arXiv subjects

Kourosh Nozari

Publications and source records attributed to Kourosh Nozari.

At least 55 records · Page 3Linked to original sources

arXiv2020

Quantum Corrections to the Accretion onto a Schwarzschild Black Hole in the Background of Quintessence

It is well known that quantum effects may lead to remove the intrinsic singularity point of back holes. Also, the quintessence scalar field is a candidate model for describing late-time acceleration expansion. Accordingly, Kazakov and Solodukhin considered the existence of back-reaction of the spacetime due to the quantum fluctuations of the background metric to deform Schwarzschild black hole, which led to change the intrinsic singularity of the black hole to a 2-sphere with a radius of the order of the Planck length. Also, Kiselev rewrote the Schwarzschild metric by taking into account the quintessence field in the background. In this study, we consider the quantum-corrected Schwarzschild black hole inspired by Kazakov-Solodukhin's work, and Schwarzschild black hole surrounded by quintessence deduced by Kiselev to study the mutual effects of quantum fluctuations and quintessence on the accretion onto the black hole. Consequently, the radial component of 4-velocity and the proper energy density of the accreting fluid have a finite value on the surface of its central 2-sphere due to the presence of quantum corrections. Also, by comparing the accretion parameters in different kinds of black holes, we infer that the presence of a point-like electric charge in the spacetime is somewhat similar to some quantum fluctuations in the background metric.

gr-qc↗

arXiv2020

IR-deformed thermodynamics of quantum bouncers and the issue of dimensional reduction

We probe the low-temperature behavior of a system of quantum bouncers as a theoretical model for ultracold neutrons within a low energy modified version of the standard quantum mechanics, due to the gravitational effects. Working in one dimension, the energy spectrum and bound states of a deformed quantum bouncer are obtained using the first-order WKB approximation, granted the very low energy regime of the particle. In this manner, we can study energy levels of a system of ultracold neutrons as an informative probe towards exploring the low energy manifestation of semi-classical quantum gravitational effects. Our calculated energy levels of ultracold neutrons are in accordance with the observed energy levels, as obtained in the famous Nesvizhevsky \emph{et al.} experiment, with a negative constant deformation, as dependent on the deformation parameter. In advance, we tackle modified thermodynamics of a system of quantum bouncers in the infrared regime via an ensemble theory both in one dimension and also three dimensions, to seek for any trace of an effective, thermodynamic dimensional reduction in this low energy regime of semi-classical quantum gravity. While the issue of dimensional reduction has been essentially assigned to the high energy regime, here we show that there is a trace of an effective, thermodynamic dimensional reduction in infrared regime with one important difference: in the high energy regime, the dimensional reduction effectively occurs from $D=3$ to $D=1$, but here, in this low energy regime, there is a trace of thermodynamic dimensional reduction from $D=3$ to $D=2$.

hep-th↗

arXiv2020

A quantum-corrected approach to black hole radiation via a tunneling process

In the language of black hole physics, Hawking radiation is one of the most controversial subjects about which there exist lots of puzzles, including the information loss problem and the question of whether this radiation is thermal or not. In this situation, a possible way to face these problems is to bring quantum effects into play, also taking into account self-gravitational effects in the scenario. We consider a quantum-corrected form of the Schwarzschild black hole inspired by the pioneering work of Kazakov and Solodukhin to modify the famous Parikh-Wilczek tunneling process for Hawking radiation. We prove that in this framework the radiation is not thermal, with a correlation function more effective than the Parikh-Wilczek result, and the information loss problem can be addressed more successfully. Also, we realize that quantum correction affects things in the same way as an electric charge. So, it seems that quantum correction in this framework has something to do with the electric charge.

gr-qc↗

arXiv2020

Geodesic Structure of the Quantum-Corrected Schwarzschild Black Hole Surrounded by Quintessence

