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A gravastar comprises three distinct sections: the interior zone, the middle shell, and its outer region. By considering a specific extended teleparallel gravity model that incorporates conformal Killing vectors and provides the field equations. We observe that the interior part exhibits a repellent force acting on the shell. This is based on the assumption that pressure is analogous to negative energy density. The middle shell consists of ultrarelativistic plasma and pressure, which is directly proportional to the matter density and counteracts the repellent force exerted by the inner zone. In the outer zone, we compute the precise solution in a vacuum and then connect these spacetimes using junction conditions to investigate stability limits. We aim to investigate the influence of dark energy models on the stable characteristics of gravastar configurations. It is worth noting that the phantom field exhibits the highest stable configurations for all physically viable selections of physical parameters. We additionally investigate the influence of physical parameters on the correct length, entropy, and energy of the gravastar.

In this letter, we obtain two new embedded WH solutions by using the class-I approach in the background of newly fourth-order Ricci inverse gravity. We show that the combination of these newly calculated shape functions and Ricci inverse gravity provides us with the possibility of obtaining traversable wormholes. All the required wormhole properties are discussed, along with flaring out and flatness conditions. The embedded diagrams within the scope of upper and lower universes are provided under the effect of both newly calculated embedded shape functions. All the energy conditions are explored with valid and negative regions. The presence of exotic matter is confirmed due to the negative region in all the energy conditions, specifically in the null energy condition. The Doppler effect through the red-blue shifts function is also discussed. Several key findings from the current research are described that demonstrate the validity of these wormhole solutions in Ricci inverse gravity.

This study explores the effect of charge on a special astronomical object known as a gravastar, which is viewed as an alternative to a black hole. Based on the conjecture put out by Mazur and Mottola in general relativity, the study primarily focuses on the consequences of $f(Q)$ gravity. The internal domain, the intermediate shell, and the external domain are the three separate sections that make up a gravastar. Using a particular $f(Q)$ gravity model that includes conformal Killing vectors to analyze these areas, we discover that the inner domain shows a repulsive force on the spherical shell since it is assumed that pressure is equivalent to negative energy density. The intermediate shell is made up of ultrarelativistic plasma and pressure, which is proportional to energy density and balances the repulsive force from the interior domain. For exterior region, we use two appraoches as first we calculate the vacuum exact solution and secondly considered as the Reissner-Nordstr\"om metric. Then, we match these spacetimes through junction condition and explore the stability constraints for both cases. Our results show that charged gravastar solutions with non-singular physical parameters including length, energy, entropy, and equation of state parameter are physically realistic.

This current analysis offers novel wormhole solutions in the background of newly developed extended $f(\mathcal{R},\mathcal{L}_m)$ gravity. We use an anisotropic matter source and a particular type of energy density demonstrating cold dark matter halo and quantum wave dark matter halo to calculate two different wormhole solutions. The properties of the exotic matter within the wormhole geometry and the matter contents via energy conditions are studied in detail, both analytically and graphically, by showing valid and invalid regions. The calculated shape functions of wormhole geometry satisfy the required conditions in both cases. Further, we investigate the stability of the shell around wormhole structures by considering black hole solutions in the framework of cold dark matter halo and quantum wave dark matter by assuming matter contents located at the shell follow the phantom-like equations of state. Then, for quintessence and phantom energy type equations of state, the choice of cold dark matter halo has maximum stability at lower equilibrium shell radii and declines as the radius rises. More petite wormhole throats have an unstable configuration for the dark energy matter content, while higher wormhole throat radius has the lowest stability. For the choice of quantum wave dark matter, the shell around the wormhole structure is unstable for both quintessence and phantom energy, while dark energy shows a stable configuration for smaller values of wormhole throat.

This study examines five models derived from the Pacif parametrization scheme of the Hubble parameter ($H$), yielding various linear to quintic forms of the deceleration parameter (DP). Our goal is to explore the impact of these DP variations on late-time evolution and their potential to alleviate cosmological tensions. To enhance model constraints, we introduce non-diagonal elements into the covariance matrix to better capture statistical properties by simulating data point correlations. We also test the sensitivity of $H_{0}$ and $r_{d}$ to the Pacif parametrization scheme, treating the sound horizon $r_{d}$ as a free parameter to avoid imposing a CMB prior. This allows late-time data to constrain $r_{d}$ alongside other cosmological parameters, incorporating recent Baryon Acoustic Oscillations (BAO) measurements and Hubble data from Cosmic Chronometers Methods, Type Ia Supernovae (SNIa), Gamma-Ray Bursts (GRBs), and Quasars over a redshift range of \(0.106 < z < 2.33\). Our analysis provides optimal fit values for $H_{0}$ and $r_{d}$, showing notable consistency with Planck CMB data. By using the Akaike information criterion, we analyze the models and conclude that all models have good agreement with the most recent observations., Comment: 15 pages, 4 figures

