arXiv subjects

R. Myrzakulov

Publications and source records attributed to R. Myrzakulov.

At least 109 records · Page 6Linked to original sources

arXiv2013

Statefinder hierarchy of bimetric and galileon models for concordance cosmology

In this paper, we use Statefinder hierarchy method to distinguish between bimetric theory of massive gravity, galileon modified gravity and DGP models applied to late time expansion of the universe. We also carry out comparison between bimetric and DGP models using Statefinder pairs {r, s} and {r, q}. We show that statefinder diagnostic can differentiate between ΛCDM and above mentioned cosmological models of dark energy, and finally show that Statefinder S2 is an excellent discriminant of ΛCDM and modified gravity models.

gr-qc↗

arXiv2013

A Type of Levi-Civita's Solution in Modified Gauss-Bonnet Gravity

In this letter we obtain an exact solution for cylindrically symmetric modified Gauss Bonnet gravity. This metric is a generalization of the vacuum solution of the Levi-Civita in the general relativity. It describes an isotropic perfect fluid one parameter family of the gravitational configurations which can be interpreted as the exterior metric of a cosmic string. By setting the Gauss-Bonnet coupling parameter to zero, we recover the vacuum solution in the Einstein gravity as well.

gr-qc↗

arXiv2013

Holographic superconductors with Weyl Corrections via gauge/gravity duality

In this paper, we analytically compute the basic parameters of the p-wave holographic superconductors with Weyl geometrical corrections using the matching method. The explicit correspondence between the critical temperature $T_c$ and the dual charge density $ρ$ has been calculated as $T_c\proptoρ^{{1}{3}}$ and the dependence of the vacuum expectation value for the dual condensate operator $\cal{O}$ on the temperature has been found analytically in the form $<{\cal O}_{+}>\propto T_c^{{3}{2}}T^{Δ-{1}{2}}\sqrt{1-({T}{T_c})^3}$. The critical exponent ${1}{2}$ is an universal quantity according to predictions of the mean field theory and independent from the Weyl coupling $γ$. Our analytical results confirm the numerical results and also agree on computations using by the variational method.

hep-th↗

arXiv2013

Inhomogeneous viscous fluids in FRW universe

We give a brief review of some aspects of inhomogeneous viscous fluids in a flat Friedmann-Robertson-Walker Universe. In general, it is pointed out that several fluid models may bring the future Universe evolution to become singular, with the appearance of the so-called Big Rip scenario. We investigate the effects of fluids coupled with dark matter in a de Sitter Universe, by considering several cases. Due to this coupling, the coincidence problem may be solved, and if the de Sitter solution is stable, the model is also protected against the Big Rip singularity.

gr-qc↗

arXiv2013

Construction of a Holographic Superconductor in F(R) Gravity

We construct a toy model for holographic superconductor with non linear Maxwell field in the frame of modified gravity. By probe the bulk background by non linear Maxwell fields we show that superconductivity happens under a specific critical temperature. The effect of the non linear Maxwell field and non linear curvature corrections have been studied by analytical matching methods. We conclude that the non linearity in Maxwell field and curvature coupling make condensation harder.

hep-th↗

arXiv2013

Evaporation phenomena in f(T) gravity

We formulate evaporation phenomena in a generic model of generalized teleparallel gravity in Weitzenbock spacetime with diagonal and non-diagonal tetrads basis. We also perform the perturbation analysis around the constant torsion scalar solution named Nariai spacetime which is an exact solution of field equations as the limiting case of the Schwarzschild-de Sitter and in the limit where two back hole and their cosmological horizons coincide. By a carefully analysis of the horizon perturbation equation, we show that (anti)evaporation can not happen if we use a diagonal tetrad basis. This result implies that a typical black hole in any generic form of generalized teleparallel gravity is frozen in its initial state if we use the diagonal tetrads. But in the case of non-diagonal tetrads the analysis is completely different. By a suitable non trivial non-diagonal tetrad basis we investigate the linear stability of the model under perturbations of the metric and torsion simultaneously. We observe that in spite of the diagonal case, both evaporation and anti evaporation can happen. The phenomena depend on the initial phase of the horizon perturbation. In the first mode when we restrict ourselves to the first lower modes the (anti)evaporation happens. So, in non-diagonal case the physical phenomena is reasonable. This is an important advantage of using non-diagonal tetrads instead of the diagonal ones. We also see that this is an universal feature, completely independent from the form of the model.