By considering the back-reaction of the spacetime through the spherically symmetric quantum fluctuations of the background metric, Kazakov and Solodukhin removed the singularity of the Schwarzschild black hole. This regular Schwarzschild black hole has a spherical central region with a radius of the order of the Planck length. On the other hand, due to the positively accelerating expansion of the Universe, it seems that there exists a universal repulsive force known as dark energy. In the framework of quantum field theories, the quintessence field is a candidate model for investigating and modeling dark energy. Accordingly, by taking into account the quintessential matter field in the background of the Schwarzschild black hole, Kiselev gained the metric of this black hole surrounded by quintessence. By combining these two above ideas, in the present study we consider the quantum-corrected Schwarzschild black hole surrounded by quintessence to investigate null and time-like geodesics structure. Generally, this study points out that black holes are quantum-gravitational objects. We will show that the accelerated expansion of the Universe, instead of dark energy, happens because of the presence of quantum effects in this setup. Also, due to the presence of the central Planck-size sphere, the regular black hole has been possessed a shifting over radial coordinate in its inner structure.

gr-qc↗

arXiv2020

Gauss-Bonnet Inflation after Planck2018

We study the primordial perturbations and reheating process in the models where the Gauss-Bonnet term is non-minimally coupled to the canonical and non-canonical (DBI and tachyon) scalar fields. We consider several potentials and Gauss-Bonnet coupling terms as power-law, dilaton-like, $\cosh$-type, E-model and T-model. To seek the observational viability of these models, we study the scalar perturbations numerically and compare the results with the Planck2018 TT, TE, EE+lowE+lensing+BK14+BAO joint data at $68\%$ CL and $95\%$ CL. We also study the tensor perturbations in confrontation with the Planck2018 TT, TE, EE+lowE+lensing+BK14+BAO+ LIGO$\&$Virgo2016 joint data at $68\%$ CL and $95\%$ CL. In this regard, we obtain some constraints on the Gauss-Bonnet coupling parameter $β$. Another important process in the early universe is the reheating phase after inflation which is necessary to reheat the universe for subsequent evolution. In this regard, we study the reheating process in these models and find some expressions for the e-folds number and temperature during that era. Considering that from Planck TT,TE,EE+lowEB+lensing data and BICEP2/Keck Array 2014, based on the $Λ$CDM$+r+\frac{dn_{s}}{d\ln k}$ model, we have $n_{s}=0.9658\pm 0.0038$ and $r<0.072$, we obtain some constraints on the e-folds number and temperature. From the values of the e-folds number and the effective equation of state and also the observationally viable value of the scalar spectral index, we explore the capability of the models in explaining the reheating phase.

astro-ph.CO↗

arXiv2019

Mimetic DBI Inflation in Confrontation with Planck2018 data

We study mimetic gravity in the presence of a DBI-like term which is a non-canonical setup of the scalar field's derivatives. We consider two general cases with varying and constant sound speeds and construct the potentials for both the DBI and Mimetic DBI models. By considering the power-law scale factor as $a=a_{0}\,t^{n}$, we seek for the observational viability of these models. We show that, the Mimetic DBI model in some ranges of the parameters space is free of ghost and gradient instabilities. By studying $r-n_{s}$ and $α_{s}-n_{s}$ behavior in confrontation with Planck2018 data, we find some constraints on the model's parameters. We show that for the case with varying sound speed, although power-law DBI inflation is not consistent with Planck2018 TT, TE, EE+low E+lensing data, but the Mimetic DBI inflation is consistent with Planck2018 TT, TE, EE+low E+lensing data at 95$\%$ CL, in some ranges of the model's parameters space as $40\leq n \leq 55$ where the model is instabilities-free in these ranges of parameters too. For the constant sound speed, by adopting some sample values of $c_{s}$, we study both DBI and Mimetic DBI model numerically and find $n\sim 10^{2}$ for DBI model and $n\sim 10$ for Mimetic DBI model. We also compare the results with Planck2018 TT, TE, EE+low E+lensing+BK14+BAO data and see that the DBI and Mimetic DBI model with varying sound speed are ruled out with these joint data. However, these models with constant sound speed are consistent with Planck2018 TT, TE, EE+low E+lensing+BK14+BAO data with $n\sim 10^{2}$ for DBI model and $n\sim 10$ for Mimetic DBI model. In this case, we find some tighter constraints on the corresponding sound speed.

