHL. As experimental test of our calculations, we choose mass $M$ of 60 type of most massive observed galactic black holes as gravitational lens and study quantum matter fields effects on the angle of bending light rays in presence of the cosmological constant effects. We calculate locations of non-relativistic images and corresponding magnifications. Numerical diagrams show that the quantum matter effects cause to be reduce absolute values of the quantum deflection angle with respect to the classical ones. Sign of the quantum deflection angle is changed with respect to the classical values in presence of the cosmological constant. This means dominance of anti-gravity counterpart of the cosmological horizon on the bending light rays angle with respect to absorbing effects of local 60 type of observed most massive black holes. Variations of the image positions and magnifications are negligible by increasing dimensionless cosmological constant $ε=\frac{16ΛM^2}{3}.$ Deflection angle takes positive (negative) values for CLBHL (QLBHL) and they decrease very fast (slow) by increasing closest distance $x_0$ of bending light ray and/or dimensionless cosmological parameter for sample giant black holes $0.001<ε<0.01$.

physics.gen-ph↗

arXiv2013

Evaporating Quantum Lukewarm Black Holes Final State From Back-Reaction Corrections of Quantum Scalar Fields

We obtain renormalized stress tensor of a mass-less, charge-less dynamical quantum scalar field, minimally coupled with a spherically symmetric static Lukewarm black hole. In two dimensional analog the minimal coupling reduces to the conformal coupling and the stress tensor is found to be determined by the nonlocal contribution of the anomalous trace and some additional parameters in close relation to the work presented by Christensen and Fulling. Lukewarm black holes are a special class of Reissner- Nordström-de Sitter space times where its electric charge is equal to its mass. Having the obtained renormalized stress tensor we attempt to obtain a time-independent solution of the well known metric back reaction equation. Mathematical derivations predict that the final state of an evaporating quantum Lukewarm black hole reduces to a remnant stable mini black hole with moved locations of the horizons. Namely the perturbed black hole (cosmological) horizon is compressed (extended) to scales which is smaller (larger) than the corresponding classical radius of the event horizons. Hence there is not obtained an deviation on the cosmic sensor-ship hypothesis.

physics.gen-ph↗