arXiv subjects

Ratbay Myrzakulov

Publications and source records attributed to Ratbay Myrzakulov.

At least 145 records · Page 8Linked to original sources

arXiv2013

More on Superconductors via Gauge/Gravity Duality with Nonlinear Maxwell Field

We have developed the recent investigations on the second-order phase transition in the holographic superconductor using the probe limit for a nonlinear Maxwell field strength coupled to a massless scalar field. By analytic methods, based on the variational Sturm- Liouville minimization technique, we study the effects of the space-time dimension and the non-linearity parameter on the critical temperature and the scalar condensation of the dual operators on the boundary. Further, as a motivated result, we analytically deduce the DC conductivity in the low and zero temperatures regime. Especially in the zero temperature limit and in two dimensional toy model, we thoroughly compute the conductivity analytically. Our work clarifies more features of the holographic superconductors both in different space dimensions and on the effect of the non-linearity in Maxwell's strength field.

physics.gen-ph↗

arXiv2013

Reconstruction of the equation of state for the cyclic universes in homogeneous and isotropic cosmology

We study the cosmological evolutions of the equation of state (EoS) for the universe in the homogeneous and isotropic Friedmann-Lemaître-Robertson-Walker (FLRW) space-time. In particular, we reconstruct the cyclic universes by using the Weierstrass and Jacobian elliptic functions. It is explicitly illustrated that in several models the universe always stays in the non-phantom (quintessence) phase, whereas there also exist models in which the crossing of the phantom divide can be realized in the reconstructed cyclic universes.

gr-qc↗

arXiv2013

Cosmological evolution of equation of state for dark energy in G-essence models

We explore the cosmological evolution of equation of state (EoS) for dark energy in g-essence models, the action of which is described by a function of both the canonical kinetic term of both the scalar and fermionic fields. We examine g-essence models with realizing the crossing of the phantom divide line of $w_{\mathrm{DE}}=-1$ as well as the models in which the universe always stays in the non-phantom (quintessence) phase ($w_{\mathrm{DE}}>-1$). In particular, we find an explicit g-essence model with the crossing from the non-phantom phase to the phantom one ($w_{\mathrm{DE}}<-1$). This transition behavior is consistent with the recent observational data analyses.

physics.gen-ph↗

arXiv2013

Topological Static Spherically Symmetric vacuum Solutions in $\mathcal F(R,G)$ Gravity

The Lagrangian derivation of the Equations of Motion for topological static spherically symmetric metrics in $\mathcal F (R,G)$-modified gravity is presented and the related solutions are discussed. In particular, a new topological solution for the model $\mathcal F (R,G)=R+\sqrt{G}$ is found. The black hole solutions and the First Law of thermodynamic are analyzed. Furthermore, the coupling with electromagnetic field is also considered and a Maxwell solution is derived.

gr-qc↗

arXiv2013

Knot Universes in Bianchi Type I Cosmology

We investigate the trefoil and figure-eight knot universes from Bianchi-type I cosmology. In particular, we construct several concrete models describing the knot universes related to the cyclic universe and examine those cosmological features and properties in detail. Finally some examples of unknotted closed curves solutions (spiky and Mobius strip universes) are presented.

physics.gen-ph↗

arXiv2012

Wormholes in a viable f(T) gravity

In this paper, we derive some new exact solutions of static wormholes in f(T) gravity. We discuss independent cases of the pressure components including isotropic and anisotropic pressure. Lastly we consider radial pressure satisfying a barotropic equation of state. We also check the behavior of null energy condition (NEC) for each case and observe that it is violated for the anisotropic case, while it is satisfied for isotropic and barotropic cases.

gr-qc↗

arXiv2012

Noether Gauge Symmetry of Modified Teleparallel Gravity Minimally Coupled with a Canonical Scalar Field

