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Ali Shojai

Publications and source records attributed to Ali Shojai.

At least 19 recordsLinked to original sources
arXiv2026

On the Emergence of the Hysteresis Effect in Gravitational Systems

Previously, it was shown that the interplay between the gravitational memory effect and the thermodynamics of an ensemble of spinning objects gives rise to a hysteresis effect, which can be termed Gravo-Thermo memory. This effect can be studied either through a statistical approach or via kinetic theory. Here, we present a general kinetic theory model for the general case of a fluid composed of spinning extended objects in the presence of gravity.

gr-qc↗
arXiv2025

On the Dynamics of Extended Bodies and the GravoThermo Memory Effect

The non-linearity of the theory of gravity induces a hysteresis effect in both the systems interacting with gravity and in the gravitational field. The effect is usually referred to as the memory effect. In this paper, we explore this phenomenon in the context of the dynamics of extended rotating objects in the presence of a gravitational wave pulse. Then, we consider an ensemble of such objects and show that a redistribution of spin orientations takes place as a gravitational wave passes through. We examine how this redistribution, changes the partition function and the thermodynamic quantities including entropy and energy of the system. The result shows that some information about the source of the gravitation wave is encoded in the thermodynamics of the system.

gr-qc↗
arXiv2024

On the Gravitational Hysteresis in the Kinetic Theory

General theory of relativity is non--linear in nature and therefore can result in hysteresis-like effects and cause systems to remember the footprint of the gravitational field. Here we have investigated this effect using the Kinetic theory in curved spacetime. It is shown that the entropy rate experiences this hysteresis behavior. The effect is then considered for some special spacetimes, including Schwarzschild black hole, Friedmann--Lema\^itre--Robertson--Walker cosmological solution, and the flat Minkowski spacetime perturbed by a gravitational wave pulse. It is shown that there is some hysteresis effect for the entropy rate.

gr-qc↗
arXiv2023

On the Gravitational Precession Memory Effect for an Ensemble of Gyroscopes

We study the thermodynamic properties of a freely falling ensemble of gyroscopes after the passage of a weak gravitational wave. Due to the precession memory effect, the thermodynamic quantities will experience a change because of the space-time perturbation. We discuss that this GravoThermo memory effect potentially can be used for the detection of the gravitational waves.

gr-qc↗
arXiv2022

On the quantum gravastar

In this paper, we show that it is possible to have pure quantum gravastar solution to the quantum improved Einstein equations. Such an object is essentially a gravastar in which the thin layer of matter is replaced by quantum effects. The metric of a pure quantum gravastar is obtained and its stability is discussed.

gr-qc↗
arXiv2021

On the Quantum Improved Affine Gravity

Improvement of the classical gravity with the running gravitational coupling obtained from asymptotically safe gravity, is a good way of considering the effects of quantum gravity. This is usually done for metric theories of gravity. Here we investigate the effects of such an improvement for pure affine theories of gravity. To motivate the approach, we first consider the effects of quantum improvement on the connection using metric theory and investigate the effects on the causal structure of black hole solution. Next in the framework of Schrödinger-Eddington affine theory, the general way of affine improvement is presented and a spherically symmetric solution is obtained and compared with other ways of improvement.

gr-qc↗
arXiv2012

Cosmological solutions of Tensor-Vector theories of gravity with varying the space-time-matter coupling constant

We consider tensor-vector theories with varying the space-time-matter coupling constant (varying Einstein velocity) in a spatially flat FRW universe. We examine the dynamics of this model by dynamical system method assuming a ΛCDM background and we find some exact solutions by considering the character of critical points of the theory and their stability conditions. Then we reconstruct the potential V(A^2) and the coupling Z(A^2) by demanding a background ΛCDM cosmology. Also we set restrictions on the varying Einstein velocity to solve the horizon problem. This gives a selection rule for choosing the appropriate stable solution. We will see that it is possible to produce the background expansion history H(z) indicated by observations. Finally we will discuss the behavior of the speed of light (c_E) for those solutions.

physics.gen-ph↗
arXiv2011

Some static spherically symmetric interior solutions of $f(R)$ gravity

We have found some new exact static spherically symmetric interior solutions of metric $f(R)$ gravitational theories describing the equilibrium configuration of a star. Then the solution is matched to the exterior solution and thus gives a complete description of a star in $R^{1-n/2}$ theory.

gr-qc↗
arXiv2011

Non-minimal Quintessence: Dynamics and coincidence problem

Brans--Dicke scalar--tensor theory provides a conformally coupling of the scalar field with gravity in Einstein's frame. This model is equivalent to an interacting quintessence in which dark matter is coupled to dark energy. This provides a natural mechanism to alleviate the coincidence problem. We investigate the dynamics of this model and show that it leads to comparable dark energy and dark matter densities today.

gr-qc↗
arXiv2010

f(R) cosmological solutions with varying speed of light

We consider f(R) modified gravity theories for describing varying speed of light in a spatially flat FRW model, and find some exact solutions. Also we examine the dynamics of this model by dynamical system method assuming a LambdaCDM background and we find some exact solutions by considering the character of critical points of the theory in both formalisms. The behaviour of the speed of light is obtained.

gr-qc↗
arXiv2009

Speed of light in the extended gravity theories

We shall investigate the possibility of formulation of varying speed of light (VSL) in the framework of Palatini non-linear Ricci scalar and Ricci squared theories. Different speeds of light including the causal structure constant, electromagnetic, and gravitational wave speeds are discussed. We shall see that two local frames are distinguishable and discuss about the velocity of light in these two frames. We shall investigate which one of these local frames is inertial.

gr-qc↗
arXiv2008

Geodesic Congruences in the Palatini f(R) Theory

We shall investigate the properties of a congruence of geodesics in the framework of Palatini f(R) theories. We shall evaluate the modified geodesic deviation equation and the Raychaudhuri's equation and show that f(R) Palatini theories do not necessarily lead to attractive forces. Also we shall study energy condition for f(R) Palatini gravity via a perturbative analysis of the Raychaudhuri's equation.

gr-qc↗
arXiv2008

Causal loop quantum cosmology in momentum space

We shall show that it is possible to make a causal interpretation of loop quantum cosmology using the momentum as the dynamical variable. We shall show that one can derive Bohmian trajectories. For a sample cosmological solution with cosmological constant, the trajectory is plotted.

gr-qc↗
arXiv2008

f(R) Quantum Cosmology

We have quantized a flat cosmological model in the context of the metric f(R) models, using the causal Bohmian quantum theory. The equations are solved and then we have obtained how the quantum corrections influence the classical equations.

gr-qc↗
arXiv2006

Variational Methods in Loop Quantum Cosmology

An action functional for the loop quantum cosmology difference equation is presented. It is shown that by guessing the general form of the solution and optimizing the action functional with respect to the parameters in the guessed solution one can obtain approximate solutions which are reasonably good.

gr-qc↗
arXiv2005

Causal Loop Quantum Gravity and Cosmological Solutions

We shall present here the causal interpretation of canonical quantum gravity in terms of new variables. Then we shall apply it to the minisuperspace of cosmology. A vacuum solution of quantum cosmology is obtained and the Bohmian trajectory is investigated. At the end a coherent state with matter is considered in the cosmological model.

gr-qc↗
arXiv2004

Constraint Algebra in Causal Loop Quantum Gravity

de-Broglie--Bohm causal interpretation of canonical quantum gravity in terms of Ashtekar new variables is built. The Poisson brackets of (deBroglie--Bohm) constraints are derived and it is shown that the Poisson bracket of Hamiltonian with itself would change with respect to its classical counterpart.

gr-qc↗