Gravastar Solutions Research Articles

Charged Gravastar Solutions in the Finch‐Skea Framework with f(Q)$f(\mathbb {Q})$ Gravity

AbstractIn this study, the features of a charged gravastar are investigated within the framework of gravity ( represents nonmetricity) using the Finch–Skea metric. This metric is applied to both the interior and shell regions of the charged gravastar and the field equations are derived accordingly. For the exterior regions, various black holes are considered, i.e., Reissner–Nordström, Bardeen, and Hayward regular black holes. The interior and exterior layers are matched using the Israel junction conditions, which help to determine the surface energy density and surface pressure for these black holes. Some physical properties such as proper length, entropy, energy, and the equation of state parameter are examined. The stability of the developed gravastar model is discussed through the effective potential, the causality condition and adiabatic index. It is concluded that the compact gravastar structure could be a viable alternative to black holes within this framework.

Constraining study of charged gravastars solutions in symmetric teleparallel 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 the exterior region, we use two approaches first we calculate the vacuum exact solution, and second, consider the Reissner–Nordström metric. Then, we match these spacetimes through junction conditions 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.

Novel gravastar solutions: Investigating stability, energy, and entropy in the presence of cloud of strings and quintessence

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 presents 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.

New gravastar model in generalised cylindrically symmetric space–time and prediction of mass limit

We present a class of new Gravastar solutions following the works of Mazur and Mottola for a Gravitational Bose–Einstein Condensate (GBEC) star in generalised cylindrically symmetric space–time. A stable gravastar consists of 3 distinct regions, namely: (i) an interior de-Sitter space (p=−ρ), which exerts an outwards repulsive force at all points on the thin shell, (ii) an intermediate thin shell with a slice of finite length separating the interior and exterior regions is supposed to be consisting of an ultra-relativistic stiff fluid, with the equation of state p=ρ and (iii) an exterior vacuum region. This thin shell, which is considered as the critical surface for the quantum phase transition, replaces both the classical de-Sitter space and Schwarzschild event horizon. The new solutions are free from any singularities. The energy density, total energy, proper length, and the entropy of this shell are explored in this model. From the thin shell solution and using Lanczos equations for stress energy density, we have predicted the mass contained into the gravastar shell. The stability of the gravastar model is analysed through the consideration of gravitational surface redshift and entropy calculation. We have also obtained a constraint on the possible mass of the thin shell without violating the condition for stable gravastar configuration. All these features indicate that the present model is physically viable.

Gravastar model in Randall–Sundrum braneworld

In this work we derive a gravastar model in Randall–Sundrum II braneworld scenario. Gravastars (or gravitationally vacuum stars) were proposed by Mazur and Mottola as systems of gravitational collapse alternative to black holes. The external region of the gravastar is described by a Schwarzschild space-time, while its internal region is filled by dark energy. In between there is a thin shell surface with ultrarelativistic matter (sometimes referred to as stiff matter). We obtain solutions for some physical quantities of the braneworld gravastars. Those are compared to original Mazur–Mottola results as well as with some gravastar solutions in alternative gravity theories. The consequences of the braneworld setup in gravastar physics is deeply discussed.

Stable gravastars: Guilfoyle’s electrically charged solutions

Compelling alternatives to black holes, namely, gravitational vacuum stars (gravastars), which are multilayered compact objects, have been proposed to avoid a number of theoretical problems associated with event horizons and singularities. In this work, we construct a spherically symmetric thin-shell charged gravastar model where the vacuum phase transition between the de Sitter interior and the external Reissner–Nordström spacetime (RN) are matched at a junction surface, by using the cut-and-paste procedure. Gravastar solutions are found among the Guilfoyle exact solutions where the gravitational potential W2 and the electric potential field ϕ obey a particular relation in a simple form a(b−ϵϕ)2 + b1, where a, b and b1 are arbitrary constants. The simplest ansatz of Guilfoyle’s solution is implemented by the following assumption: that the total energy density is constant, where Q(r) is the electric charge up to a certain radius r. We show that, for certain ranges of the parameters, we can avoid the horizon formation, which allows us to study the linearized spherically symmetric radial perturbations around static equilibrium solutions. To lend our solution theoretical support, we also analyze the physical and geometrical properties of gravastar configurations.

