arXiv subjects

Kairat Myrzakulov

Publications and source records attributed to Kairat Myrzakulov.

At least 19 recordsLinked to original sources

arXiv2026

Static Magnetic Brane Solutions in Quartic Quasi-Topological Gravity with Power-Law Maxwell Nonlinear Electrodynamics

In this paper, we investigate static magnetic brane solutions in (n+1)-dimensional quartic quasi-topological gravity coupled to power-law Maxwell nonlinear electrodynamics. We derive the corresponding solutions and analyze their geometric and physical properties. We observe that the solutions are horizonless and free of curvature singularities in the physically accessible region, while the spacetime generally exhibits a conical singularity with a nonzero angular deficit at the lower radial endpoint. We demonstrate that the endpoint is independent of the higher-curvature couplings lambda, mu, and c, whereas the near-endpoint geometry is mainly controlled by the magnetic parameter q and the nonlinear parameter s. At large radial distances, we find that the higher-curvature couplings govern the asymptotic behavior of the metric. We also establish that finite asymptotic solutions require 1/2 < s <= 2 and analyze the dependence of the angular deficit on q, s, and the endpoint parameter rho_+. Using the counterterm method, we calculate the conserved mass density and show that the static magnetic brane carries no net electric charge. Finally, we verify that the power-law Maxwell theory recovers linear Maxwell electrodynamics at s=1 and Einstein gravity when lambda=mu=c=0.

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arXiv2026

Systematic Light Propagation Bias from the Heliosphere and Its Impact on the Hubble Tension

The Hubble tension (HT) represents one of the most significant discrepancies in modern cosmology, with local distance measurements yielding $H_0 = 73.5 \pm 1.4$ km/s/Mpc while cosmic microwave background (CMB) observations predict $H_0 = 67.4 \pm 0.5$ km/s/Mpc. We propose that this tension arises from systematic effects introduced by our Solar System's heliospheric (HS) environment on local distance measurements. The HS creates a complex medium of heated plasma and energetic neutral atoms (ENAs) beyond the heliopause (HP), where interstellar medium (ISM) temperatures rise significantly. This thermal gradient and particle environment may systematically affect observations of Cepheid variables and Type Ia supernovae (SNe~Ia) used in the local cosmic distance ladder (CDL), biasing distance measurements and artificially inflating the measured Hubble constant. We present theoretical calculations showing how HS effects could account for up to $\sim 8\%$ of the observed 8--9\% discrepancy, with the realistic contribution lying in the $\sim 3$--$8\%$ range once anisotropy, partial calibration cancellation, and chromatic suppression are included, and discuss observational tests to validate this hypothesis.

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arXiv2026

Thermodynamics, Shadow, and Quasinormal Modes of AdS Ayón--Beato--García Massive Black Hole

We investigate the thermodynamics, photon sphere, and dynamical stability of an AdS Ayón--Beato--García (ABG) massive black hole with graviton mass and magnetic charge. The Gibbs free energy exhibits distinct limiting behaviors: it reduces to that of an AdS massive black hole when magnetic charge vanishes, to that of an AdS ABG black hole when graviton mass is zero, and smoothly interpolates to the AdS massive Reissner-Nordström case in the asymptotic regime. Furthermore, the photon sphere and shadow analysis indicate that increasing the graviton mass expands their radii, while increasing the magnetic charge causes contraction, in agreement with earlier studies of black hole spacetimes. Quasinormal mode (QNM) calculations further confirm dynamical stability, as the imaginary part remains negative, ensuring decay of perturbations. Additionally, the real part of the frequency decreases with graviton mass, while the imaginary part initially grows before saturating at higher values. Together, these results provide meaningful insights into the interplay between graviton mass, magnetic charge, and stability, thereby enriching the understanding of black holes in modified gravity theories.

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arXiv2026

More Exact Thermodynamic Analysis of Topological Black Holes in $R^2$ Gravity

This study investigates the thermodynamics of topological black hole solutions in $R^{2}$ gravity, incorporating the effects of small statistical fluctuations up to first-order corrections. We precisely calculate entropy, internal energy, Helmholtz free energy, specific heat, enthalpy, and Gibbs free energy, accounting for perturbative thermal corrections. Our results reveal that the internal energy of small black holes diverges asymptotically due to these fluctuations. The corrected Gibbs free energy attains asymptotically high values for small horizon radii. In contrast, the equilibrium Gibbs free energy approaches zero. Additionally, we assess the stability of the black hole in the presence of these thermal fluctuations. We find that, in contrast to the equilibrium state, the thermal fluctuation introduces a double phase transition to the stability of the black hole. Our analysis reveals that the influence of fluctuations is notably significant, primarily for small black holes. These findings offer new insights into the thermodynamic properties of topological black holes in the presence of thermal fluctuations.

