This paper discusses the phenomenon of evolving spherically symmetric cluster of stars in the presence of an exotic matter. To discuss evolutionary mechanism, we use the Starobinsky model of ( ) gravity as exotic matter and the structure scalars as evolutional parameters. We study various evolution modes such as isotropic pressure, quasi-homologous evolution, density homogeneity, and geodesic nature. The stability of homogeneous density of baryonic and non-baryonic matter is discussed using dissipation, tidal forces, anisotropic pressures, expansion and shear-effects. It is observed that the dark matter has remarkable impact on the evolutionary changes. Also, it is shown that the dissipation factor produces density inhomogeneity in expanding clusters with shear effects. We observe that high curvature geometry enhances quasi-homologous evolution in the presence of shear. We use star Her X-1 as test star to discuss physical behavior of Starobinsky model. It is found that the density of dark matter overcomes the density of matter for large values of dark matter parameter .

Pakistan

Wasee Shahid

Rubab Manzoor

Saadia Mumtaz

Syed Mardan

China

Adnan Malik

Stability of evolving cluster of stars and exotic matter

In the present paper, we investigate the thermodynamics of a new perturbed black hole solution in symmetric teleparallel gravity. Our main focus is on the solution for a nonlinear model like <math><mi>Q</mi><mo>+</mo><mi>α</mi><msup><mrow><mi>Q</mi></mrow><mrow><mn>2</mn></mrow></msup></math>. We study new features of null geodesics and quasi-normal modes that occur for the current perturbed black hole solution. We inspect the critical behavior of thermal fluctuation and phase transitions in the background of a considered black hole solution and get the Davies points. To complete this study, we also explore the interesting aspects of the emission energy and quantum fluctuations. We highlight that our results are in good agreement with the thermodynamics of the previous black hole solutions and assumptions presented in the literature.

Faisal Javed

G. Mustafa

Uzbekistan

Farruh Atamurotov

Thermal analysis with emission energy of perturbed black hole in ( ) Gravity

This study explores new wormhole solutions in the background of teleparallel gravity. All the energy conditions are investigated for two different new calculated shape functions. The presence of exotic matter is confirmed due to the violation of the energy conditions. Thin-shell around the wormhole geometry is obtained by using the cut and paste approach taking the Schwarzschild black hole as an exterior manifold. The stability of thin-shell is explored by using linearized radial perturbation about equilibrium shell radius for both choices of calculated shape functions. It is concluded that stable regions and the position of the expected event horizon depend on the choice of physical parameters.

Ibrar Hussain

Wu-Ming Liu

New wormhole models with stability analysis via thin-shell in teleparallel gravity

We examine the evolution of cavities within spherically symmetric cluster of stars in high curvature gravity. For this purpose, we use f(R) gravity through the Starobinsky model to incorporate dark matter effects in the discussion. In particular, we check the physical significance of the f(R) model by associating it with the observational data of stellar object 4U182030. For evolution of the cavity, we consider the purely areal evolutionary phase by assuming that the proper distance (in a radial direction) among neighboring stars remains constant. The analytical solutions are obtained among which a few solutions fulfill the Darmois conditions. It is found that the evolution of the cavity in a cluster of stars is highly controlled by the influence of dark matter.

Daoud Intizar

Dynamics of evolving cavity in cluster of stars

In this paper, we study the phase space analysis of locally rotationally symmetric Bianchi type I universe model by taking interactions between dark matter and scalar field models. We define normalized dimensionless variables to develop an autonomous system of equations. We also find the corresponding critical points in order to study the dynamics of the system. The dynamical analysis indicates that all the critical points correspond to accelerated cosmic expansion for tachyon coupled field. We observe that positive values of <math id="M1"><mrow><mi>m</mi></mrow></math> provide more stable future attractors as compared to its negative values. We also analyze the behavior of power-law scale factor which shows different cosmological phases. It is found that the region for decelerated expansion gets larger for the phantom coupled matter by increasing <math id="M2"><mrow><mi>m</mi></mrow></math> while this region decreases for tachyon coupled field.

M. Sharif

Stability of the Universe Model Coupled with Phantom and Tachyon Fields

This paper is devoted to the phase space analysis of an isotropic and homogeneous model of the universe by taking a noninteracting mixture of the electromagnetic and viscous radiating fluids whose viscous pressure satisfies a nonlinear version of the Israel–Stewart transport equation. We establish an autonomous system of equations by introducing normalized dimensionless variables. In order to analyze the stability of the system, we find corresponding critical points for different values of the parameters. We also evaluate the power-law scale factor whose behavior indicates different phases of the universe in this model. It is concluded that the bulk viscosity as well as electromagnetic field enhances the stability of the accelerated expansion of the isotropic and homogeneous model of the universe.

Stability of the accelerated expansion in nonlinear electrodynamics

The aim of this paper is to construct regular Hayward thin-shell wormholes and analyze their stability. We adopt Israel formalism to calculate surface stresses of the shell and check the null and weak energy conditions for the constructed wormholes. It is found that the stress-energy tensor components violate the null and weak energy conditions leading to the presence of exotic matter at the throat. We analyze the attractive and repulsive characteristics of wormholes corresponding to <math id="M1"><msup><mrow><mi>a</mi></mrow><mrow><mi>r</mi></mrow></msup><mo>&gt;</mo><mn>0</mn></math> and <math id="M2"><msup><mrow><mi>a</mi></mrow><mrow><mi>r</mi></mrow></msup><mo>&lt;</mo><mn>0</mn></math>, respectively. We also explore stability conditions for the existence of traversable thin-shell wormholes with arbitrarily small amount of fluid describing cosmic expansion. We find that the space-time has nonphysical regions which give rise to event horizon for <math id="M3"><mn>0</mn><mo>&lt;</mo><msub><mrow><mi>a</mi></mrow><mrow><mn>0</mn></mrow></msub><mo>&lt;</mo><mn>2.8</mn></math> and the wormhole becomes nontraversable producing a black hole. The nonphysical region in the wormhole configuration decreases gradually and vanishes for the Hayward parameter <math id="M4"><mi>l</mi><mo>=</mo><mn>0.9</mn></math>. It is concluded that the Hayward and Van der Waals quintessence parameters increase the stability of thin-shell wormholes.

Stability of the Regular Hayward Thin-Shell Wormholes

This paper is devoted to construct Schwarzschild-de Sitter and anti-de Sitter thin-shell wormholes by employing Visser’s cut and paste technique. The Darmois-Israel formalism is adopted to formulate the surface stresses of the shell. We analyze null and weak energy conditions as well as attractive and repulsive characteristics of thin-shell wormholes. We also explore stable and unstable solutions against linear perturbations by taking two different Chaplygin gas models for exotic matter. It is concluded that the stress-energy tensor components violate the null and weak energy conditions indicating the existence of exotic matter at the wormhole throat. Finally, we find unstable and stable configurations for the constructed thin-shell wormholes.