astro-ph.CO↗

arXiv2019

Black Hole Production in the Presence of a Maximal Momentum in Horizon Wave Function Formalism

We study the Horizon Wave Function (HWF) description of a generalized uncertainty principle (GUP) black hole in the presence of two natural cutoffs as a minimal length and a maximal momentum. This is motivated by a metric which allows the existence of sub-Planckian black holes, where the black hole mass $m$ is replaced by $M=m\Big(1+\frac{β^{2}}{2}\frac{M_{pl}^{2}}{m^{2}}-β\frac{M_{pl}}{m}\Big)$. Considering a wave-packet with a Gaussian profile, we evaluate the HWF and the probability that the source might be a (quantum) black hole. By decreasing the free parameter the general form of probability distribution, ${\mathcal{P}}_{BH}$, is preserved , but this resulted in reducing the probability for the particle to be a black hole accordingly. The probability for the particle to be a black hole grows when the mass is increasing slowly for larger positive $β$, and for a minimum mass value it reaches to $0$. In effect, for larger $β$ the magnitude of $M$ and $r_{H}$ increases, matching with our intuition that either the particle ought to be more localized or more massive to be a black hole. The scenario undergoes a change for some values of $β$ significantly, where there is a minimum in ${\mathcal{P}}_{BH}$ , so this expresses that every particle can have some probability of decaying to a black hole. In addition, for sufficiently large $β$ we find that every particle could be fundamentally a quantum black hole.

physics.gen-ph↗

arXiv2019

The role of an invariant IR cutoff in late time cosmological dynamics

We study the role of an invariant infra-red cutoff in the late time cosmological dynamics. This low energy cutoff originates from the existence of a minimal measurable uncertainty as a result of the curvature of background manifold, and can be encoded in an extended uncertainty relation. Inspired by black hole entropy-area relation, we extend the analysis to the thermodynamics of apparent horizon of the universe. By treating both the Newtonian and general relativistic cosmologies, we show that the contribution of infra-red cutoff in the equations of dynamics can be interpreted as an effective fluid which is capable of explaining late time cosmic speed up and even transition to a phantom phase of expansion. We use the latest observational data from PLANCK2018 to constrain the parameter of an extended uncertainty relation.

gr-qc↗

arXiv2018

Black hole thermodynamics in Snyder phase space

By defining a noncommutative symplectic structure, we study thermodynamics of Schwarzschild black hole in a Snyder noncommutative phase space for the first time. Since natural cutoffs are the results of compactness of symplectic manifolds in phase space, the physics of black holes in such a space would be affected mainly by these cutoffs. In this respect, this study provides a basis for more deeper understanding of the black hole thermodynamics in a pure mathematical viewpoint.

gr-qc↗

arXiv2018

On the viability of Planck scale cosmology with quartessence

In this paper, using a qualitative theory of dynamical systems, we study the stability of a unified dark matter-dark energy framework known as quartessence Chaplygin model (QCM) with three different equation-of-states within ultraviolet (UV) deformed Friedmann-Robertson-Walker (FRW) cosmologies without Big-Bang singularity. The UV deformation is inspired by the non-commutative (NC) Snyder spacetime approach in which by keeping the transformation groups and rotational symmetry there is a dimensionless, Planck scale characteristic parameter $μ_0$ with dual implications dependent on its sign that addresses the required invariant cutoffs for length and momentum in nature, in a separate manner. Our stability analysis is done in the $(H,ρ)$ phase space at a finite domain concerning the hyperbolic critical points. According to our analysis, due to constraints imposed on the signs of $μ_0$ from the phenomenological parameters involved in quartessence models $(Ω_m^*, c_s^2, ρ_*)$, for an expanding and accelerating late universe, all three QCMs can be stable in the vicinity of the critical points. The requirement of stability for these quartessence models in case of admission of a minimum invariant length, can yield a flat as well as non-flat expanding and accelerating universe in which Big-Bang singularity is absent. This feedback also phenomenologically credits to braneworld-like framework versus loop quantum cosmology-like one as two possible scenarios which can be NC Snyder spacetime generators (correspond to $μ_0<0$ and $μ_0>0$, respectively). As a result, our analysis show that between quartessence models with Chaplygin gas equation-of-states and accelerating FRW backgrounds occupied by a minimum invariant length, there is a possibility of viability.