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arXiv2025

Refined Thermodynamic Analysis of Perfect Fluid Dark Matter Black Holes in a Phantom Background

In this study, we examine the thermodynamics of black holes immersed in perfect fluid dark matter (PFDM) by employing the Misner-Sharp energy framework. We extend the analysis to include the thermal fluctuations of these black holes when size reduces to small size, redefining key thermodynamic variables such as pressure, volume, temperature, internal energy, and entropy within the context of PFDM. Using these newly defined variables, we systematically calculate the enthalpy, Gibbs free energy, and specific heat of PFDM black holes, incorporating perturbative thermal corrections. To assess the stability of these black holes, we perform a detailed graphical analysis of the specific heat as a function of the horizon radius, providing insights into the thermal stability of PFDM black holes. This comprehensive approach enhances our understanding of the thermodynamic properties and stability of black holes in the presence of dark matter.

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arXiv2025

Exploring non-perturbative effects on quasi-topological black hole thermodynamics

We examine the impact of non-perturbative quantum corrections to the entropy of both charged and charged rotating quasi-topological black holes, with a focus on their thermodynamic properties. The negative-valued correction to the entropy for small black holes is found to be unphysical. Furthermore, we analyze the effect of these non-perturbative corrections on other thermodynamic quantities, including internal energy, Gibbs free energy, charge density, and mass density, for both types of black holes. Our findings indicate that the sign of the correction parameter plays a crucial role at small horizon radii. Additionally, we assess the stability and phase transitions of these black holes in the presence of non-perturbative corrections. Below the critical point, both the corrected and uncorrected specific heat per unit volume are in an unstable regime. This instability leads to a first-order phase transition, wherein the specific heat transitions from negative to positive values as the system reaches a stable state.

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arXiv2025

Bulk viscous matter in $f(T)$ gravity: A path to cosmic acceleration

In this paper, we investigate the effects of varying bulk viscosity coefficients $ζ(t)=ζ_{0}+ζ_{1}H$ on cosmic evolution within the framework of $f(T)$ teleparallel gravity. We focus on two cases: (i) $ζ_{1} \neq0$ and (ii) $ζ_{1} =0$, deriving the Hubble parameter $H$ as a function of redshift $z$ using a linear $f(T)$ model ($f(T) = αT$ where $α\neq 0$). Using the combined $H(z)+Pantheon^{+}+BAO$ dataset, we obtain observational constraints on model parameters. For Case I ($ζ_1 \neq 0$), best-fit values are $H_0=60.0^{+2.0}_{-1.9}$ km/s/Mpc, $α=1.01^{+0.10}_{-0.098}$, $ζ_0=40.1^{+1.9}_{-2.0}$, and $ζ_1=0.123^{+0.093}_{-0.088}$, while for Case II ($ζ_1 = 0$), they are $H_0=67.5^{+1.3}_{-1.3}$ km/s/Mpc, $α=0.94^{+0.14}_{-0.13}$, and $ζ_0=34.7^{+2.0}_{-2.0}$. The analysis reveals a transition in the deceleration parameter, indicating a shift from deceleration to acceleration of the universe's expansion, with present-day values of $q_{0} \approx -0.49$ and $q_{0} \approx -0.32$ for the respective cases. The jerk parameter $j(z)$ and effective EoS for the cosmic viscous fluid also support the cosmic acceleration, with trajectories aligning with the quintessence scenario. These findings underscore the potential of our $f(T)$ model dominated by bulk viscous matter in explaining cosmic acceleration.

astro-ph.CO↗

arXiv2024

Thermodynamics of a newly constructed black hole coupled with nonlinear electrodynamics and cloud of strings

This paper finds an exact singular black hole solution in the presence of nonlinear electrodynamics as the source of matter field surrounded by a cloud of strings in $4D$ $AdS$ spacetime. Here, the presence of the cloud of string, the usual Bardeen solution, becomes singular. The obtained black hole solution interpolates with the $AdS$ Letelier black hole in the absence of both the deviation parameter and magnetic charge and interpolates with the $AdS$ Bardeen black hole in the absence of the deviation parameter and a cloud of strings parameter. We analyse the horizon structure and thermodynamics properties, including the stability of the resulting black hole, numerically and graphically. Thermodynamical quantities associated with the black hole get modified due to the nonlinear electrodynamics and cloud of strings. Moreover, we study the effect of a cloud of strings parameter, magnetic charge and deviation parameter on critical points and phase transition of the obtained black hole where the cosmological constant is treated as the thermodynamics pressure. The critical radius increases with increasing deviation parameter values and magnetic charge values. In contrast, the critical pressure and temperature decrease with increasing deviation parameters and magnetic charge values.

