arXiv subjects

Siamak Akhshabi

Publications and source records attributed to Siamak Akhshabi.

14 recordsLinked to original sources

arXiv2024

The Role of Torsion in Trans-Planckian Physics

The torsion of spacetime, if exists, plays an important role at the very early universe when the spin density of particles was very high. It is generally believed that in extremely high energies of the early universe a new physics called trans-Planckian physics should be considered. Since the initial conditions for inflation are probably a result of this new physics, here we consider spin and torsion as trans- Planckian effects and analyze their influence on the power spectrum of scalar and tensor perturbations at the end of the inflationary era.

gr-qc↗

arXiv2024

Gravitational Slip parameter and Gravitational Waves in Einstein-Cartan theory

We study the evolution of scalar and tensor cosmological perturbations in the framework of the Einstein-Cartan theory of gravity. The value of the gravitational slip parameter which is defined as the ratio of the two scalar potentials in the Newtonian gauge, can be used to determine whether or not the gravity is modified. We calculate the value of slip parameter in the Einstein-Cartan cosmology and show that it falls within the observed range. We also discuss the evolution of the cosmic gravitational waves as another measure of the modification of gravity.

gr-qc↗

arXiv2023

Cosmological Distances And Hubble Tension In Einstein-Cartan Theory

We analyze the measurement of cosmological distances in the presence of torsion in both Einstein-Cartan and Poincare gauge theory of gravity. Using the modified cosmological distance measurements, we use the observed time delays in gravitational lensing systems to determine the Hubble parameter. The results show the measured Hubble parameter from a lensing system can be less than its expected value in General Relativity for certain models of torsion and its associated density parameter. This can reduce the tension between late-time and early-universe measurements of the Hubble parameter, the so-called Hubble tension.

gr-qc↗

arXiv2021

Gravitational lensing by a black hole in Poincar{é} gauge theory of gravity

Using a recently found black hole solution in the framework of the Poincar{é} gauge theory of gravity, we study gravitational lensing for a system where the lens is a static spherically symmetric black hole. By analyzing the equations of motion for light rays in a space-time with torsion, we derive the deflection angle as the light emitted from a source pass through near the black hole and numerically solve the resulting integral. We also study the effects of torsion on the position of images. The results show that the presence of torsion slightly alters both the deflection angle and position of images in this setup.

gr-qc↗

arXiv2018

Light propagation and optical scalars in torsion theories of gravity

We investigate the propagation of light rays and evolution of optical scalars in gauge theories of gravity where torsion is present. Recently the modified Raychaudhuri equation in the presence of torsion has been derived. We use this result to derive the basic equations of geometric optics for several different interesting solutions of the Poincar{é} gauge theory of gravity. The results show that the focusing effects for neighboring light rays will be different than general relativity. This in turn has practical consequences in the study of gravitational lensing effects and also determining the angular diameter distance for cosmological objects.

gr-qc↗

arXiv2017

Inflation by spin and torsion in Poincare gauge theory of gravity

In Poincare gauge theory of gravity, in addition to mass-energy content, spin is also a source for gravitational interactions. Although the effects of spin are negligible at low energies, they can play a crucial role at very early universe when the spin density was very high. In this paper by choosing a suitable Lagrangian for Poincare gauge theory of gravity, and suitable energy-momentum and spin density tensors, we show that the effects of spin and torsion can lead to a inflationary phase without the need for any additional fields. No fine tuning of parameters is required in this setup. We also calculate the scalar spectral index at the end of inflation and show that it agrees with the most recent observational data.

gr-qc↗

arXiv2011

Noncommutative Geometry Inspired Entropic Inflation

Recently Verlinde proposed that gravity can be described as an emergent phenomena arising from changes in the information associated with the positions of material bodies. By using noncommutative geometry as a way to describe the microscopic, microstructure of quantum spacetime, we derive modified Friedmann equation in this setup and study the entropic force modifications to the inflationary dynamics of early universe.

