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Farhad Darabi

Publications and source records attributed to Farhad Darabi.

13 recordsLinked to original sources
arXiv2021

$α$-Attractors and Reheating in a Non-Minimal Inflationary Model

We study $α$-attractor models with both E-model and T-model potential in an extended Non-Minimal Derivative (NMD) inflation where a canonical scalar field and its derivatives are non-minimally coupled to gravity. We calculate the evolution of perturbations during this regime. Then by adopting inflation potentials of the model we show that in the large N and small $α$ limit, value of the scalar spectral index ns and tensor to scalar ratio r are universal. Next, we study reheating after inflation in this formalism. We obtain some constraints on the model's parameter space by adopting the results with Planck 2018.

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

Time Dependent Geometry in Massive Gravity

In this paper, we will analyze a time dependent geometry in a massive theory of gravity. This will be done by analyzing Vaidya space-time in such a massive theory of gravity. As gravitational collapse is a time dependent system, we will analyze it using the Vaidya space-time in massive gravity. The Vainshtein and dRGT mechanisms are used to obtain a ghost free massive gravity, and construct such time dependent solutions. Singularities formed, their nature and strength will be studied in detail. We will also study the thermodynamical aspects of such a geometry by calculating the important thermodynamical quantities for such a system, and analyzing the thermodynamical behavior of such quantities.

physics.gen-ph↗
arXiv2017

Vaidya Spacetime in Massive Gravity's Rainbow

In this paper, we will analyze the energy dependent deformation of massive gravity using the formalism of massive gravity's rainbow. So, we will use the Vainshtein mechanism and the dRGT mechanism for the energy dependent massive gravity, and thus analyze a ghost free theory of massive gravity's rainbow. We study the energy dependence of a time-dependent geometry, by analyzing the radiating Vaidya solution in this theory of massive gravity's rainbow. The energy dependent deformation of this Vaidya metric will be performed using suitable rainbow functions.

gr-qc↗
arXiv2012

Signature Change in Noncommutative FRW Cosmology

The conditions for which the no boundary proposal may have a classical realization of a continuous change of signature, are investigated for a cosmological model described by FRW metric coupled with a self interacting scalar field, having a noncommutative phase space of dynamical variables. The model is then quantized and a good correspondence is shown between the classical and quantum cosmology indicating that the noncommutativity does not destruct the classical-quantum correspondence. It is also shown that the quantum cosmology supports a signature transition where the bare cosmological constant takes a vast continuous spectrum of negative values. The bounds of bare cosmological constant are limited by the values of noncommutative parameters. Moreover, it turns out that the physical parameters are constrained by the noncommutativity parametres.

gr-qc↗
arXiv2011

Quantum Corrections for a Braneworld Black Hole

By using the quantum tunneling approach over semiclassical approximations, we study the quantum corrections to the Hawking temperature, entropy and Bekenstein-Hawking entropy-area relation for a black hole lying on a brane.

gr-qc↗
arXiv2010

Correspondence between Jordan-Einstein frames and Palatini-metric formalisms

We discuss the conformal symmetry between Jordan and Einstein frames considering their relations with the metric and Palatini formalisms for modified gravity. Appropriate conformal transformations are taken into account leading to the evident connection between the gravitational actions in the two mentioned frames and the Hilbert-Einstein action with a cosmological constant. We show that the apparent differences between Palatini and metric formalisms strictly depend on the representation while the number of degrees of freedom is preserved. This means that the dynamical content of both formalism is identical.

gr-qc↗
arXiv2010

Self-gravitational corrections to the Cardy-Verlinde formula of charged BTZ black hole

The entropy of the charged BTZ black hole horizon is described by the Cardy-Verlinde formula. We then compute the self-gravitational corrections to the Cardy-Verlinde formula of the charged BTZ black hole in the context of Keski-Vakkuri, Kraus and Wilczek (KKW) analysis. The self-gravitational corrections to the entropy as given by the Cardy-Verlinde formula are found to be positive. This result provides evidence in support of the claim that the holographic bound is not universal in the framework of two-dimensional gravity models.

physics.gen-ph↗
arXiv2010

Equivalence between Palatini and metric formalisms of f(R)-gravity by divergence free current

The equivalence between metric and Palatini formalisms in f(R)-gravity can be achieved in the general context of theories with divergence free current. This equivalence is a necessary result of a symmetry which is included in a particular conservation equation of the current. In fact the conservation equation, by an appropriate redefinition of the introduced auxiliary field, may be encoded in a massless scalar field equation.

gr-qc↗
arXiv2000

de Broglie oscillation, rest mass and inertia

The model of a relativistic free massive point particle is investigated in the context of a Hamiltonian constraint system. It is shown that de Broglie oscillation, rest mass and inertia may be described within this model of Hamiltonian constraint system.

gr-qc↗
arXiv1999

Large scale-small scale duality and cosmological constant

We study a model of quantum cosmology originating from a classical model of gravitation where a self interacting scalar field is coupled to gravity with the metric undergoing a signature transition. We show that there are dual classical signature changing solutions, one at large scales and the other at small scales. It is possible to fine-tune the physics in both scales with an infinitesimal effective cosmological constant.

gr-qc↗
arXiv1998

Inertia as Emerging of a Hamiltonian Constraint System

The issue of inertia as opposition to acceleration of a massive point particle in Minkowski space-time is investigated in the context of a Hamiltonian constraint system. It is shown that the inertia as a locally-originating force in Minkowski space-time may emerge due to a global constraint.

physics.gen-ph↗