arXiv subjects

H. Shabani

Publications and source records attributed to H. Shabani.

5 recordsLinked to original sources

arXiv2024

Gravitational Collapse without Singularity Formation in Brans-Dicke Theory

In the present work we study collapse process of a homogeneous and isotropic fluid in Brans-Dicke ({BD}) theory with non-vanishing spacetime torsion. In this theory, torsion can be generated by the {BD} scalar field as well as the intrinsic angular momentum (spin) of matter. Assuming the matter content of the collapsing body to be a Weyssenhoff fluid, which is a generalization of perfect fluid in general relativity ({GR}) in order to include the spin effects, we find that in BD theory with torsion, the existence of spin effects could avoid the spacetime singularity that forms in the original version of this theory (Scheel et al. in Phys Rev D 51:4208, 1995, Scheel et al. in Phys Rev D 51:4236, 1995). Numerical simulations of collapse model show that the spacetime singularity is replaced by a non-singular bounce, the spacetime event at which the collapse process halts at a minimum radius and then turns into an expanding phase. Moreover, the model parameters can be set so that the apparent horizon will never meet the boundary of the collapsing body so that the bounce event can be detectable by external observers in the Universe.

gr-qc↗

arXiv2022

The galaxy rotation curves in the $f(R,T)$ modified gravity formalism

Astronomical data have shown that the galaxy rotation curves are mostly flat in the far distance of the galactic cores, which reveals the insufficiency of our knowledges about how gravity works in these regimes. In this paper we introduce a resolution of this issue from the $f(R,T)$ modified gravity formalism perspective. By investigating two classes of models with separable (minimal coupling model) and inseparable (non-minimal coupling model) parts of the Ricci scalar $R$ and trace of the energy-momentum tensor $T$, we find that only in the latter models it is possible to attain flat galaxy rotation curves.

gr-qc↗

arXiv2021

Quantum gravity and the square of Bell operators

The Bell's inequality is a strong criterion to distinguish classical and quantum mechanical aspects of reality. Its violation is the net effect of the existence of non-locality in systems, an advantage for quantum mechanics (QM) over classical physics. The quantum mechanical world is under the control of the Heisenberg uncertainty principle (HUP) that is generalized by quantum gravity (QG) scenarios, called generalized uncertainty principle (GUP). Here, the effects of GUP on the square of Bell operators of qubits and qutrits are studied. The achievements claim that the violation quality of the square of Bell inequalities may be a tool to get a better understanding of the quantum features of gravity. In this regard, it is obtained that the current accuracy of the Stern-Gerlach experiments implies upper bounds on the values of the GUP parameters.

quant-ph↗

arXiv2021

Effects of Rastall parameter on perturbation of dark sectors of the Universe

In recent years, Rastall gravity is undergoing a considerable surge in popularity. This theory purports to be a modified gravity theory with a non-conserved energy-momentum tensor ({\rm EMT}) and an unusual non-minimal coupling between matter and geometry. The present work looks for the evolution of homogeneous spherical perturbations within the Universe in the context of Rastall gravity. Using the spherical Top-Hat collapse model we seek for exact solutions in linear regime for density contrast of dark matter (\rm DM) and dark energy ({\rm DE}). We find that the Rastall parameter affects crucially the dynamics of density contrasts for {\rm DM} and {\rm DE} and the fate of spherical collapse is different in comparison to the case of general relativity ({\rm GR}). Numerical solutions for perturbation equations in non-linear regime reveal that {\rm DE} perturbations could amplify the rate of growth of {\rm DM} perturbations depending on the values of Rastall parameter.

physics.gen-ph↗

arXiv2020

Structure Formation in Generalized Rastall Gravity

Recently a modified version of Rastall theory of gravity has been introduced in which a varying coupling parameter could act as dark energy (DE) and thus, it can be held responsible for the current accelerated expansion of the Universe. Motivated by this modification, we study here the evolution of linear and non-linear perturbations in the matter content of the Universe, utilizing spherically symmetric top-hat collapse scenario. The exact solutions we obtain in linear regime show that as the Universe evolves, matter density perturbations grow and reach a maximum value at a certain redshift after which these perturbations start decreasing towards a finite positive value at the present time. Depending on model parameters, exact oscillatory solutions can be also found representing that matter perturbations could experience either overdense and underdense regions during the dynamical evolution of the Universe. Numerical solutions in non-linear regime show that the amplitude of perturbations grow much faster than the linear one and diverges at a critical redshift. However, the formation of collapsed structures is delayed as compared to $Λ${\rm CDM} model. It is found that the running mutual interaction between matter and geometry, encoded in the variable Rastall coupling parameter, could drastically affect the dynamics of matter perturbations and their growth rate during the evolution of the Universe.

physics.gen-ph↗