arXiv subjects

Ratbay Myrzakulov

Publications and source records attributed to Ratbay Myrzakulov.

At least 163 records · Page 9Linked to original sources

arXiv2012

Stability of a non-minimally conformally coupled scalar field in F(T) cosmology

In this paper, we introduce a non-minimally conformally coupled scalar field and dark matter in F(T) cosmology and study their dynamics. We investigate the stability and phase space behavior of the parameters of the scalar field by choosing an exponential potential and cosmologically viable form of F(T). We found that the dynamical system of equations admits two unstable critical points; thus no attractor solutions exist in this cosmology. Furthermore, taking into account the scalar field mimicking quintessence and phantom energy, we discuss the corresponding cosmic evolution for both small and large times. We investigate the cosmological implications of the model via the equation of state and deceleration parameters of our model and show that the late-time Universe will be dominated by phantom energy and, moreover, phantom crossing is possible. Our results do not lead to explicit predictions for inflation and the early Universe era.

gr-qc↗

arXiv2012

Phase space analysis of interacting dark energy in f(T) cosmology

In this paper, we examine the interacting dark energy model in $f(T)$ cosmology. We assume dark energy as a perfect fluid and choose a specific cosmologically viable form $f(T)= β\sqrt{T} $. We show that there is one attractor solution to the dynamical equation of $f(T)$ Friedmann equations. Further we investigate the stability in phase space for a general $f(T)$ model with two interacting fluids. By studying the local stability near the critical points, we show that the critical points lie on the sheet $u^*=(c-1)v^*$ in the phase space, spanned by coordinates $(u,v,Ω,T)$. From this critical sheet, we conclude that the coupling between the dark energy and matter $c\in (-2,0)$.

gr-qc↗

arXiv2012

Thermodynamics of a Schwarzschild Black Hole in Phantom Cosmology with Entropy Corrections

Motivated by some earlier works \cite{pavon,sadjadi} dealing with the study of generalized second law (GSL) of thermodynamics for a system comprising of a Schwarzschild black accreting a test non-self-gravitating fluid namely phantom energy in FRW universe, we extend them when the entropy of horizons of black hole and the cosmological undergo quantum corrections. Two types of such corrections are relevant here including logarithmic and power-law, while both are motivated from different theoretical backgrounds. We obtain general mathematical conditions for the validity of GSL in each case. Further we find that GSL restricts the mass of black hole for accretion of phantom energy. As such we obtain upper bounds on the mass of black hole above which the black hole cannot accrete the phantom fluid, otherwise the GSL is violated.

gr-qc↗

arXiv2012

Ekpyrotic universes in $F(R)$ Hořava-Lifshitz gravity

The Ekpyrotic scenario is studied in the context of some extensions of Hořava-Lifshitz gravity. Some particular solutions that lead to cyclic Hubble parameters are analyzed, where the corresponding gravitational actions are reconstructed by using several techniques and auxiliary fields. Its comparison with standard $F(R)$ gravity is performed. In addition, the so-called Little Rip, a stage of the universe evolution when some bounded systems may be dissolute, is also studied in this frame of theories.

gr-qc↗

arXiv2012

Reconstruction of $f(T)$ gravity: Rip cosmology, finite-time future singularities and thermodynamics

We demonstrate that there appear finite-time future singularities in $f(T)$ gravity with $T$ being the torsion scalar. We reconstruct a model of $f(T)$ gravity with realizing the finite-time future singularities. In addition, it is explicitly shown that a power-low type correction term $T^β$ ($β>1$) such as a $T^2$ term can remove the finite-time future singularities in $f(T)$ gravity. Moreover, we study $f(T)$ models with realizing inflation in the early universe, the $Λ$CDM model, Little Rip cosmology and Pseudo-Rip cosmology. It is demonstrated that the disintegration of bound structures for Little Rip and Pseudo-Rip cosmologies occurs in the same way as in gravity with corresponding dark energy fluid. We also discuss that the time-dependent matter instability in the star collapse can occur in $f(T)$ gravity. Furthermore, we explore thermodynamics in $f(T)$ gravity and illustrate that the second law of thermodynamics can be satisfied around the finite-time future singularities for the universe with the temperature inside the horizon being the same as that of the apparent horizon.