physics.gen-ph↗

arXiv2013

Planar Symmetry in f(T) gravity

Exact solutions for planar Weitzenboch geometries in generalized teleparallel gravity with torsion is presented. The model predicted the existence of Moller, Kattler-Wittaker and planar de-Sitter geometries for different forms of action.

physics.gen-ph↗

arXiv2013

Integrable Heisenberg Ferromagnet Equations with self-consistent potentials

In this paper, we consider some integrable Heisenberg Ferromagnet Equations with self-consistent potentials. We study their Lax representations. In particular we give their equivalent counterparts which are nonlinear Schrödinger type equations. We present the integrable reductions of the Heisenberg Ferromagnet Equations with self-consistent potentials. These integrable Heisenberg Ferromagnet Equations with self-consistent potentials describe nonlinear waves in ferromagnets with some additional physical fields.

physics.gen-ph↗

arXiv2013

Exploring Cylindrical Solutions in Modified f(G) Gravity

We present cylindrically symmetric solutions for a type of the Gauss-Bonnet gravity, in details. We derive the full system of the field equations and show that there exist seven families of exact solutions for three forms of viable models. By applying the method based on the effective fluid energy momentum tensor components, we evaluate the mass per unit length for the solutions. From dynamical point of the view, by evaluating the null energy condition for these configurations, we show that in some cases the azimuthal pressure breaks the energy condition. This violation of the null energy condition predicts the existence of a cylindrical wormhole.

gr-qc↗

arXiv2013

The universe evolution in exponential $F(R)$-gravity

A generic feature of viable exponential $F(R)$-gravity is investigated. An additional modification to stabilize the effective dark energy oscillations during matter era is proposed and applied to two viable models. An analysis on the future evolution of the universe is performed. Furthermore, a unified model for early and late-time acceleration is proposed and studied.

gr-qc↗

arXiv2012

Thermodynamics in Little Rip cosmology in the framework of a type of f(R,T) gravity

Cosmological reconstruction of Little Rip model in $f(R, T)$ gravity is investigated, where $R$ is the curvature scalar and $T$ the trace of the energy momentum tensor. The model perfectly reproduces the present stage of the universe, characterized by the $ΛCDM$ model, without singularity at future finite-time (without the Big Rip). The input parameters are determined according to Supernovae Cosmology data and perfectly fit with the WMAP around the Little Rip. Moreover, the thermodynamics is considered in this Little Rip cosmology and it is illustrated that the second law of thermodynamics is always satisfied around the Little Rip universe for the temperature inside the horizon being the same as that of the apparent horizon. Moreover, we show the existence of a stable fixed point in the Little Rip universe which confirms that this is actually a late-time attractor in the phantom-dominated universe. The linear perturbation analysis is performed around the critical points, showing that the Little Rip model obtained is stable.

gr-qc↗

arXiv2012

Holographic superconductors in the AdS black hole with a magnetic charge

In this work we study the analytical properties of a 2+1 dimensional magnetically charged holographic superconductor in $AdS_4$. We obtain the critical chemical potential $μ_c$ analytically, using the Sturm-Liouville variational approach. Also, the obtained analytic result can be used to back up the numerical computations in the holographic superconductor in the probe limit.

physics.gen-ph↗

arXiv2012

Violation of First Law of Thermodynamics in f(R,T) Gravity

In this Letter, we derived the first law of thermodynamics using the method proposed by Wald. Treating the entropy as Noether charge and comparing with the usual first law of thermodynamics, we obtained the expression of entropy explicitly which contains infinitely many non-local terms (i.e. the integral terms). We have proved, in general, that the first law of black bole thermodynamics is violated for $f(R,T)$ gravity. But there might exist some special cases in which the first law for $f(R,T)$ gravity is recovered.