gr-qc↗

arXiv2018

Observational Viability of an Inflation Model with E-Model non-Minimal Derivative Coupling

By starting with a two-fields model in which the fields and their derivatives are nonminimally coupled to gravity, and then by using a conformal gauge, we obtain a model in which the derivatives of the canonically normalized field are nonminimally coupled to gravity. By adopting some appropriate functions, we study two cases with constant and E-model nonminimal derivative coupling, while the potential in both cases is chosen to be E-model one. We show that in contrary to the single field $α$-attractor model that there is an attractor \textit{point} in the large $N$ and small $α$ limits, in our setup and for both mentioned cases there is an attractor \emph{line} in these limits that the $r-n_{s}$ trajectories tend to. By studying the linear and nonlinear perturbations in this setup and comparing the numerical results with Planck2015 observational data, we obtain some constraints on the free parameter $α$. We show that by considering the E-model potential and coupling function, the model is observationally viable for all values of $M$ (mass scale of the model). We use the observational constraints on the tensor-to-scalar ratio and the consistency relation to obtain some constraints on the sound speed of the perturbations in this model. As a result, we show that in a nonminimal derivative $α$-attractor model, it is possible to have small sound speed and therefore large non-Gaussianity.

astro-ph.CO↗

arXiv2018

More on the Non-Gaussianity of Perturbations in a Non-Minimal Inflationary Model

We study nonlinear cosmological perturbations and their possible non-Gaussian character in an extended non-minimal inflation where gravity is coupled non-minimally to both the scalar field and its derivatives. By expansion of the action up to the third order, we focus on the non-linearity and non-Gaussianity of perturbations in comparison with recent observational data. By adopting an inflation potential of the form V(phi) = (1/n)(lambda)(phi)^n, we show that for n = 4, for instance, this extended model is consistent with observation if 0.013 <lambda< 0.095 in appropriate units. By restricting the equilateral amplitude of non-Gaussianity to the observationally viable values, the coupling parameter lambda is constraint to the values lambda< 0.1.

gr-qc↗

arXiv2018

Planck scale effects on the stochastic gravitational wave background generated from cosmological hadronization transition: A qualitative study

We reconsider the stochastic gravitational wave background spectrum produced during the first order hadronization process, in presence of ultraviolet cutoffs suggested by the generalized uncertainty principle as a promising signature towards the Planck scale physics. Unlike common perception that the dynamics of QCD phase transition and its phenomenological consequences are highly influenced by the critical temperature, we find that the underlying Planck scale modifications can affect the stochastic gravitational spectrum arising from the QCD transition without a noteworthy change in the relevant critical temperature. Our investigation shows that incorporating the natural cutoffs into MIT bag equation of state and background evolution leads to a growth in the stochastic gravitational power spectrum, while the relevant redshift of the QCD era, remains unaltered. These results have double implications from the point of view of phenomenology. Firstly, it is expected to enhance the chance of detecting the stochastic gravitational signal created by such a transition in future observations. Secondly, it gives a hint on the decoding from the dynamics of QCD phase transition.

gr-qc↗

arXiv2018

Lowering the self-coupling of the scalar field in a generalized Higgs inflation

We study cosmological dynamics of a generalized Higgs inflation. By expanding the action up to the second and third order in the small perturbations, we study the primordial perturbation and its non-Gaussian distribution. We study the non-Gaussian feature in both the equilateral and orthogonal configurations. By adopting a quartic potential, we perform a numerical analysis on the model's parameter space and compare the results with Planck2015 observational data. To obtain some observational constraint, we focus on the self-coupling and the non-minimal coupling parameters. We show that, in the presence of the non-minimal coupling and the Galileon-like interaction, the self-coupling parameter can be reduced to the order of $10^{-6}$ which is much larger than the value that CMB normalization suggests for this self-coupling.

