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Hossein Ghaffarnejad

Publications and source records attributed to Hossein Ghaffarnejad.

36 records · Page 2Linked to original sources
arXiv2020

Hairy black holes and holographic heat engine

By considering AdS charged black hole in the context of extended thermodynamic as the working substance we use it as a heat engine. We investigate the effect of hairy charge on the evolution of efficiency and Carnot efficiency along with electric charge. Because of interesting thermodynamic behavior of hairy black holes it would be natural to know their effects when we use black hole as a heat engine. We show that the hairy charge increases the efficiency, and so maximum temperature would be happened for bigger Maxwell charge when this hairy charge grows. For the fixed electric charges, the efficiency has a minimum value. In fact all critical points describe physical states except when the charge removed. If the electric charge takes a zero value then the hairy charge must be negative. We also seek behavior of the system for large charges which is provided a model with low-temperature thermodynamics.

hep-th↗
arXiv2019

Effects of quintessence dark energy on the action growth and butterfly velocity

In this work we are about to investigate the effects of quintessence dark energy on evolution of the computational complexity relating to the AdS/CFT correspondence. We use $"complexity=action"$ conjecture for a charged AdS black hole surrounded by the dark energy at the quintessence regime. Then we try to find some conditions on the quintessence parameters where the Lloyd bound is satisfied in presence of affects of the quintessence dark energy on the complexity growth at the late time approximations. We compare late time approximation of the action growth by perturbed geometry in small limits of shift function. Actually we investigate the evolution of complexity when thermofield double state on the boundaries is perturbed by local operator corresponding to a shock wave geometry as holographically. Furthermore we seek spread of local shock wave on the black hole horizon in presence of the quintessence dark energy.

hep-th↗
arXiv2019

Canonical quantization of anisotropic Bianchi I cosmology from scalar vector tensor Brans Dicke gravity

We applied a generalized scalar-vector-tensor Brans Dicke gravity model to study canonical quantization of an anisotropic Bianchi I cosmological model. Regarding an anisotropic Harmonic Oscillator potential we show that the corresponding Wheeler de Witt wave functional of the system is described by Hermit polynomials. We obtained a quantization condition on the ADM mass of the cosmological system which raises versus the quantum numbers of the Hermit polynomials. Our calculations show that the inflationary expansion of the universe can be originate from the big bang with no naked singularity due to the uncertainty principle.

physics.gen-ph↗
arXiv2019

Galaxy rotation curves and preferred reference frame effects

As an alternative to dark matter models we use generalized Jordan-Brans-Dicke scalar-vector-tensor (JBD-SVT) gravity model to study the behavior of the rotational velocities of test particles moving around galaxies. To do so we consider an interaction potential $U(ϕ,N_μ)$ between the Brans-Dicke scalar field $ϕ$ and time like dynamical four-vector field $N_μ$ which plays as four velocity of a preferred reference frame. We show that at in weak field limits metric solution of the galaxy under consideration reaches to a modified Schwarzschild-de Sitter space in which mass of the vector field plays as an effective cosmological constant. In fact the present work proposes modification on the formulation of Newton's gravitational acceleration. This is used to explain circular velocity of galaxies without postulating dark matter. We also check our theoretical results with empirical baryonic Tully Fisher relation which states a linear relations between the rotational speed of galaxies and their mass. Mathematical calculation predict a good correspondence between our theoretical results and experimental observations for a set of 12 spiral galaxies.

gr-qc↗
arXiv2019

Thermodynamic phase transition for Quintessence Dyonic Anti de Sitter Black Holes

We study thermodynamic of a dyonic AdS black hole surrounded by quintessence dark energy where negative cosmological constant of AdS space behaves as pressure of the black hole. We choose grand canonical ensemble of the black hole where its magnetic charge $Q_M$ and electric potential $Φ_E$ are hold as constant. Our goal in this work is study physical effects of the magnetic charge and electric potential on the thermodynamic phase transition of the black hole in presence of quintessence dark energy. When barotropic index of the quintessence is $ω=-\frac{7}{9}$ we obtained that compressibility factor of the black hole reduces to $Z_c=\frac{3}{8}$ which corresponds to the Van der Waals fluid. We obtained analogy between the small/large black hole phase transition and liquid/gas phase transition of the Van der Waals fluid. Numerical calculations predict that the black hole may born $plasma$ phase which is fourth different state of the matter which does not appear in the Van der Waals fluid.

physics.gen-ph↗
arXiv2018

Holographic thermalization in AdS-Gauss-Bonnet gravity for small entangled regions