Gravastars in f(G,T) gravity

This work proposes a stellar model under Gauss-Bonnet $f(\mathcal{G},T)$ gravity with the conjecture theorized by Mazur and Mottola, well known as the gravitational vacuum stars (gravastars). By taking into account the $f(\mathcal{G},T)$ stellar model, the structure of the gravastar with its exclusive division of three different regions, namely, (i) the core interior region, (ii) the junction region (shell), and (iii) the exterior region, has been investigated with reference to the existence of energy density, pressure, ultrarelativistic plasma, and repulsive forces. The different physical features, like the equation of state parameter, length of the shell, entropy, and energy-thickness relation of the gravastar shell model, have been discussed. Also, some other physically valid aspects have been presented with the connection to nonsingular and event-horizon-free gravastar solutions, which in contrast to a black hole solution, might be stable without containing any information paradox.

Stability analysis of lower dimensional gravastars in noncommutative geometry

The Ba\~{n}ados, Teitelboim and Zanelli \cite{BTZ1992}, black hole solution is revamped from the Einstein field equations in (2 + 1)-dimensional anti-de Sitter spacetime, in a context of noncommutative geometry \cite{Rahaman(2013)}. In this article, we explore the exact gravastar solutions in three-dimension anti-de Sitter space given in the same geometry. As a first step we derive BTZ solution assuming the source of energy density as point-like structures in favor of smeared objects, where the particle mass M, is diffused throughout a region of linear size $\sqrt{\alpha}$ and is described by a Gaussian function of finite width rather than a Dirac delta function. We matched our interior solution to an exterior BTZ spacetime at a junction interface situated outside the event horizon. Furthermore, stability analysis is carried out for the dynamic case for the specific case when $\chi < 0. 214$ under radial perturbations about static equilibrium solutions. To give theoretical support we also trying to explore their physical properties and characteristics.

Linearized stability analysis of gravastars in noncommutative geometry

In this work, we find exact gravastar solutions in the context of noncommutative geometry, and explore their physical properties and characteristics. The energy density of these geometries is a smeared and particle-like gravitational source, where the mass is diffused throughout a region of linear dimension $\sqrt{(\alpha)}$ due to the intrinsic uncertainty encoded in the coordinate commutator. These solutions are then matched to an exterior Schwarzschild spacetime. We further explore the dynamical stability of the transition layer of these gravastars, for the specific case of $\beta=M^2/\alpha<1.9$, where M is the black hole mass, to linearized spherically symmetric radial perturbations about static equilibrium solutions. It is found that large stability regions exist and, in particular, located sufficiently close to where the event horizon is expected to form.

Radial stability analysis of the continuous pressure gravastar

Radial stability of the continuous pressure gravastar is studied using the conventional Chandrasekhar method. The equation of state for the static gravastar solutions is derived and Einstein equations for small perturbations around the equilibrium are solved as an eigenvalue problem for radial pulsations. Within the model there exists a set of parameters leading to a stable fundamental mode, thus proving the radial stability of the continuous pressure gravastar. It is also shown that the central energy density possesses an extremum in ρc(R) curve which represents a splitting point between stable and unstable gravastar configurations. As such the ρc(R) curve for the gravastar mimics the famous M(R) curve for a polytrope. Together with the former axial stability calculations this work completes the stability problem of the continuous pressure gravastar.

Gravastar solutions with continuous pressures and equation of state

We study the gravitational vacuum star (gravastar) configuration as proposed by Cattoen et al (2005 Class. Quantum Grav. 22 4189) in a model where the interior de Sitter spacetime segment is continuously extended to the exterior Schwarzschild spacetime. The multilayered structure of Mazur and Mottola (2001 Preprint gr-qc/0109035, 2003 Proc. 6th Workshop on Quantum Field Theory Under the Influence of External Conditions (Oklahoma) (Princeton, NJ: Rinton), Preprint gr-qc/0405111 (2004 Proc. Natl Acad. Sci. 111 9545) is replaced by a continuous stress–energy tensor at the price of introducing anisotropy in the (fluid) model of the gravastar. Either with an ansatz for the equation of state connecting the radial pr and tangential pt pressure or with a calculated equation of state with non-homogeneous energy/fluid density, solutions are obtained which in all aspects satisfy the conditions expected for an anisotropic gravastar (Cattoen et al 2005 Class. Quantum Grav. 22 4189). Certain energy conditions have been shown to be obeyed and a polytropic equation of state has been derived. Stability of the solution with respect to possible axial perturbation is shown to hold.

Stable gravastars with generalized exteriors

New spherically symmetric gravastar solutions, stable to radial perturbations, are found by utilizing the construction of Visser and Wiltshire. The solutions possess an anti-de Sitter or de Sitter interior and a Schwarzschild–(anti)- de Sitter or Reissner–Nordström exterior. We find a wide range of parameters which allow stable gravastar solutions, and present the different qualitative behaviour of the equation of state for these parameters.