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arXiv2024

Observational analysis of late-time acceleration in $f(Q, L_m)$ gravity

In this study, we explored late-time cosmology within an extended class of theories based on $f(Q, L_m)$ gravity. This theory generalizes $f(Q)$ gravity by incorporating a non-minimal coupling between the non-metricity $Q$ and the matter Lagrangian $L_m$, analogous to the $f(Q,T)$ theory. The coupling between $Q$ and $L_m$ leads to the non-conservation of the matter energy-momentum tensor. We first investigated a cosmological model defined by the functional form $f(Q, L_m) = αQ + βL_m^n$, where $α$, $β$, and $n$ are constants. The derived Hubble parameter $H(z) = H_0 (1+z)^{\frac{3n}{2(2n-1)}}$ indicates that $n$ significantly influences the scaling of $H(z)$ over cosmic history, with $n > 2$ suggesting accelerated expansion. We also examined the simplified case of $n = 1$, leading to the linear form $f(Q, L_m) = αQ + βL_m$, consistent with a universe dominated by non-relativistic matter. Using various observational datasets, including $H(z)$ and Pantheon, we constrained the model parameters. Our analysis showed that the $f(Q, L_m)$ model aligns well with observational results and exhibits similar behavior to the $Λ$CDM model. The results, with $q_0 = -0.22 \pm 0.01$ across all datasets, indicate an accelerating universe, highlighting the model's potential as an alternative to $Λ$CDM.

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arXiv2024

Possible wormholes in $f(R)$ gravity sourced by solitonic quantum wave and cold dark matter halos and their repulsive gravity effect

In this paper, we present new generalized wormhole (WH) solutions within the context of $f(R)$ gravity. Specifically, we focus on $f(R)$ gravitational theories formulated in the metric formalism, with our investigation centered on a power-law form represented by $f(R) = εR^χ$. Here, $ε$ is an arbitrary constant, and $χ$ is a real number. Notably, this form possesses the advantageous property of reducing to Einstein gravity when $ε=1$ and $χ=1$. To obtain these novel WH solutions, we establish the general field equations for any $f(R)$ theory within the framework of Morris-Thorne spacetime, assuming metric coefficients that are independent of time. By utilizing an anisotropic matter source and a specific type of energy density associated with solitonic quantum wave (SQW) and cold dark matter (CDM) halos, we calculate two distinct WH solutions. We thoroughly investigate the properties of the exotic matter (ExoM) residing within the WH geometry and analyze the matter contents through energy conditions (ECs). Both analytical and graphical methods are employed in this analysis to examine the validity of different regions. Notably, the calculated shape functions for the WH geometry satisfy the necessary conditions in both scenarios, emphasizing their reliability. This ExoM is characterized by an energy-momentum tensor that violates the null energy condition (NEC) and, consequently, the weak energy condition as well, in the vicinity of the WH throats. Furthermore, we investigated the repulsive effect of gravity and discovered that its presence results in a negative deflection angle for photons following null geodesics. Importantly, we observed that the deflection angle consistently exhibits negative values across all $r_0$ values in both scenarios, indicating the manifestation of the repulsive gravity effect.

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arXiv2023

More Exact Thermodynamics of Nonlinear Charged AdS Black Holes in 4D Critical Gravity

In this paper, we investigate nonlinearly charged AdS black holes in four-dimensional critical gravity and study more exact black hole thermodynamics under the effect of small statistical fluctuations. We compute the correction to the thermodynamics of nonlinearly charged AdS black hole up to the leading order. We discuss the stability of black holes under the circumstances of fluctuation and find that fluctuation causes instability in the black holes. Moreover, both the isothermal and adiabatic compressibilities are also derived. Finally, we estimate the role of small fluctuations on the equation of states and study the $P-v$ diagram of nonlinearly charged AdS black hole.

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arXiv2023

Quasinormal Modes and Phase Structure of Regular $AdS$ Einstein-Gauss-Bonnet Black Holes

In this paper, we present an exact regular black hole solution in Einstein-Gauss-Bonnet coupled with nonlinear matter fields. It is a generalization of a regular Einstein-Gauss-Bonnet black hole in $5D$ $AdS$ spacetime. The causal structure of the obtained solution identifies with Boulware-Deser black hole solution, except for the curvature singularity at the center. It incorporates the Boulware-Deser black holes in the absence of deviation parameters. We also study the thermodynamic properties of the solution that satisfies a modified first law of thermodynamics. Furthermore, we discuss the stability of the obtained black hole solution and, in this regard, a double phase transition occurs. Within context, we find that phase transition exists at the point where the heat capacity diverges and, incidentally, the temperature attains the maximum value. We discuss the fluid nature of the black hole also exhibiting critical points. The quasinormal modes of the black hole solution and their dependencies on Gauss-Bonnet coupling and deviation parameters are also analysed in terms of null geodesics.