This study deals with astrophysical accretion onto the charged black hole solution, which is sourced by the dilation, spin, and shear charge of matter in metric affine gravity. The metric affine gravity defines the link between torsion and nonmetricity in space-time geometry. In the current analysis, we study the accretion process of various perfect fluids that are accreting near the charged black hole in the framework of metric affine gravity. Within the domain of accretion, multiple fluids have been examined depending on the value of $f_1$. The ultra-stiff, ultra-relativistic, and sub-relativistic fluids are considered to discuss the accretion. In the framework of equations of state, we consider isothermal fluids for this investigation. Further, we explore the effect of polytropic test fluid in relation to accretion discs, and it is presented in phase diagrams. Some important aspects of the accretion process are investigated. Analyzing the accretion rate close to a charged black hole solution, typical behavior is created and discussed graphically., Comment: 22 pages, 10 figures, 6 Tables

This study examines a recently hypothesized black hole. We study the Joule-Thomson coefficient, the inversion temperature and also the isenthalpic curves in the $T_i -P_i$ plane. A comparison is made between the Van der Waals fluid and the black hole to study their similarities and differences. The Joule-Thomson coefficient, the inversion curves and the isenthalpic curves are discussed inAdS black holes surrounded by Chaplygin dark fluid. In $T -P$ plane, the inversion temperature curves and isenthalpic curves are obtained with different parameters. Next, we explore the radial timelike geodesics that leads us to explore the tidal force effects for a radially in-falling particle in such black hole spacetime. We also numerically solve the geodesic deviation equation for two nearby radial geodesics for a freely falling particle. Our analysis shows that contrary to the Schwarzschild spacetime, the tidal forces don't become zero at spatial infinity due to the lack of asymptotic flatness because of the presence of a non-zero cosmological constant. The geodesic separation profile shows an oscillating trend and depends on the dynamic spacetime parameters $q, B$ and $\Lambda$., Comment: 15 pages, 13 figures

Gravastars, theoretical alternatives to black holes, have captured the interest of scientists in astrophysics due to their unique properties. This paper aims to further investigate the exact solution of a novel gravastar model based on the Mazur-Mottola (2004) method within the framework of general relativity, specifically by incorporating the cloud of strings and quintessence. By analyzing the gravitational field and energy density of gravastars, valuable insights into the nature of compact objects in the universe can be gained. Understanding the stability of gravastars is also crucial for our comprehension of black holes and alternative compact objects. For this purpose, we present the Einstein field equations with the modified matter source and calculate the exact solutions for the inner and intermediate regions of gravastars. The exterior region is considered as a black hole surrounded by the cloud of strings and quintessence, and the spacetimes are matched using the Darmoise-Israel formalism. {An investigation is conducted on the stability of gravastars using linearized radial perturbation. Additionally, the proper length, energy content, and entropy of the shell are computed. The stability of gravastars is positively correlated with the enhancement of the cloud of strings parameter, while it is negatively correlated with the growth in the quintessence field parameter.} The paper concludes with a summary of the findings and their implications in the field of astrophysics and cosmology., Comment: 14 pages, 3 figures

This study examines a recently hypothesized black hole, which is a perfect solution of metric-affine gravity with a positive cosmological constant, and its thermodynamic features as well as the Joule-Thomson expansion. We develop some thermodynamical quantities, such as volume, Gibbs free energy, and heat capacity, using the entropy and Hawking temperature. We also examine the first law of thermodynamics and thermal fluctuations, which might eliminate certain black hole instabilities. In this regard, a phase transition from unstable to stable is conceivable when the first law order corrections are present. Besides that, we study the efficiency of this system as a heat engine and the effect of metric-affine gravity for physical parameters $q_e$, $q_m$, $\kappa_{\mathrm{s}}$, $\kappa_{\mathrm{d}}$ and $\kappa_{\mathrm{sh}}$. Further, we study the Joule-Thomson coefficient, and the inversion temperature and also observed the isenthalpic curves in the $T_i -P_i$ plane. In metric-affine gravity, a comparison is made between the Van der Waals fluid and the black hole to study their similarities and differences.

The current study explores the generalized embedded wormhole solutions in the background of $f(\mathcal{R},\mathcal{G})$ gravity, where $\mathcal{R}$ represents the Ricci scalar and $\mathcal{G}$ denotes the Gauss-Bonnet invariant. To investigate the necessary structures of the wormhole solutions we thoroughly analyzed the energy conditions under $f(\mathcal{R},\mathcal{G})$ gravity within the anisotropic source of matter. To meet this aim, we consider spherically symmetric geometry with the most generic gravity model of the gravity. A modified version of the field equations is calculated for two different embedded wormhole solutions. All the energy conditions are calculated and shown graphically with the regional ranges of the model parameter. Further, the invalid region of the energy conditions confirms the presence of exotic matter. Finally, we have concluding remarks.

This paper explores the role of nonlinear electrodynamics on the stable configuration of thin-shell wormholes formulated from two equivalent geometries of Reissner-Nordstr\"om black hole with nonlinear electrodynamics. For this purpose, we use cut and paste approach to eliminate the central singularity and event horizons of the black hole geometry. Then, we explore the stability of the developed model by considering different types of matter distribution located at thin-shell, i.e., barotropic model and variable equations of state (phantomlike variable and Chaplygin variable models). We use linearized radial perturbation to explore the stable characteristics of thin-shell wormholes. It is interesting to mention that Schwarzschild and Reissner-Nordstr\"om black holes show the unstable configuration for such type of matter distribution while Reissner-Nordstr\"om black hole with nonlinear electrodynamics expresses stable regions. It is found that the presence of nonlinear electrodynamics gives the possibility of a stable structure for barotropic as well as variable models. It is concluded that stable region increases for these models by considering higher negative values of coupling constant $\alpha$ and the real constant $n$.

