arXiv subjects

Kourosh Nozari

Publications and source records attributed to Kourosh Nozari.

At least 109 records · Page 6Linked to original sources

arXiv2011

Observational Constraints on the Normal Branch of a Warped DGP Cosmology

We investigate observational constraints on the normal branch of the warped DGP braneworld cosmology by using observational data from Type Ia Supernovae (SNIa), Baryon Acoustic Oscillations (BAO), Cosmic Microwave Background (CMB) and Baryon Gas Mass Fraction of cluster of galaxies. The best fit values of model free parameters are: $Ω_m=0.240^{+0.050}_{-0.130}$ and $Ω_{r_c}=0.000^{+0.014}$ at $1σ$ confidence interval by using Gold sample SNIa+CMB shift parameter+BAO+Gas mass fraction of baryons in cluster of galaxies. The results for essence sample SNIa combined with CMB shift parameter, BAO and Baryon Gas mass fraction correspont to: $Ω_m= 0.220^{+0.020}_{-0.170}$ and $Ω_{r_c}=0.000^{+0.025}$ at 68.3% confidence interval. We determine the age of the universe by using these best fit values. We also study the effective cosmological dynamics on the brane via an effective equation of state parameter and the deceleration parameter to conclude that an effective phantom-like behavior arises in this scenario.

astro-ph.CO↗

arXiv2011

Minimal length, maximal momentum and the entropic force law

Different candidates of quantum gravity proposal such as string theory, noncommutative geometry, loop quantum gravity and doubly special relativity, all predict the existence of a minimum observable length and/or a maximal momentum which modify the standard Heisenberg uncertainty principle. In this paper, we study the effects of minimal length and maximal momentum on the entropic force law formulated recently by E. Verlinde.

gr-qc↗

arXiv2011

On the cosmological viability of the Hu-Sawicki type modified induced gravity

It has been shown recently that the normal branch of a DGP braneworld scenario self-accelerates if the induced gravity on the brane is modified in the spirit of $f(R)$ modified gravity. Within this viewpoint, we investigate cosmological viability of the Hu-Sawicki type modified induced gravity. Firstly, we present a dynamical system analysis of a general $f(R)$-DGP model. We show that in the phase space of the model, there exist three standard critical points; one of which is a de Sitter point corresponding to accelerating phase of the universe expansion. The stability of this point depends on the effective equation of state parameter of the curvature fluid. If we consider the curvature fluid to be a canonical scalar field in the equivalent scalar-tensor theory, the mentioned de Sitter phase is unstable, otherwise it is an attractor, stable phase. We show that the effective equation of state parameter of the model realizes an effective phantom-like behavior. A cosmographic analysis shows that this model, which admits a stable de Sitter phase in its expansion history, is a cosmologically viable scenario.

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↗

arXiv2011

$Λ$CDM, $Λ$DGP and extended phantom-like cosmologies

In this paper we compare outcomes of some extended phantom-like cosmologies with each other and also with $Λ$CDM\, and $Λ$DGP. We focus on the variation of the luminosity distances, the age of the universe and the deceleration parameter versus the redshift in these scenarios. In a dynamical system approach, we show that the accelerating phase of the universe in the $f(R)$-DGP scenario is stable if one consider the \emph{curvature fluid} as a phantom scalar field in the equivalent scalar-tensor theory, otherwise it is a transient and unstable phenomenon. Up to the parameters values adopted in this paper, the extended $F(R,ϕ)$-DGP scenario is closer to the $Λ$CDM scenario than other proposed models. All of these scenarios explain the late-time cosmic speed-up in their normal DGP branches, but the redshift at which transition to the accelerating phase occurs are different: while the $Λ$DGP model transits to the accelerating phase much earlier, the $F(R,ϕ)$-DGP model transits to this phase much later than other scenarios. Also, within the parameter spaces adopted in this paper, the age of the universe in the $f(R)$-DGP model is larger than $Λ$CDM, but this age in $F(G,ϕ)$-DGP is smaller than $Λ$CDM.

