arXiv subjects

Ulbossyn Ualikhanova

Publications and source records attributed to Ulbossyn Ualikhanova.

10 recordsLinked to original sources

arXiv2026

Reconstruction of f(G) Gravity from an Interacting Viscous Generalized QCD Ghost Dark Energy Model: Cosmology and Thermodynamics: Cosmology and Thermodynamics

In this work, we investigate an interacting viscous generalized QCD ghost dark energy model in the framework of reconstructed $f(G)$ gravity proposed in Phys.\ Lett.\ B 631, 1--6 (2005). The interaction between dark matter and dark energy together with bulk viscosity is incorporated to describe a more realistic cosmic evolution. A hybrid expansion law is adopted to reconstruct the modified Gauss-Bonnet function, which naturally connects the early matter-dominated epoch with the present accelerated expansion of the universe. Since an exact analytical reconstruction is difficult, the $f(G)$ function is obtained numerically in both the early- and late-time regimes. Motivated by the numerical reconstruction, a reconstruction-inspired power-law form of $f(G)$ is also considered to examine the cosmological implications of the model. The results show that the reconstructed $f(G)$ function evolves smoothly throughout the cosmic history, while the effective equation of state gradually approaches the de Sitter phase at late times. The thermodynamic behavior of the model is further examined using Barrow entropy following Eur.\ Phys.\ J.\ C 81, 644 (2021). The non-negative evolution of the total entropy shows the validity of the generalized second law of thermodynamics. The study finally concludes that the interacting viscous generalized QCD ghost dark energy model in reconstructed $f(G)$ gravity provides a viable and thermodynamically consistent framework for explaining the late-time accelerated expansion of the universe.

gr-qc↗

arXiv2026

A Combined Barrow Entropy and QCD Ghost Mechanism for Late-Time Cosmic Acceleration

We investigate a unified dark-energy scenario based on the combined effects of Barrow entropy corrections and the QCD ghost mechanism, referred to as the BH--QCDGDE model. The dark-energy density is constructed in a generalized holographic form that incorporates both Barrow-deformed entropy corrections and low-energy QCD vacuum effects within a single framework. The cosmological dynamics are analyzed in a spatially flat Friedmann--Lema\^ıtre--Robertson--Walker background. The model exhibits a smooth transition from a decelerated matter-dominated era to a late-time accelerated phase without crossing the phantom divide, indicating a viable background evolution. An equivalent scalar-field description of the effective dark-energy sector is reconstructed and shown to admit a quintessence-like behavior. The thermodynamic viability is examined by testing the generalized second law at the apparent horizon, which is found to be satisfied throughout the parameter space. The classical stability of the model is further investigated through the squared speed of sound, revealing the role of model parameters in shaping stable cosmological regimes. Overall, the BH--QCDGDE framework provides a consistent and physically viable description of late-time cosmic acceleration.

physics.gen-ph↗

arXiv2025

Dynamical systems approach and cosmological attractors in newer general relativity

We study the cosmological dynamics of a class of symmetric teleparallel gravity theories known as ``newer general relativity'' using the methods of dynamical systems, restricted to the case of vacuum solutions with a spatially flat Friedmann-Lemaître-Robertson-Walker metric. For the most general class of theories, we study generic properties of the solutions, in particular their fixed points, asymptotic behavior and effective dark energy. We then apply this approach to two phenomenologically motivated subclasses of theories, which we study in full detail. For these theories, we derive the complete space of solutions and cosmological attractors, which we display in a number of phase diagram. Depending on the particular theory at hand, we find different possible scenarios, including a turnaround followed by a big crunch, a big rip and an eternally expanding universe whose Hubble parameter asymptotically approaches zero. It turns that this different behavior can be explained by the effective dark energy barotropic index, which shows either phantom or non-phantom behavior, depending on the theory, but does not change dynamically between these two possibilities.

gr-qc↗

arXiv2025

Post-Newtonian limit of generalized scalar-teleparallel theories of gravity

We propose a general class of scalar-teleparallel theories, which are based on a scalar field which is coupled to a flat connection with torsion and nonmetricity, and study its post-Newtonian limit using the parametrized post-Newtonian formalism. We find that among this class there are theories whose post-Newtonian limit fully agrees with general relativity; for others only the parameters $β$ and $γ$ deviate from their general relativity values $β= γ= 1$, while all other parameters remain the same, thus preserving total momentum conservation, local Lorentz invariance and local position invariance; finally, we also find theories whose post-Newtonian limit is pathological. Our main result is a full classification of the proposed theories into these different cases. We apply our findings to a number of simpler classes of theories and show that for these a subset of the aforementioned cases can be found.

gr-qc↗

arXiv2024

Holographic reconstruction of k-essence model with Tsallis and the most generalized Nojiri-Odintsov version of holographic dark energy