In this work, we present the Einstein field equations (EFE) in the framework of a modified matter source and thus find out the solutions for the Wormhole. To obtain characteristic solutions set, we employ two kinds of equations of states (EOS), i.e., the van der Waals and polytropic EOS to the EFE. We adopt embedding class as a general technique for the system and discuss several physical attributes of the stellar system under embedded wormhole solutions. Detailed discussion about the matter contents of the thin-shell developed around the obtained wormhole solutions in the background of Schwarzschild BH surrounded by cloud and quintessence-type fluid distribution by using a well-known cut and paste approach. We have also performed stability analysis using the linearized radial perturbation method for the van der Waals EOS and polytropic EOS. Several interesting points have evolved from the entire investigation along with the features of the thin shell around the wormhole., Comment: 24 figures and 4 tables

In this paper, we are interested to explore stable configurations of $d$-dimensional gravastars constructed from the interior $d$-dimensional de Sitter and exterior $d$-dimensional Schwarzschild/Reissner-Nordstr\"om (de Sitter) black holes through cut and paste approach. We consider the linearized radial perturbation preserving the original symmetries to explore their stability by using three different types of matter distributions. The resulting frameworks represent unstable structures for barotropic, Chaplygin, and phantomlike models for every considered choice of exterior geometries. However, matter contents with variable equations of state have a remarkable role to maintain the stability of gravastars. We conclude that stable structures of gravastars are obtained only for generalized variable models with exterior $d$-dimensional Schwarzschild/Reissner-Nordstr\"om-de Sitter black holes., Comment: 21 pages, 12 figures, to appear in Chinese J. Physics

This paper develops a new solution of gravitational vacuum star in the background of charged Kiselev black holes as an exterior manifold. We explore physical features and stability of thin-shell gravastars with radial perturbation. The matter thin layer located at thin-shell greatly affects stable configuration of the developed structure. We assume three different choices of matter distribution such as barotropic, generalized Chaplygin gas and generalized phantomlike equation of state. The last two models depend on the shell radius, also known as variable equation of state. For barotropic model, the structure of thin-shell gravastar is mostly unstable while it shows stable configuration for such type of matter distribution with extraordinary quintessence parameter. The resulting gravastar structure indicates stable behavior for generalized Chaplygin gas but unstable for generalized phantomlike model. It is also found that proper length, entropy and energy within the shell show linear relation with thickness of the shell., Comment: 23 pages, 9 figures

This paper explores the possible existence of stable regions of bounded excursion gravastars in the background of regular black holes (Bardeen and Bardeen-de Sitter black holes). For this purpose, we match internal de Sitter geometry with an external regular black hole through an intermediate shell with an equation of state. The matter surface located at the shell greatly affects the dynamical configuration of the shell. We consider stiff and dust fluids to discuss the outcomes of the developed structures. We then determine the critical values of physical parameters at which the shell neither collapses nor expands for both cases with different choices of external and internal geometries. It is found that there does not exist any possible region of stable bounded excursion gravastar for the interior flat and exterior regular geometries. However, the stable bounded excursion gravastar is obtained for the interior de Sitter and exterior regular black holes with a suitable choice of physical parameters for both stiff and dust shells. We conclude that the possibility of the existence of both bounded excursion gravastars as well as black holes cannot be excluded in the dynamical model., Comment: 29 pages, 19 figures, to appear in ASTR

This paper examines the stability of thin-shell gravastars in the context of regular spacetimes (Bardeen and Bardeen-de Sitter black holes). We apply cut and paste approach to construct gravastars through the matching of interior non-singular de Sitter geometry with exterior regular black hole. This model contains three regions, i.e., interior, thin-shell and exterior. The interior and exterior regions are connected at thin-shell. We investigate physical viability of the developed model by the energy conditions and explore its stability by using radial perturbation about the equilibrium shell radius. It is found that thin-shell gravastars show large stable regions for the Bardeen-de Sitter black hole as compared to the Bardeen black hole. It is concluded that stable regions exist near the formation of expected event horizon., Comment: 17 pages, 5 figures

This paper investigates the dynamics of thin-shell in the presence of perfect fluid as well as the scalar field. We formulate the equations of motion using Israel thin-shell formalism by taking the interior and exterior regions of Schwarzschild, Kerr as well as Kerr-Newmann black hole. We find numerical solutions of equations of motion and effective potential to analyze the scalar shell for collapse and expansion. It is found that the rate of collapse and expansion of scalar shell through shell's radius depends on charge and rotation parameters. We conclude that the massive scalar shell leads to a collapse of thin-shell, while massless scalar shell indicates both collapses as well as expansion., Comment: 23 pages, 10 figures