hep-th↗

arXiv2010

Evolution of Perturbations in a Noncommutative Braneworld Inflation

Following our previous work in noncommutative braneworld inflation (arXiv:0911.4418), in this paper we use the smeared, coherent state picture of noncommutativity to study evolution of perturbations in a noncommutative braneworld scenario. We show that in this setup, the early stage of the universe evolution has a phantom evolution with imaginary effective sound speed. We show also that the amplitude of perturbations in the commutative regime decays faster than the noncommutative regime with the same parameter values, and as a result we need smaller number of e-folds in the noncommutative regime to have a successful braneworld inflation.

hep-th↗

arXiv2010

One more step toward the noncommutative brane inflation

Recently a new approach to inflation proposal has been constructed via the smeared coherent state picture of spacetime noncommutativity. Here we generalize this viewpoint to a Randall-Sundrum II braneworld scenario. This model realizes an inflationary, bouncing solution without recourse to any axillary scalar or vector fields. There is no initial singularity and the model has the potential to produce scale invariant spectrum of scalar perturbations.

hep-th↗

arXiv2009

Effects of the Generalized Uncertainty Principle on the Inflation Parameters

We investigate the effects of the generalized uncertainty principle on the inflationary dynamics of the early universe in both standard and braneworld viewpoint. We choose the Randall-Sundrum II model as our underlying braneworld scenario. We find that the quantum gravitational effects lead to a spectral index which is not scale invariant. Also, the amplitude of density fluctuations is reduced by increasing the strength of quantum gravitational corrections. However, the tensor-to-scalar ratio increases by incorporation of these quantum gravity effects. We outline possible manifestations of these quantum gravity effects in the recent and future observations.

gr-qc↗

arXiv2009

Chaotic inflation on the Randall-Sundrum 2-brane model

We construct an inflation model on the Randall-Sundrum I (RSI) brane where a bulk scalar field stabilizes the inter-brane separation. We study impact of the bulk scalar field on the inflationary dynamics on the brane. We proceed in two different approaches: in the first approach, the stabilizing field potential is directly appeared in the Friedmann equation and the resulting scenario is effectively a two-field inflation. In the second approach the stabilization mechanism is considered in the context of a warp factor so that there is just one field present that plays the roles of both inflaton and stabilizer. We study constraints imposed on the model parameters from recent observations.

hep-th↗

arXiv2008

Bouncing universe with the non-minimally coupled quintom matter on the warped DGP Brane

We construct a quintom dark energy model with two non-minimally coupled scalar fields, one quintessence and the other phantom field, confined on the warped DGP brane. We study some important issues such as phantom divide line crossing, existence of the bouncing solutions and the stability of the solutions in this framework. We show that this model accounts for crossing of the phantom divide line and realization of the bouncing solutions. This model allows for stability of the solutions in separate regions of the $ω$-$ω'$ phase-plane.

hep-th↗

arXiv2007

Orbits of particles in noncommutative Schwarzschild spacetime

By considering particles as smeared objects, we investigate the effects of space noncommutativity on the orbits of particles in Schwarzschild spacetime. The effects of space noncommutativity on the value of the precession of the perihelion of particle orbit and deflection of light ray in Schwarzschild geometry are calculated and the stability of circular orbits is discussed.

gr-qc↗

arXiv2006

On the Stability of Planetary Circular Orbits in Noncommutative Spaces

We investigate the effects of space noncommutativity and the generalized uncertainty principle on the stability of circular orbits of particles in both a central force potential and Schwarzschild spacetime. We find noncommutative form of the effective potential which up to first order of noncommutativity parameter contains an angular momentum dependent extra term. This angular momentum dependent extra term affects the stability of circular orbits in such a way that the radius of a stable circular orbit in noncommutative space is larger than its commutative counterpart. In the case of large angular momentum, the condition for stability of circular orbits in noncommutative space differs considerably from commutative case.

gr-qc↗