gr-qc↗

arXiv2011

Black hole solutions in the warped DGP braneworld

We study the static, analytical solution of black holes in the warped DGP braneworld scenario. We show that the linearized field equations and matching conditions lead to solutions that are not compatible with Schwarzschild-(A)dS$_{(4)}$ solutions on the brane. This incompatibility is similar to vDVZ discontinuity in massive gravity theory. Following the standard procedure to remove this discontinuity, which firstly was proposed by Vainshtein, we keep some appropriate nonlinear terms in the field equations. This strategy has its origin in the fact that the spatial extrinsic curvature of the brane plays a crucial role in the nonlinear nature of the solutions and also in recovering the well-measured predictions of General Relativity (GR) at small scales. Using this feature, we obtained an interesting black string solution in the bulk when it is compatible with 4D GR solutions on the brane.

hep-th↗

arXiv2011

The effects of minimal length and maximal momentum on the transition rate of ultra cold neutrons in gravitational field

The existence of a minimum observable length and/or a maximum observable momentum is in agreement with various candidates of quantum gravity such as string theory, loop quantum gravity, doubly special relativity and black hole physics. In this scenario, the Heisenberg uncertainty principle is changed to the so-called Generalized (Gravitational) Uncertainty Principle (GUP) which results in modification of all Hamiltonians in quantum mechanics. In this paper, following a recently proposed GUP which is consistent with quantum gravity theories, we study the quantum mechanical systems in the presence of both a minimum length and a maximum momentum. The generalized Hamiltonian contains two additional terms which are proportional to $αp^3$ and $α^2 p^4$ where $α\sim 1/M_{Pl}c$ is the GUP parameter. For the case of a quantum bouncer, we solve the generalized Schrodinger equation in the momentum space and find the modified energy eigenvalues and eigenfunctions up to the second-order in GUP parameter. The effects of the GUP on the transition rate of ultra cold neutrons in gravitational spectrometers are discussed finally.

hep-th↗

arXiv2010

Minimal Length and Bouncing Particle Spectrum

In this paper we study the effects of the Generalized Uncertainty Principle (GUP) on the spectrum of a particle that is bouncing vertically and elastically on a smooth reflecting floor in the Earth's gravitational field (a quantum bouncer). We calculate energy levels and corresponding wave functions of this system in terms of the GUP parameter. We compare the outcomes of our study with the results obtained from elementary quantum mechanics. A potential application of the present study is discussed finally.

hep-th↗

arXiv2010

Observational constraints on Chaplygin cosmology in a braneworld scenario with induced gravity and curvature effect

We study cosmological dynamics and late-time evolution of an extended induced gravity braneworld scenario. In this scenario, curvature effects are taken into account via the Gauss-Bonnet term in the bulk action and there is also a Chaplygin gas component on the brane. We show that this model mimics an effective phantom behavior in a relatively wider range of redshifts than previously formulated models. It also provides a natural framework for smooth crossing of the phantom-divide line due to presence of the Chaplygin gas component on the brane. We confront the model with observational data from type Ia Supernovae, Cosmic Microwave Background and Baryon Acoustic Oscillations to constraint the model parameters space.

astro-ph.CO↗

arXiv2010

Non-Minimal Warm Inflation and Perturbations on the Warped DGP Brane with Modified Induced Gravity

We construct a warm inflation model with inflaton field non-minimally coupled to induced gravity on a warped DGP brane. We incorporate possible modification of the induced gravity on the brane in the spirit of $f(R)$-gravity. We study cosmological perturbations in this setup. In the case of two field inflation such as warm inflation, usually entropy perturbations are generated. While it is expected that in the case of one field inflation these perturbations to be removed, we show that even in the absence of the radiation field, entropy perturbations are generated in our setup due to non-minimal coupling and modification of the induced gravity.