In this paper we study the propagation of entanglement entropy after a global instantaneous quench on the CFT boundary of AdS bulk. We consider the Gauss-Bonnet model as a higher curvature gravity model for which we correct the RT(HRT) proposal to compute the holographic entanglement entropy(HEE). To obtain an analytical solution we perform an approximation approach which bounds our computations to the small subregions and we compare its thermalization regimes to the result of large subsystem case. We can see tsunami picture where the evolution of entanglement breaks down for the large systems and so its details depends just on the shape and size of entangled region and also the used gravity model. We can see the phase transition in this regime is always continuous regardless the shape and size, in contrary with large subregions.

hep-th↗
arXiv2018

Complexity growth and shock wave geometry in AdS-Maxwell-power-Yang-Mills theory

We study effects of non-abelian gauge fields on the holographic characteristics for instance the evolution of computational complexity. To do so we choose Maxwell-power-Yang-Mills theory defined in the AdS space-time. Then we seek the impact of charge of the YM field on the complexity growth rate by using $complexity=action$ (CA) conjecture. We also investigate the spreading of perturbations near the horizon and the complexity growth rate in local shock wave geometry in presence of the YM charge. At last we check validity regime of Lloyd bound.

gr-qc↗
arXiv2018

Effects of a cloud of strings on the extended phase space of Einstein-Gauss-Bonnet AdS black holes

In this paper we study the thermodynamics of Einstein-Gauss-Bonnet (EGB)-AdS black holes minimally coupled to a cloud of strings in an extended phase space where the cosmological constant is treated as pressure of the black holes and its conjugate variable is the thermodynamical volume of the black holes. To investigate the analogy between EGB black holes surrounded by a cloud of strings and liquid-gas system we derive the analytical solutions of the critical points and probe the effects of a cloud of strings on $P-V$ criticality. There is obtained resemblance between "small black hole/large black hole" (SBH/LBH) phase transition and the liquid-gas phase transition. We see that impact of a cloud of strings can bring Van der Waals-like behavior, in absence of the Gauss-Bonnet (GB) counterpart. In the other words, in the EGB black hole with $α\to 0$ and when it is surrounded by a cloud of strings the Hawking-Page phase transition would be disappeared and SBH/LBH phase transition recovers. Also there is not happened Joule-Thomson effect.

gr-qc↗
arXiv2018

The Last Lost Charge And Phase Transition In Schwarzschild AdS Minimally Coupled to a Cloud of Strings

In this paper we study the Schwarzschild AdS black hole with a cloud of string background in an extended phase space and investigate a new phase transition related to the topological charge. By treating the topological charge as a new charge for black hole solution we study its thermodynamics in this new extended phase space. We treat by two approaches to study the phase transition behavior via both $T-S$ and $P-v$ criticality and we find the results confirm each other in a nice way. It is shown a cloud of strings affects the critical physical quantities and it could be observed an interesting Van der Waals-like phase transition in the extended thermodynamics. The swallow tail-like behavior is also observed in Free Energy-Temperature diagram. We observe in $a\to 0$ limit the small/large black hole phase transition reduces to the Hawking-Page phase transition as we expects. We can deduce that the impact of cloud of strings in Schwarzschild black hole can bring Van der Waals-like black hole phase transition.

hep-th↗
arXiv2018

Schwarzschild-de Sitter black hole in canonical quantization

We solve Wheeler-De Witt (WDW) metric probability wave equation on the apparent horizon hypersurface of the Schwarzschild de Sitter (SdS) black hole. To do so we choose radial dependent mass function $M(r)$ for its internal regions in the presence of a dynamical massless quantum matter scalar field $ψ(r)$ and calculate canonical supper hamiltonian constraint on $t-$constant hypersurface near the horizon $r=M(r)$. In this case $M(r)$ become geometrical degrees of freedom while $ψ(r)$ is matter degrees of freedom of the apparent horizon. However our solution is obtained versus the quantum harmonic oscillator which defined against the well known hermit polynomials. In the latter case we obtain quantized mass of the SdS quantum black hole as $\sqrtΛM(n)=\big(\frac{2n+1}{12\sqrt{2}}\big)^{\frac{1}{3}}$ in which $Λ$ is the cosmological constant and $n=0,1,2,\cdots $ are quantum numbers of the hermit polynomials. This shows that a quantized SdS in its ground state has a nonzero value for the mass $M(0)=0.38914/\sqrtΛ$. Thus one can infer that the latter result satisfies Penrose hypotheses for cosmic censorship where a causal singularity may be covered by a horizon surface.

gr-qc↗
arXiv2016

Spherically symmetric curved space times from quantum fields backreaction corrections in two dimensional analogue