This paper is devoted to the study of Noether gauge symmetries of $f(T)$ gravity minimally coupled with a canonical scalar field. We explicitly determine the unknown functions of the theory $f(T),V(ϕ), W(ϕ)$. We have shown that there are two invariants for this model, one of which defines the Hamiltonian $H$ under time invariance (energy conservation) and the other is related to scaling invariance. We show that the equation of state parameter in the present model can cross the cosmological constant boundary. The behavior of Hubble parameter in our model closely matches to that of $Λ$CDM model, thus our model is an alternative to the later.

astro-ph.CO↗

arXiv2012

Energy conditions in generalized teleparallel gravity models

In this paper, we investigate the energy conditions (including null, weak, strong, dominant) in generalized teleparallel gravities including pure $F(T)$, teleparallel gravity with non-minimally coupled scalar field and $F(T)$ with non-minimally coupled scalar field models. In particular, we apply them to Friedmann-Robertson-Walker (FRW) cosmology and obtain some corresponding results. Using two specific phenomenological forms of $F(T)$, we show that some of the energy conditions are violated.

physics.gen-ph↗

arXiv2012

Cosmology of F(T) gravity and k-essence

This a brief review on $F(T)$ gravity and its relation with k-essence. Modified teleparallel gravity theory with the torsion scalar has recently gained a lot of attention as a possible explanation of dark energy. We perform a thorough reconstruction analysis on the so-called $F(T)$ models, where $F(T)$ is some general function of the torsion term, and deduce the required conditions for the equivalence between of $F(T)$ models with pure kinetic k-essence models. We present a new class of models of $F(T)$-gravity and k-essence.

gr-qc↗

arXiv2012

Generalized Second Law of Thermodynamics in $f(T)$ Gravity with Entropy Corrections

We study the generalized second law (GSL) of thermodynamics in $f(T)$ cosmology. We consider the universe as a closed bounded system filled with $n$ component fluids in the thermal equilibrium with the cosmological boundary. We use two different cosmic horizons: the future event horizon and the apparent horizon. We show the conditions under which the GSL will be valid in specific scenarios of the quintessence and the phantom energy dominated eras. Further we associate two different entropies with the cosmological horizons: with a logarithmic correction term and a power-law correction term. We also find the conditions for the GSL to be satisfied or violated by imposing constraints on model parameters.

physics.gen-ph↗

arXiv2012

FRW Cosmology in F(R,T) gravity

In this paper, we consider a theory of gravity with a metric-dependent torsion namely the $F(R,T)$ gravity, where $R$ is the curvature scalar and $T$ is the torsion scalar. We study a geometric root of such theory. In particular we give the derivation of the model from the geometrical point of view. Then we present the more general form of $F(R,T)$ gravity with two arbitrary functions and give some of its particular cases. In particular, the usual $F(R)$ and $F(T)$ gravity theories are the particular cases of the $F(R,T)$ gravity. In the cosmological context, we find that our new gravitational theory can describes the accelerated expansion of the universe.

gr-qc↗

arXiv2012

F(T) gravity and k-essence

Modified teleparallel gravity theory with the torsion scalar have recently gained a lot of attention as a possible explanation of dark energy. We perform a thorough reconstruction analysis on the so-called $F(T)$ models, where $F(T)$ is some general function of the torsion term, and derive conditions for the equivalence between of $F(T)$ models with purely kinetic k-essence. We present a new class models of $F(T)$-gravity and k-essence. We also proposed some new models of generalized gases and knot universes as well as some generalizations of $F(T)$ gravity.

physics.gen-ph↗

arXiv2012

Statefinder Analysis of f(T) Cosmology

In this paper, we intend to evaluate and analyze the statefinder parameters in $f(T)$ cosmology. Friedmann equation in $f(T)$ model is taken, and the statefinder parameters ${r,s}$ are calculated. We consider a model of $f(T)$ which contains a constant, linear and non-linear form of torsion. We plot $r$ and $s$ in order to characterize this model in the ${r,s}$ plane. We found that our model $f(T)=2C_1 \sqrt{-T} +αT+C_2,$ predicts the decay of dark energy in the far future while its special case namely teleparallel gravity predicts that dark energy will overcome over all the energy content of the Universe.