gr-qc↗

arXiv2012

Reconstruction of some cosmological models in f(R,T) gravity

In this paper, we reconstruct cosmological models in the framework of $f(R,T)$ gravity, where $R$ is the Ricci scalar and $T$ is the trace of the stress-energy tensor. We show that the dust fluid reproduces $Λ$CDM, phantom-non-phantom era and the phantom cosmology. Further, we reconstruct different cosmological models including, Chaplygin gas, scalar field with some specific forms of $f(R,T)$. Our numerical simulation for Hubble parameter shows good agreement with the BAO observational data for low redshifts $z<2$.

physics.gen-ph↗

arXiv2012

Quantized Black Hole and Heun function

Following the simple proposal by He and Ma for quantization of a black hole(BH) by Bohr's idea about the atoms, we discussed the solvability of the wave equation for such a BH. We superficial solved the associated Schrodinger equation. The eigenfunction problem reduces to HeunB $H$ differential equation which is a natural generalization of the hypergeometric differential equation. In other words, the spectrum can be determined by solving the Heun's differential equation.

physics.gen-ph↗

arXiv2012

Attractor Solutions in f(T) Cosmology

In this paper, we explore the cosmological implications of interacting dark energy model in a torsion based gravity namely $f(T)$. Assuming dark energy interacts with dark matter and radiation components, we examine the stability of this model by choosing different forms of interaction terms. We consider three different forms of dark energy: cosmological constant, quintessence and phantom energy. We then obtain several attractor solutions for each dark energy model interacting with other components. This model successfully explains the coincidence problem via the interacting dark energy scenario.

physics.gen-ph↗

arXiv2012

Generalized Second Law of Thermodynamics for FRW Cosmology with Power-Law Entropy Correction

In this work, we have considered the power law correction of entropy on the horizon. If the flat FRW Universe is filled with the n components fluid with interactions, the GSL of thermodynamics for apparent and event horizons have been investigated for equilibrium and non-equilibrium cases. If we consider a small perturbation around the de Sitter space-time, the general conditions of the validity of GSL have been found. Also if a phantom dominated Universe has a polelike type scale factor, the validity of GSL has also been analyzed. Further we have obtained constraints on the power-law parameter α in the phantom and quintessence dominated regimes. Finally we obtain conditions under which GSL breaks down in a cosmological background.

gr-qc↗

arXiv2012

Statefinder Parameters for Different Dark Energy Models with Variable G Correction in Kaluza-Klein Cosmology

In this work, we have calculated the deceleration parameter, statefinder parameters and EoS parameters for different dark energy models with variable $G$ correction in homogeneous, isotropic and non-flat universe for Kaluza-Klein Cosmology. The statefinder parameters have been obtained in terms of some observable parameters like dimensionless density parameter, EoS parameter and Hubble parameter for holographic dark energy, new agegraphic dark energy and generalized Chaplygin gas models.

physics.gen-ph↗

arXiv2011

Einstein-Cartan gravity with scalar-fermion interactions

In this paper, we have considered the g-essence and its particular cases, k-essence and f-essence, within the framework of the Einstein-Cartan theory. We have shown that a single fermionic field can give rise to the accelerated expansion within the Einstein-Cartan theory. The exact analytical solution of the Einstein-Cartan-Dirac equations is found. This solution describes the accelerated expansion of the Universe with the equation of state parameter $w=-1$ as in the case of $Λ$CDM model.