hep-th↗

arXiv2010

Non-Minimal Inflation Revisited

We reconsider an inflationary model that inflaton field is non-minimally coupled to gravity. We study parameter space of the model up to the second ( and in some cases third ) order of the slow-roll parameters. We calculate inflation parameters in both Jordan and Einstein frames and the results are compared in these two frames and also with observations. Using the recent observational data from combined WMAP5+SDSS+SNIa datasets, we study constraint imposed on our model parameters especially the nonminimal coupling $ξ$.

gr-qc↗

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↗

arXiv2010

Nonminimal Inflation on the Randall-Sundrum II Brane with Induced Gravity

We study an inflation model that inflaton field is non-minimally coupled to the induced scalar curvature on the Randall-Sundrum (RS) II brane. We investigate the effects of the non-minimal coupling on the inflationary dynamics of this braneworld model. Our study shows that the number of e-folds decreases by increasing the value of the non-minimal coupling. We compare our model parameters with the minimal case and also with recent observational data. In comparison with recent observation, we obtain a constraint on the values that the non-minimal coupling attains.

gr-qc↗

arXiv2009

Holographic Dark Energy from a Modified GBIG Scenario

We construct a holographic dark energy model in a braneworld setup that gravity is induced on the brane embedded in a bulk with Gauss-Bonnet curvature term. We include possible modification of the induced gravity and its coupling with a canonical scalar field on the brane. Through a perturbational approach to calculate the effective gravitation constant on the brane, we examine the outcome of this model as a candidate for holographic dark energy.

gr-qc↗

arXiv2009

Phantom-like effects in asymmetric brane embedding with induced gravity and the Gauss-Bonnet term in the Bulk

We construct an asymmetric braneworld embedding with induced gravity on the brane, where stringy effects are taken into account by incorporation of the Gauss-Bonnet term in the bulk action. We derive the effective Friedmann equation of the brane and then we investigate the possible realization of the phantom-like behavior in this setup. We show that in the absence of the Gauss-Bonnet term in the bulk action (a pure induced gravity scenario), the phantom-like behavior in asymmetric case can be realized in smaller redshift than the corresponding symmetric case. We show also that in the general case with curvature effect, the phantom-like behavior can be realized in two subcases: in a symmetric subcase and also in an asymmetric branch of the solutions. In either cases this phantom-like behavior happens without introducing any phantom fields neither on the brane nor in the bulk.

gr-qc↗

arXiv2009

Phantom Mimicry on the Normal Branch of a DGP-inspired Braneworld Scenario with Curvature Effect

It has been shown recently that phantom-like effect can be realized on the normal branch of the DGP setup without introduction of any phantom matter neither in the bulk nor on the brane and therefore without violation of the null energy condition. It has been shown also that inclusion of the Gauss-Bonnet term in the bulk action modifies this picture via curvature effects. Here, based on the Lue-Starkman conjecture on the dynamical screening of the brane cosmological constant in the DGP setup, we extend this proposal to a general DGP-inspired $f(R,ϕ)$ model that stringy effects in the ultra-violet sector of the theory are taken into account by inclusion of the Gauss-Bonnet term in the bulk action. We study cosmological dynamics of this setup, especially its phantom-like behavior and possible crossing of the phantom divide line especially with a non-minimally coupled quintessence field on the brane. In this setup, scalar field and curvature quintessence are treated in a unified framework.

gr-qc↗

arXiv2009

Phantom-Like Behavior of a DGP-Inspired Scalar-Gauss-Bonnet Gravity

We study the phantom-like behavior of a DGP-inspired braneworld scenario where curvature correction on the brane is taken into account. We include a possible modification of the induced gravity on the brane by incorporating higher order curvature terms of Gauss-Bonnet type. We investigate the cosmological implications of the model and we show that the normal branch of the scenario self-accelerates in this modified scenario without introducing any dark energy component. Also, a phantom-like behavior can be realized in this model without introducing any phantom field that suffers from serious difficulties such as violation of the null energy condition.

gr-qc↗