Aim of the paper is to obtain 2d analogue of the backreaction equation which will be useful to study final state of quantum perturbed spherically symmetric curved space times. Thus we take Einstein-massless-scalar $ψ$ tensor gravity model described on class of spherically symmetric curved space times. We rewrite the action functional in 2d analogue in terms of dimensionless dilaton-matter field $(χ=Φψ)$ where dilaton field $Φ$ is conformal factor of 2-sphere. Then we seek renormalized expectation value of quantum dilaton-matter field stress tensor operator by applying Hadamard rennormalization prescription. Singularity of the Green function is assumed to be has logarithmic form. Covariantly conservation condition on the renormalized quantum dilaton-matter stress tensor demands to input a variable cosmological parameter $λ(x)$. Energy conditions (weak, strong and null) is studied on the obtained renormalized stress tensor leading to dynamical equations for $λ(x), Φ$ and quantum vacuum state $W_0(x)=<0|\hatχ^2|0>_{ren}.$ In weak quantum field limits our obtained trace anomaly corresponds to one which obtained from zeta function regularization method. Setting null-like apparent horizon equation $\nabla_cΦ\nabla^cΦ=0,$ our procedure predicts that physically correct value of the parameter in the anomaly trace $\frac{1}{24π}\{R-α\frac{\nabla_{c}\nabla^{c}Φ}Φ+(α-6)\frac{\nabla_cΦ\nabla^cΦ}{Φ^2} \}$ should be $α=6.$ At last we solved the backreaction equation and obtained explicitly metric field solution in the slow varying limits of the quantum and dilaton fields which has black holes topology and its singularity is covered by apparent horizon hypersurface.

hep-th↗
arXiv2016

Dilatonic Brans-Dicke Anisotropic Collapsing Fluid Sphere And de Broglie Quantum Wave Motion

Two dimensional (2D) analogue of vacuum sector of the Brans Dicke (BD) gravity [1] is studied to obtain dynamics of anisotropic spherically symmetric perfect fluid. Our obtained static solutions behave as dark matter with state equation $γ=\frac{p(ρ)}{\varrho}=-0.25$ but in non-static regimes behave as regular perfect fluid with barotropic index $γ>0.$ Positivity property of total mass of the fluid causes that the BD parameter to be $ω>\frac{2}{3}$ and/or $ω<-1$. Locations of the event and apparent horizons of the collapsing fluid are obtained in its static regime. In case $ω>0$ the apparent horizon is covered by event horizon where the cosmic censorship hypothesis is still valid. According to the model [1], we obtain de Broglie pilot wave of our metric solution which describes particles ensemble which become distinguishable via different values of $ω.$ Incident current density of particles ensemble on the horizons is evaluated which describe the `Hawking radiation`. The de Brogle -Bohm quantum potential effect is calculated also on the event (apparent) horizon which is independent (dependent) to values of $ω.$

gr-qc↗
arXiv2015

Classical and Quantum Reissner-Nordström Black Hole Thermodynamics and first order Phase Transition

First we consider CRNBH metric which is obtained by solving Einstein-Maxwell metric equation for a point electric charge $e$ inside of a spherical static body with mass $M$. It has 2 interior and exterior horizons. Using Bekenestein-Hawking entropy theorem we calculate interior and exterior entropy, temperature, Gibbs free energy and heat capacity at constant electric charge. We calculate first derivative of the Gibbs free energy with respect to temperature which become a singular function having a singularity at critical point with corresponding temperature $T_c=\frac{1}{24π\sqrt{3}|e|}.$ Hence we clime first order phase transition is happened there. Temperature same as Gibbs free energy takes absolutely positive (negative) values on the exterior (interior) horizon. The Gibbs free energy takes two different positive values synchronously for $0<T<T_c$ but not for negative values which means the system is made from two subsystem. For negative temperatures entropy reaches to zero value at $T\to-\infty$ and so takes Bose-Einstein condensation single state. Entropy increases monotonically in case $0<T<T_c$. Regarding results of the work presented at Ref. \citep{Bob01} we calculate again the mentioned thermodynamical variables for remnant stable final state of evaporating QRNBH and obtained results same as one in case of the CRNBH. Finally, we solve mass loss equation of QRNBH against advance Eddington-Finkelstein time coordinate and derive luminosity function. We obtain switching off of QRNBH evaporation before than the mass completely vanishes. It reaches to a could Lukewarm type of RN black hole which its final remnant mass is $m_{final}=|e|$ in geometrical units. Its temperature and luminosity vanish but not in Schwarzschild case of evaporation. Our calculations can be takes some acceptable statements about information loss paradox .

physics.gen-ph↗
arXiv2015

Anisotropic Spherically Symmetric Collapsing Star From Higher Order Derivative Gravity Theory

