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A. Shojai

Publications and source records attributed to A. Shojai.

15 recordsLinked to original sources
arXiv2022

Observational assessment of the viability of de Sitter-Gödel-de Sitter phase transition

de Sitter--Gödel--de Sitter phase transition(dGd) is a possible geometrical phase transition in the very early universe. It induces fluctuations with possibly observable traces on matter and radiation fields. Here we present a simulation based on dGd to investigate possible perturbations which could be along with the standard inflationary fluctuations in the cosmic microwave background (CMB) and distribution of the large-scale structure. The power spectrum of perturbations is characterized by a parameter pair, labeled here as ($p_1, p_2$). With {\it Planck} observations we find $p_1=0.008^{+0.003}_{-0.008}$ and $p_2= 0.002^{+0.001}_{-0.002}$ consistent with pure inflationary power spectrum and no hint for the dGd transition. Also, it is estimated future large-scale surveys such as Euclid and SKA can further tighten the constraints up to an order of magnitude and probe the physics of the early universe with much higher precision.

gr-qc↗
arXiv2021

Induced rotation from de Sitter-Godel-de Sitter phase transition

Therotationofthecosmicobjectsisauniversalphenomenonanditsoriginisstillanopenquestion.Here a model for the origin of rotation is presented. After an investigation of the phase transition of a scalar field in de Sitter and Gödel backgrounds, the motion of a test particle during the phase transitions is studied. Then using computer simulation for a congruence of particles, we show that although the local induced rotation is nonzero, the global rotation is below the observational limit.

gr-qc↗
arXiv2020

Traversability of quantum improved wormhole solution

We have investigated the problem of traversability of wormholes in the framework of quantum improvement of gravity theory arising from functional renormalization group methods to describe the asymptotic safe quantum gravity. We have shown that improved pseudospherical wormholes could be traversable with non--exotic matter, while spherical ones could not. This is done within a specific model of improvement.

gr-qc↗
arXiv2020

Geodesic congruences in quantum improved spacetimes

We investigate the geodesic deviation equation in the context of quantum improved spacetimes. The improved Raychaudhuri equation is derived, and it is shown that the classical strong energy condition does not necessarily lead to the convergence of geodesics in a congruence in the quantum improved spacetime.

gr-qc↗
arXiv2020

On the quantum improved Schwarzschild black hole

Deriving the gravitational effective action directly from exact renormalization group is very complicated, if not impossible. Hence, to study the effects of running gravitational coupling which tends to a non--Gaussian UV fixed point (as it is supposed by the asymptotic safety conjecture), two steps are usually adopted. Cutoff identification and improvement of the gravitational coupling to the running one. As suggested in [1], a function of all independent curvature invariants seems to be the best choice for cutoff identification of gravitational quantum fluctuations in curved spacetime and makes the action improvement, which saves the general covariance of theory, possible. Here, we choose Ricci tensor square for this purpose and then the equation of motion of improved gravitational action and its spherically symmetric vacuum solution are obtained. Indeed, its effect on the massive particles' trajectory and the black hole thermodynamics are studied.

gr-qc↗
arXiv2019

Casimir force in the Gödel space-time and its possible induced cosmological inhomogeneity

The Casimir force between two parallel plates in the Gödel universe is computed for a scalar field at finite temperature. It is observed that when the plates separation is comparable with the scale given by the rotation of the space-time, the force becomes repulsive and then approaches zero. Since it has been shown previously that the universe may experience a Godel phase for a small period of time, the induced inhomogeneities from the Casimir force are also studied.

hep-th↗
arXiv2019

On the cutoff identification and the quantum improvement in asymptotically safe gravity

Applying the exact renormalization group method to search the nonGaussian fixed points of gravitational coupling, is frequently followed by two steps: cutoff identification and improvement. Although there are various models for identifying the cutoff momentum by some physical length, saving the general covariance should be considered as an important property in the procedure. In this paper, use of an arbitrary function of curvature invariants for cutoff identification is suggested. It is shown that the field equations for this approach differs from the ones obtained from the conventional cutoff identification and improvement, even for nonvacuum solutions of the improved Einstein equations. Indeed, it is concluded that these two steps are correlated to each other.

gr-qc↗
arXiv2018

On the effect of renormalization group improvement on the cosmological power spectrum

Asymptotically safe quantum gravity predicts running gravitational and cosmological constants, while it remains a meaningful quantum field theory because of the existence of non-Gaussian ultraviolet fixed points. Here we have investigated the effect of such running couplings on the cosmological perturbations. We have obtained the improved Mukhanov-Sassaki equation and solved it for two models. The effect of asymptotically safe quantum gravity on the cosmological power spectrum is also studied.

gr-qc↗
arXiv2015

The equivalence principle and the relative velocity of local inertial frames

In this paper we explicitly show that in general relativity, the relative velocity of two local inertial frames is always less than the velocity of light. This fact is a by-product of the equivalence principle. The general result is then illustrated within two examples, the FLRW cosmological model and the Schwarzschild metric.

gr-qc↗
arXiv2014

An f(R) model for dark matter: rotation curves and gravitational lensing

There should be two ways to describe the flat rotation curves of galaxies and cluster of galaxies. Either one can introduce a dark matter component for the matter filling the halo, or by modifying the gravity theory and give the flat rotation curve a geometrical nature. Here we adopt an f(R) model suitable for describing the effect. After matching the solution with the exterior solution, the effective density, radial and tangential pressures are obtained. Then the energy conditions and lensing effect is investigated.

gr-qc↗
arXiv2013

Notes on the compatibility of type Ia supernovae data and varying--$G$ cosmology

Observational data for type Ia supernovae, shows that the expansion of the universe is accelerated. This accelerated expansion can be described by a cosmological constant or by dark energy models like quintessence. An interesting question may be raised here. Is it possible to describe the accelerated expansion of universe using varying--$G$ cosmological models? Here we shall show that the price for having accelerated expansion in slow--varying--$G$ models (in which the dynamical terms of $G$ are ignored) is to have highly non--conserved matter and also that it is in contradiction with other data.

gr-qc↗
arXiv2009

Statefinder diagnosis of nearly flat and thawing non-minimal quintessence

Non-minimally coupled scalar field models of dark energy are equivalent to an interacting quintessence in the Einstein's frame. Considering two special important choices of the potential of the scalar field, i.e. nearly flat and thawing potential, one has an analytical expression for the equation of state parameter as a function of the density parameter of the scalar field for any choice. Here we investigate the non-minimal quintessence model by applying the method of statefinder diagnosis to it and plotting the evolutionary trajectories of the statefinder parameters.

gr-qc↗