physics.gen-ph↗

arXiv2012

Cosmic history of viable exponential gravity: Equation of state oscillations and growth index from inflation to dark energy era

A generic feature of viable $F(R)$ gravity is investigated: It is demonstrated that during the matter dominated era the large frequency oscillations of the effective dark energy may influence the behavior of higher derivatives of the Hubble parameter with the risk to produce some singular unphysical solutions at high redshift. This behavior is explicitly analyzed for realistic $F(R)$ models, in particular, exponential gravity and a power form model. To stabilize such oscillations, we consider the additional modification of the models via a correction term which does not destroy the viability properties. A detailed analysis on the future evolution of the universe and the evolution history of the growth index of the matter density perturbations are performed. Furthermore, we explore two applications of exponential gravity to the inflationary scenario. We show how it is possible to obtain different numbers of $e$-folds during the early-time acceleration by making different choices of the model parameters in the presence of ultrarelativistic matter, which destabilizes inflation and eventually leads to the exit from the inflationary stage. We execute the numerical analysis of inflation in two viable exponential gravity models. It is proved that at the end of the inflation, the effective energy density and curvature of the universe decrease and thus a unified description between inflation and the $Λ$CDM-like dark energy dominated era can be realized.

gr-qc↗

arXiv2012

Cylindrical Solutions in Modified f(T) Gravity

We investigate static cylindrically symmetric vacuum solutions in Weyl coordinates in the framework of f(T) theories of gravity, where T is the torsion scalar. The set of modified Einstein equations is presented and the fourth coming equations are established. Specific physical expressions are assumed for the algebraic function f(T) and solutions are obtained. Moreover, general solution is obtained with finite values of u(r) on the axis r = 0, and this leads to a constant torsion scalar. Also, cosmological constant is introduced and its relation to Linet-Tian solution in GR is commented.

physics.gen-ph↗

arXiv2012

Analysis of F(R,T) Gravity Models Through Energy Conditions

This paper is devoted to study the energy conditions in F(R,T) gravity for FRW universe with perfect fluid, where $R$ is the Ricci scalar and $T$ is the torsion scalar. We construct the general energy conditions in this theory and reduce them in F(R) as well as F(T) theory of gravity. Further, we assume some viable models and investigate bounds on their constant parameters to satisfy the energy condition inequalities. We also plot some of the cases using present-day values of the cosmological parameters. It is interesting to mention here that the model F(R,T)=μR+ νT satisfies the energy conditions for different ranges of the parameters.

gr-qc↗

arXiv2012

Noether symmetry of F(T) cosmology with quintessence and phantom scalar fields

In this paper, we investigate the Noether symmetries of $F(T)$ cosmology involving matter and dark energy. In this model, the dark energy is represented by a canonical scalar field with a potential. Two special cases for dark energy are considered including phantom energy and quintessence. We obtain $F(T)\sim T^{3/4},$ and the scalar potential $V(ϕ)\simϕ^2$ for both models of dark energy and discuss quantum picture of this model. Some astrophysical implications are also discussed.

physics.gen-ph↗

arXiv2012

Turbulence and Little Rip Cosmology

A variety of conditions is considered under which the cosmic dark fluid may be able to develop a future Big Rip or Little Rip singularity. Both one-component and two-component models are considered. In the last-mentioned case we present a way in which the fluid can be decomposed into two components, one non-turbulent (ideal) and one turbulent part, obeying two different equations of state. For the non-turbulent part, the thermodynamical parameter, commonly called w, is assumed to be less than -1 throughout. For the turbulent part, it turns out that it is sufficient that w{turb} lies in the quintessence region in order to lead to a singularity. Both Big Rip and Little Rip behaviour for dark energy are found. In the one-component case, we examine how the universe may develop from a viscous era with constant bulk viscosity into a turbulent era, the turbulence in effect protecting the universe from encountering the singularity at all. The equivalent description of the same cosmology in terms of inhomogeneous (imperfect) fluid is also presented.

gr-qc↗