arXiv subjects

Y. Heydarzade

Publications and source records attributed to Y. Heydarzade.

24 records · Page 2Linked to original sources

arXiv2016

Emergent universe in Horava-Lifshitz-like F(R) gravity

We investigate the Einstein static universe (ESU) and the emergent universe scenario in the framework of Hořava-Lifshitz-like $F(R)$ gravity. We first perform a dynamical analysis in the phase space, and amongst others we show that a spatially open universe filled with matter satisfying the strong energy condition can exhibit a stable static phase. Additionally, we examine the behavior of the scenario under scalar perturbations and extract the conditions under which it is free of perturbative instabilities, showing that the obtained background ESU solutions are free of such instabilities. However, in order for the Einstein static universe to give rise to the emergent universe scenario we need to have an exotic matter sector that can lead the universe to depart from the stable static state and enter into its usual expanding thermal history.

gr-qc↗

arXiv2015

On the Stability of Einstein Static Universe in Doubly General Relativity Scenario

By presenting a relation between average energy of the ensemble of probe photons and energy density of the Universe, in the context of {\it gravity's rainbow} or {\it doubly general relativity} scenario, we introduce a rainbow FRW Universe model. By analyzing the fixed points in flat FRW model modified by two well known rainbow functions, we find that the finite time singularity avoidance (i.e. Big-Bang) may still remain as a problem. Then, we follow the "Emergent Universe" scenario in which there is no beginning of time and consequently there is no Big-Bang singularity. Moreover, we study the impact of a high energy quantum gravity modifications related to the gravity's rainbow on the stability conditions of an "Einstein static Universe" (ESU). We find that independent of a particular rainbow function, the positive energy condition dictates a positive spatial curvature for the Universe. In fact, without raising a nonphysical energy condition in the quantum gravity regimes, we can address an agreement between gravity's rainbow scenario and basic assumption of modern version of "Emergent Universe". We show that in the absence and presence of an energy-dependent cosmological constant $Λ(ε)$, a stable Einstein static solution is available versus the homogeneous and linear scalar perturbations under the variety of obtained conditions. Also, we explore the stability of ESU against the vector and tensor perturbations.

gr-qc↗

arXiv2015

Stability of Einstein Static Universe over Lyra Geometry

The existence and stability conditions of Einstein static universe against homogeneous scalar perturbations in the context of Lyra geometry is investigated. The stability condition is obtained in terms of the constant equation of state parameter $ω=p/ρ$ depending on energy density $ρ_0$ and scale factor $a_0$ of the initial Einstein static universe. Also, the stability against vector and tensor perturbations is studied. It is shown that a stable Einstein static universe can be found in the context of Lyra geometry against scalar, vector and tensor perturbations for suitable range and values of physical parameters.

gr-qc↗

arXiv2015

Phantom Wormhole Solutions in a Generic Cosmological Constant Background

There are a number of reasons to study wormholes with generic cosmological constant $Λ$. Recent observations indicate that present accelerating expansion of the universe demands $Λ>0$. On the other hand, some extended theories of gravitation such as supergravity and superstring theories posses vacuum states with $Λ<0$. Even within the framework of general relativity, a negative cosmological constant permits black holes with horizons topologically different from the usual spherical ones. These solutions are convertible to wormhole solutions by adding some exotic matter. In this paper, the asymptotically flat wormhole solutions in a generic cosmological constant background are studied. By constructing a specific class of shape functions, mass function, energy density and pressure profiles which support such a geometry are obtained. It is shown that for having such a geometry, the wormhole throat $r_0$, the cosmological constant $Λ$ and the equation of state parameter $ω$ should satisfy two specific conditions. The possibility of setting different values for the parameters of the model helps us to find exact solutions for the metric functions, mass functions and energy-momentum profiles. At last, the volume integral quantifier, which provides useful information about the total amount of energy condition violating matter is discussed briefly.

gr-qc↗

arXiv2015

Induced Matter Brane Gravity and Einstein Static Universe

We investigate stability of the Einstein static universe against the scalar, vector and tensor perturbations in the context of induced matter brane gravity. It is shown that in the framework of this model, the Einstein static universe has a positive spatial curvature. In contrast to the classical general relativity, it is found that a stable Einstein static universe against the scalar perturbations does exist provided that the variation of time dependent geometrical equation of state parameter is proportional to the minus of the variation of the scale factor, $δω_{g}(t)=-Cδa(t)$. We obtain neutral stability against the vector perturbations, and the stability against the tensor perturbations is guaranteed due to the positivity of the spatial curvature of the Einstein static universe in induced matter brane gravity.

gr-qc↗

arXiv2014

Einstein Static Universe in Braneworld Scenario

The stability of Einstein static universe against homogeneous scalar perturbations in the context of braneworld scenario is investigated. The stability regions are obtained in terms of the constant geometric linear equation of state parameter $ω_{extr}=p_{extr}/ρ_{extr}$ and are studied for each evolutionary era of the universe. The results are discussed for the case of closed, open or flat universe in each era under the obtained restricting conditions. We also briefly investigate the stability against vector and tensor perturbations. Contrary to the classical general relativity, it is found that a stable Einstein static universe may exist in a braneworld theory of gravity against scalar, vector and tensor perturbations for some suitable values and ranges of the cosmological parameters.

gr-qc↗