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arXiv2023

Weak Deflection Angle, Greybody Bound and Shadow for Charged Massive BTZ Black Hole

We provide a discussion on a light ray in a charged black hole solution in massive gravity. To serve the purpose, we exploit the optical geometry of the black hole solution and find the Gaussian curvature in weak gravitational lensing. Furthermore, we discuss the deflection angle of the light ray in both plasma and non-plasma mediums using the Gauss-Bonnet theorem on the black hole. We also analyze the Regge--Wheeler equation and derive rigorous bounds on the greybody factors of linearly charged massive BTZ black hole. We also study the shadow or silhouette generated by charged massive BTZ black holes. The effects of charge and cosmological constant on the radius of the shadow are also discussed.

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arXiv2023

Solving the $H_{0}$ tension in $f(T)$ Gravity through Bayesian Machine Learning

Bayesian Machine Learning~(BML) and strong lensing time delay~(SLTD) techniques are used in order to tackle the $H_{0}$ tension in $f(T)$ gravity. The power of BML relies on employing a model-based generative process which already plays an important role in different domains of cosmology and astrophysics, being the present work a further proof of this. Three viable $f(T)$ models are considered: a power law, an exponential, and a squared exponential model. The learned constraints and respective results indicate that the exponential model, $f(T)=αT_{0}\left(1-e^{-p T / T_{0}}\right)$, has the capability to solve the $H_{0}$ tension quite efficiently. The forecasting power and robustness of the method are shown by considering different redshift ranges and parameters for the lenses and sources involved. The lesson learned is that these values can strongly affect our understanding of the $H_{0}$ tension, as it does happen in the case of the model considered. The resulting constraints of the learning method are eventually validated by using the observational Hubble data(OHD).

astro-ph.CO↗

arXiv2020

General slow-roll inflation in $f(R)$ gravity under the Palatini approach

Slow-roll inflation is analyzed in the context of modified gravity within the Palatini formalism. As shown in the literature, inflation in this framework requires the presence of non-traceless matter, otherwise it does not occur just as a consequence of the non-linear gravitational terms of the action. Nevertheless, by including a single scalar field that plays the role of the inflaton, slow-roll inflation can be performed in these theories, where the equations lead to an effective potential that modifies the dynamics. We obtain the general slow-roll parameters and analyze a simple model to illustrate the differences introduced by the gravitational terms under the Palatini approach, and the modifications on the spectral index and the tensor to scalar ratio predicted by the model.

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arXiv2020

Cosmological applications of Myrzakulov gravity

We investigate the cosmological applications of Myrzakulov $F(R,T)$ gravity. In this theory ones uses a specific but non-special connection, and thus both curvature and torsion are dynamical fields related to gravity. We introduce a parametrization that quantifies the deviation of curvature and torsion scalars form their corresponding values obtained using the special Levi-Civita and Weitzenb{ö}ck connections, and we extract the cosmological field equations following the mini-super-space procedure. Even for the simple case where the action of the theory is linear in $R$ and $T$, we find that the Friedmann equations contain new terms of geometrical origin, reflecting the non-special connection. Applying the theory at late times we find that we can acquire the thermal history of the universe, where dark energy can be quintessence-like or phantom-like, or behave exactly as a cosmological constant and thus reproducing $Λ$CDM cosmology. Furthermore, we show that these features are obtained for other Lagrangian choices, too. Finally, early-time application leads to the de Sitter solution, as well as to an inflationary realization with the desired scale-factor evolution.

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arXiv2020

Curvaton: Perturbations and reheating

We study chaotic and runaway potential inflationary models in the curvaton scenario. In particular, we address the issue of large tensor-to-scalar ratio and red-tilted spectrum in chaotic models and reheating in runaway model in the light of latest Planck results. We show that curvaton can easily circumvent these problems and is well applicable to both type of models. For chaotic models, the observable non-Gaussianity put strong constraints on the decay epoch of curvaton as well as on its field value around the horizon exit. Besides, it can also explain the observed red-tilt in the spectrum as a consequence of its negative mass-squared value. As for the runaway inflationary models, curvaton by sudden decay into the background radiation provides an efficient reheating mechanism, whereas the inflaton rolls down from its potential and enters into the kinetic regime. To this effect, we consider the generalized exponential potential and obtain the allowed parametric space for model parameters. From the estimates on inflaton parameters, we constrained the curvaton mass and then the reheating temperature. We constrained the latter for both dominating and sub-dominating case and show that it agrees with the nucleosynthesis constraint.

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arXiv2019

Braneworld inflation with effective α-attractor potential

In this paper, we study inflation with α-attractor potential in the RS braneworlds where high energy corrections to the Friedmann equation facilitate slow roll. In this scenario, we numerically investigate the inflationary parameters and show that the high energy brane corrections have significant effect on the parameter α, namely, the lower values of the parameter are preferred by observation in this limit. We find that the sub-Planckian values of the field displacement can be achieved by suitably constraining the brane tension.

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