Adding linear combinations $R^2,R_{μν}R^{μν}$ and $R_{μνηδ}R^{μνηδ}$ with Einstein-Hilbert action we obtain interior metric of an an-isotropic spherically symmetric collapsing (ASSC) stellar cloud. We assume stress tensor of the higher order geometrical terms to be treat as an-isotropic imperfect fluid with time dependent density function $ρ(t)$ and radial and tangential pressures $p_r(t)$ and $p_t(t)$ respectively. We solved linearized metric equation via perturbation method and obtained 12 different kinds of metric solutions. Calculated Ricci and Kretschmann scalars of our metric solutions are non-singular at beginning of the collapse for 2 kinds of them only. Event and apparent horizons are formed at finite times for two kinds of singular metric solutions while 3 metric solutions exhibit with event horizon only with no formed apparent horizon. There are obtained 3 other kinds of the metric solutions which exhibit with apparent horizon with no formed event horizon. Furthermore 3 kinds of our metric solutions do not exhibit with horizons. Barotropic index of all 12 kinds of metric solutions are calculated also. They satisfy different regimes such as domain walls (6 kinds), cosmic string (2 kinds), dark matter (2 kinds), anti-matter (namely negative energy density) (1 kind) and stiff matter (1 kind). Time dependent radial null geodesics expansion parameter $Θ(t)$ is also calculated for all 12 kinds of our metric solutions. In summary 4 kinds of our solutions take absolutely positive value $Θ>0$ which means the collapse ended to a naked singularity but 8 kinds of our metric solutions ended to a covered singularity at end of the collapse where $Θ\leq0$ and so trapped surfaces are appeared.

physics.gen-ph↗
arXiv2015

Weak Gravitational lensing from regular Bardeen black holes

In this article we study weak gravitational lensing of regular Bardeen black hole which has scalar charge $g$ and mass $m.$ We investigate the angular position and magnification of non-relativistic images in two cases depending on the presence or absence of photon sphere. Defining dimensionless charge parameter $q=\frac{g}{2m}$ we seek to disappear photon sphere in the case of $|q|>{24\sqrt5}/{125}$ for which the space time metric encounters strongly with naked singularities. We specify the basic parameters of lensing in terms of scalar charge by using the perturbative method and found that the parity of images is different in two cases: (a) The strongly naked singularities is present in the space time. (b) singularity of space time is weak or is eliminated (the black hole lens).

gr-qc↗
arXiv2015

Wave function of the Universe, Preferred reference frame effects and metric signature transition

Gravitational model of non-minimally coupled Brans Dicke (BD) scalar field $ϕ$ with dynamical unit time-like four vector field is used to study flat Robertson Walker (RW) cosmology in the presence of variable cosmological parameter $V(ϕ)=Λϕ.$ Aim of the paper is to seek cosmological models which exhibit metric signature transition. The problem is studied in both classical and quantum cosmological approach with large values of BD parameter $ω>>1$. Scale factor of RW metric is obtained as $R(t)=6\sqrt{\frac{3}Λ}\cosh\big(\frac{t}{4}\sqrt{\fracΛ{3}}\big)$ which describes nonsingular inflationary universe in Lorentzian signature sector. Euclidean signature sector of our solution describes a re-collapsing universe and is obtained from analytic continuation of the Lorentzian sector by exchanging $t\to it$ as $R(t)=6\sqrt{\frac{3}Λ}\cos\big(\frac{t}{4}\sqrt{\fracΛ{3}}\big).$ Dynamical vector field together with the BD scalar field are treated as fluid with time dependent barotropic index. They have regular (dark) matter dominance in the Euclidean (Lorentzian) sector. We solved Wheeler De Witt (WD) quantum wave equation of the cosmological system. Assuming a discrete non-zero ADM mass $M_j=4\sqrt{2}(2j+1)\sqrt{\fracΛ{3}}$ with $j=0,1,2,\cdots$, we obtained solutions of the WD equation as simple harmonic quantum Oscillator eigen functionals described by Hermite polynomials. Absolute values of these eigen functionals have nonzero values on the hypersurface $R=6\sqrt{\frac{3}Λ}$ in which metric field has signature degeneracy. Our eigen functionals describe nonzero probability of the space time with Lorentzian (Euclidean) signature for $R>6\sqrt{\frac{3}Λ}$ ($R<6\sqrt{\frac{3}Λ}$). Maximal probability corresponds to the ground state $j=0.$

gr-qc↗
arXiv2014

Spherically symmetric Jordan-Brans-Dicke quantum gravity with de Broglie Bohm pilot wave perspective

We obtain two dimensional analogue of the Jordan-Brans-Dicke (JBD) gravity action described in four dimensional spherically symmetric curved space time metric. There will be two scalar fields, namely, the Brans Dicke (BD) $ϕ$ and scale factor of 2-sphere part of the space time $ψ.$ We obtained suitable duality transformations between $(ψ,ϕ)$ and $(ρ,S)$ where $ρ$ and $S$ is respectively amplitude and phase part of the corresponding de Broglie pilot wave function $Ψ(ρ,S)=\sqrtρe^{iS}.$ Covariant conservation of mass-energy current density of particles ensemble $J_a=ρ\partial_aS,$ is established by applying a particular dynamical conformal frame described by $(ρ,S).$

gr-qc↗