physics.gen-ph↗

arXiv2012

Thermodynamics of an Evolving Lorentzian Wormhole with Entropy Corrections

In this work, we study the generalized second law of thermodynamics (GSL) at the apparent horizon of an evolving Lorentzian wormhole. We obtain the expressions of thermal variables at the apparent horizon. Choosing the two well-known entropy functions i.e. power-law and logarithmic, we obtain the expressions of GSL. We have analyzed the GSL using a power-law form of scale factor $a(t)=a_0t^n$ and the special form of shape function $b(r)=\frac{b_{0}}{r}$. It is shown that GSL is valid in the evolving wormhole spacetime for both choices of entropies if the power-law exponent is small, but for large values of $n$, the GSL is satisfied at initial stage and after certain stage of the evolution of the wormhole, it violates.

gr-qc↗

arXiv2012

Power-Law Entropy Corrected Ricci Dark Energy and Dynamics of Scalar Fields

Motivated by the holographic principle, it has been suggested that the Dark Energy (DE) density can be inversely proportional to the area $A$ of the event horizon of the universe. However, this kind of model would have a casuality problem. In this work, we study the power-law entropy corrected holographic DE (PLECHDE) model in the non-flat Friedmann-Robertson-Walker universe, with the future event horizon replaced by the average radius of the Ricci scalar curvature. We derive the equation of state parameter $ω_Λ$, the deceleration parameter $q$ and the evolution of energy density parameter $Ω_D'$ in presence of interaction between DE and Dark Matter (DM). We consider the correspondence between our Ricci-PLECHDE model and the Modified Chaplygin Gas (MCG) and the tachyon, K-essence, dilaton and quintessence scalar fields. The potential and the dynamics of the scalar field models have been reconstructed according to the evolutionary behaviour of the interacting entropy-corrected holographic DE model.

physics.gen-ph↗

arXiv2012

Periodic Cosmological Evolutions of Equation of State for Dark Energy

We demonstrate two periodic or quasi-periodic generalizations of the Chaplygin gas (CG) type models to explain the origins of dark energy as well as dark matter by using the Weierstrass $\wp(t)$, $σ(t)$ and $ζ(t)$ functions with two periods being infinite. If the universe can evolve periodically, a non-singular universe can be realized. Furthermore, we examine the cosmological evolution and nature of the equation of state (EoS) of dark energy in the Friedmann-Lemaître-Robertson-Walker cosmology. It is explicitly illustrated that there exist three type models in which the universe always stays in the non-phantom (quintessence) phase, whereas it always evolves in the phantom phase, or the crossing of the phantom divide can be realized. The scalar fields and the corresponding potentials are also analyzed for different types of models.

gr-qc↗

arXiv2012

Resolution of dark matter problem in f(T) gravity

In this paper, we attempt to resolve the dark matter problem in f(T) gravity. Specifically, from our model we successfully obtain the flat rotation curves of galaxies containing dark matter. Further, we obtain the density profile of dark matter in galaxies. Comparison of our analytical results shows that our torsion-based toy model for dark matter is in good agreement with empirical data-based models. It shows that we can address the dark matter as an effect of torsion of the space.

gr-qc↗

arXiv2012

A note on the relationship between solutions of Einstein, Ramanujan and Chazy equations

The Einstein equation for the Friedmann-Robertson-Walker metric plays a fundamental role in cosmology. The direct search of the exact solutions of the Einstein equation even in this simple metric case is sometime a hard job. Therefore, it is useful to construct solutions of the Einstein equation using a known solutions of some other equations which are equivalent or related to the Einstein equation. In this work, we establish the relationship the Einstein equation with two other famous equations namely the Ramanujan equation and the Chazy equation. Both these two equations play an imporatant role in the number theory. Using the known solutions of the Ramanujan and Chazy equations, we find the corresponding solutions of the Einstein equation. The relationship between the Friedmann equation and the equations of the trefoil knot and figure-eight knot is established.

physics.gen-ph↗