astro-ph.CO↗

arXiv2011

Modified Chaplygin Gas and Solvable F-essence Cosmologies

The Modified Chaplygin Gas (MCG) model belongs to the class of a unified models of dark energy and dark matter. In this paper, we have modeled MCG in the framework of f-essence cosmology. By constructing an equation connecting the MCG and the f-essence, we solve it to obtain explicitly the pressure and energy density of MCG. As special cases, we obtain both positive and negative pressure solutions for suitable choices of free parameters. We also calculate the state parameter which describes the phantom crossing.

physics.gen-ph↗

arXiv2011

Accelerating universe from F(T) gravity

It is shown that the acceleration of the universe can be understood by considering a F(T) gravity models. For these F(T) gravity models, a variant of the accelerating cosmology reconstruction program is developed. Some explicit examples of F(T) are reconstructed from the background FRW expansion history.

gr-qc↗

arXiv2011

Dark energy in some integrable and nonintegrable FRW cosmological models

One of the greatest challenges in cosmology today is to determine the nature of dark energy, the sourse of the observed present acceleration of the Universe. Besides the vacuum energy, various dark energy models have been suggested. The Friedmann - Robertson - Walker (FRW) spacetime plays an important role in modern cosmology. In particular, the most popular models of dark energy work in the FRW spacetime. In this work, a new class of integrable FRW cosmological models is presented. These models induced by the well-known Painlev$\acute{e}$ equations. Some nonintegrable FRW models are also considered. These last models are constructed with the help of Pinney, Schr$\ddot{o}$dinger and hypergeometric equations. Scalar field description and two-dimensional generalizations of some cosmological models are presented. Finally some integrable and nonintegrable $F(R)$ and $F(G)$ gravity models are constructed.

physics.gen-ph↗

arXiv2011

Entropic derivation of F=ma for circular motion

We examine the entropic picture of Newton's second law for the case of circular motion. It is shown that one must make modifications to the derivation of F=ma due to a change in the effective Unruh temperature for circular motion. These modifications present a challenge to the entropic derivation of Newton's second law, but also open up the possibility to experimentally test and constrain this model for large centripetal accelerations.

gr-qc↗

arXiv2011

FRW cosmological models with integrable and nonintegrable differential equations of state

The basic models of modern cosmology work in FRW spacetime. Hence follows that it is important to study the physical and mathematical nature of FRW cosmological models. In this work, we consider FRW models with the differential equations of state. Some new classes integrable and nonintegrable FRW cosmological models were constructed. It is remarkable that all proposed integrable and nonintegrable FRW models admit exact solutions. For some of them, such exact solutions are presented. Some artificial two-dimensional FRW models were also proposed. Finally in Appendix, we extend the obtained results for g-essence models and for its two reductions: k-essence and f-essence.

physics.gen-ph↗

arXiv2011

On the $Λ$CDM Universe in $f(G)$ gravity

In the context of the so-called Gauss-Bonnet gravity, where the gravitational action includes function of the Gauss-Bonnet invariant, we study cosmological solutions, especially the well-known $Λ$CDM model. It is shown that the dark energy contribution and even the inflationary epoch can be explained in the frame of this kind of theories with no need of any other kind of component. Other cosmological solutions are constructed and the rich properties that this kind of theories provide are explored.

gr-qc↗

arXiv2010

On the stability of spherically symmetric and wormhole solutions supported by the sine-Gordon ghost scalar field

In this paper we investigate wormhole and spherically symmetric solutions in 4D gravity plus a matter source consisting of a ghost scalar field with a sine-Gordon potential. For the wormhole solutions we also include the possibility of electric and/or magnetic charges. For both types of solutions we perform a linear stability analysis and show that the wormhole solutions are stable and that when one turns on the electric and/or magnetic field the solution remains stable. The linear stability analysis of the spherically symmetric solutions indicates that they can be stable or unstable depending on one of the parameters of the system. This result for the spherically symmetric solution is nontrivial since a previous investigation of 4D gravity plus a ghost scalar field with a $λϕ^4$ interaction found only unstable spherically symmetric solutions. Both the wormhole and spherically symmetric solutions presented here asymptotically go to anti-de-Sitter space-time.

gr-qc↗