gr-qc↗

arXiv2018

Circular orbits and accretion process in a class of Horndeski/Galileon black holes

In this paper the geodesics motion and accretion process around a subclass of Horndeski/Galileon black holes are investigated. Firstly, we present spherically symmetric geometries in a Horndeski/Galileon black hole spacetime by considering an isothermal fluid around the black hole. Then we focus on three main issues: in the first step circular orbits of test particles and their stability in equatorial plane are examined in details. Then, by treating perturbations via restoring forces, oscillations of particles around the central object are studied. Finally, the accretion process, the critical speed of the flow and accretion rate are investigated in this setup properly.

gr-qc↗

arXiv2018

Non-singular model universe from a perfect fluid scalar-metric cosmology

To seek for a singularity free model universe from a perfect fluid scalar-metric cosmology, we work in the "\emph{Emergent Cosmology}" (EC) paradigm which is a non-singular alternative for cosmological inflation. By using two methods including Linear Stability Theory and Effective Potential Formalism, we perform a classical analysis on the possible static solutions (that are called usually as Einstein Static Universes (ESU)in literature) in order to study EC paradigm in a FRW background. Our model contains a kinetic term of the scalar field minimally coupled to the background geometry without a potential term. The matter content of the model consists of a perfect fluid plus a cosmological constant $Λ$ as a separate source. In the framework of a local dynamical system analysis, we show that in the absence or presence of $Λ$, depending on some adopted values for the free parameters of the underlying cosmological model with flat and non-flat spatial geometries, one gets some static solutions which are viable under classical linear perturbations. By extending our study to a global dynamical system analysis, we show that in the presence of $Λ$ with non-flat spatial geometries there is a future global de Sitter attractor in this model. Following the second method, we derive a new static solution that represents a stable ESU but this time without dependence on the free parameters of the cosmological model at hand. As a whole, our analysis suggests the possibility of graceful realization of a non-singular EC paradigm (i.e. leaving the initial static phase and entering the inflation period as the universe is evolving) through either preserving or violation of the strong energy condition.

gr-qc↗

arXiv2018

Observational Status of Tachyon Natural Inflation and Reheating

We study observational viability of Natural Inflation with a tachyon field as inflaton. By obtaining the main perturbation parameters in this model, we perform a numerical analysis on the parameter space of the model and in confrontation with $68\%$ and $95\%$ CL regions of Planck2015 data. By adopting a warped background geometry, we find some new constraints on the width of the potential in terms of its height and the warp factor. We show that the Tachyon Natural Inflation in the large width limit recovers the tachyon model with a $ϕ^{2}$ potential which is consistent with Planck2015 observational data. Then we focus on the reheating era after inflation by treating the number of e-folds, temperature and the effective equation of state parameter in this era. Since it is likely that the value of the effective equation of state parameter during the reheating era to be in the range $0\leq ω_{eff}\leq \frac{1}{3}$, we obtain some new constraints on the tensor to scalar ratio as well as the e-folds number and reheating temperature in this Tachyon Natural Inflation model.

astro-ph.CO↗

arXiv2018

$α$-Attractor and Reheating in a Model with Non-Canonical Scalar Fields

We consider two non-canonical scalar fields (tachyon and DBI) with E-model type of the potential. We study cosmological inflation in these models to find possible $α$-attractors. We show that similar to the canonical scalar field case, in both tachyon and DBI models there is a value of the scalar spectral index in small $α$ limit which is just a function of the e-folds number. However, the value of $n_{s}$ in DBI model is somewhat different from the other ones. We also compare the results with Planck2015 TT, TE, EE+lowP data. The reheating phase after inflation is studied in these models which gives some more constraints on the model's parameters.

astro-ph.CO↗