The holographic principle, which has its roots in string theory and black hole thermodynamics, connects the maximum distance of a quantum field theory to its infrared cutoff, which is correlated with the vacuum energy. The present study explores a reconstruction scheme for the k-essence form of dark energy with the most generalized version of holographic dark energy introduced in S. Nojiri, and S. D. Odintsov (2006) (Gen. Relativ. Gravit., 38 p: 1285-1304 ) and (S. Nojiri and S. D. Odintsov, 2017, European Physical Journal C, 77, pp.1-8 ). Here,Ulbossyn Ualikhanova in the initial phase of the study, we begin with a reconstruction scheme of the k-essence model with Tsallis holographic dark energy and finally with a highly generalized version of holographic dark energy with Nojiri-Odintsov generalization. Finally, we have studied the cosmological consequences of the k-essence dark energy with the generalized versions of holographic fluid.

physics.gen-ph↗

arXiv2020

Propagation of gravitational waves in teleparallel gravity theories

We investigate propagation of gravitational waves in the most general teleparallel gravity model with second order field equations around the Minkowski background. We argue that in this case the most general Lagrangian at the first order of perturbations is given by the linear combination of quadratic invariants and hence coincides with the well-known new general relativity model. We derive the linearized field equations and analyze them using the principal polynomial and the Newman-Penrose formalism. We demonstrate that all gravitational wave modes propagate at the speed of light and there are up to six possible polarizations. We show that two tensorial modes of general relativity are always present and the number of extra polarizations depends on the free parameters of the new general relativity model.

gr-qc↗

arXiv2020

Propagation of gravitational waves in symmetric teleparallel gravity theories

Symmetric teleparallel gravity (STG) offers an interesting third geometric interpretation of gravitation besides its formulation in terms of a spacetime metric and Levi-Civita connection or its teleparallel formulation. It describes gravity through a connection which is not metric compatible, however is torsion and curvature free. We investigate the propagation velocity of the gravitational waves around Minkowski spacetime and their potential polarizations in a general class of STG theories, the so-called "newer general relativity" class. It is defined in terms of the most general Lagrangian that is quadratic in the nonmetricity tensor, does not contain its derivatives and is determined by five free parameters. In our work we employ the principal symbol method and the Newman-Penrose formalism, to find that all waves propagate with the speed of light, i.e., on the Minkowski spacetime light cone, and to classify the theories according to the number of polarizations of the waves depending on the choice of the parameters in the Lagrangian. In particular it turns out that there exist more theories than just the reformulation of general relativity which allow only for two polarization modes. We also present a visualization of the parameter space of the theory to better understand the structure of the model.

gr-qc↗

arXiv2020

Parameterized post-Newtonian limit of general teleparallel gravity theories

We derive the post-Newtonian limit of a general class of teleparallel gravity theories, whose action is given by a free function of three scalar quantities obtained from the torsion of the teleparallel connection. This class of theories is chosen to be sufficiently generic in order to include the $f(T)$ class of theories as well as new general relativity as subclasses. To derive its post-Newtonian limit, we first impose the Weitzenböck gauge, and then introduce a post-Newtonian approximation of the tetrad field around a Minkowski background solution. Our results show that the class of theories we consider is fully conservative, with only the parameters $β$ and $γ$ potentially deviating from their general relativity values. In particular, we find that the post-Newtonian limit of any $f(T)$ theory is identical to that of general relativity, so that these theories cannot be distinguished by measurements of the post-Newtonian parameters alone.

gr-qc↗

arXiv2018

Covariant formulation of scalar-torsion gravity

We consider a generalized teleparallel theory of gravitation, where the action contains an arbitrary function of the torsion scalar and a scalar field, $f(T,ϕ)$, thus encompassing the cases of $f(T)$ gravity and nonminimally coupled scalar field as subclasses. The action is manifestly Lorentz invariant when besides the tetrad one allows for flat but nontrivial spin connection. We derive the field equations and demonstrate how the antisymmetric part of the tetrad equations is automatically satisfied when the spin connection equation holds. The spin connection equation is a vital part of the covariant formulation, since it determines the spin connection compatible with a given tetrad. We discuss how the spin connection equation can be solved in general, and provide the cosmological and spherically symmetric examples. Finally we generalize the theory to an arbitrary number of scalar fields.

gr-qc↗

arXiv2017

Dynamical systems approach and generic properties of $f(T)$ cosmology

We present a systematic analysis of the dynamics of flat Friedmann-Lemaître-Robertson-Walker cosmological models with radiation and dust matter in generalized teleparallel $f(T)$ gravity. We show that the cosmological dynamics of this model is fully described by a function $W(H)$ of the Hubble parameter, which is constructed from the function $f(T)$. After reducing the phase space to two dimensions we derive the conditions on $W(H)$ for the occurrence of de Sitter fixed points, accelerated expansion, crossing the phantom divide, and finite time singularities. Depending on the model parameters it is possible to have a bounce (from contraction to expansion) or a turnaround (from expansion to contraction), but cyclic or oscillating scenarios are prohibited. As an illustration of the formalism we consider power law $f(T) = T + α(-T)^n$ models, and show that these allow only one period of acceleration and no phantom divide crossing.

gr-qc↗