Thin-shell Gravastars Research Articles

Anisotropic gravastar as horizonless regular black hole spacetime and its images illuminated by thin accretion disk

A connection between regular black holes and horizonless ultracompact objects was proposed in Carballo-Rubio et al. (JHEP 08:046, 2023, arXiv:2211.05817 [gr-qc]). In this paper, we construct a model of a horizonless compact object, specifically an anisotropic gravastar with continuous pressure, that corresponds to regular black hole spacetime in the appropriate limit. The construction begins by modeling an equation of state that satisfies the anisotropic gravastar conditions and transitions to the de Sitter (p=-ϵ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$p=-\\epsilon $$\\end{document}) upon horizon formation. The spacetime structure is similar to the Quantum Horizonless Compact Object (QHCO) described in Chen and Yokokura (Phys Rev D 109:104058, 2024, arXiv:2403.09388 [gr-qc]). Within this model, we also generate images of the corresponding objects surrounded by a thin accretion disk. The resulting images reveal that assuming that the emitting matter exists only outside the object, the inner light ring structure closely resembles that of the horizonless configuration of a regular black hole and the QHCO, yet it exhibits a distinct light ring structure compared to the thin-shell gravastar model. However, the opposite occurs when emitting matter is taken into account inside the object.

Thin-shell gravastars in the effect of graviton mass: A study on linearized stability and dynamics

In the present study we exhibit gravastar models which are effectively stable in their configuration as well as we thoroughly investigate dynamics of the physical systems. For this we have adopted the de Rham-Gabadadze-Tolly massive gravity and analyze Einstein’s general relativity (GR) in the context of the consistent nonlinear interaction terms interpreted as a graviton mass. The studies follow the structured routes of the interior geometry, exterior geometry and thin-shell gravastar model. A linearized stability of thin-shell gravastars has been established via three special models, viz. linear model, polytropic model and van der Waals model. We have also critically analyzed dynamics of thin-shell gravastars with special emphasis on massless scalar field and massive scalar field. Altogether the results are very interesting and physically viable as far as stability issue of gravastars are concerned.

Thin-shell gravastar in a noncommutative BTZ geometry

In this paper, we build a thin-shell gravastar model within a noncommutative BTZ geometry. For this, we consider a noncommutative BTZ metric in the inner region and a geometry associated with a BTZ solution in the outer region, joined by the generalized join technique. After investigating the inner spacetime, surface and outer spacetime, we observe that there is a surface energy density and surface pressure, such as to make gravastar stable. This effect persists even when the cosmological constant is zero. Besides, we found a bound for the noncommutativity parameter. In addition, we examine the thermodynamics of the noncommutative BTZ black hole in Schwarzschild-type form in three-dimensional spacetime. We also check the stability condition by calculating the specific heat capacity, as well as for the formation of black hole remnants.

Stability and Dynamics of Regular Thin-Shell Gravastars

This paper explores the stable and dynamical configuration of thin-shell gravastars developed from the matching of exterior (Hayward and Hayward anti-de Sitter black holes) and interior (de Sitter) spacetimes. These spacetimes are connected at a thin-shell by considering Visser cut and paste technique. The matter surface energy density and pressure are formulated from the Lanczos equations. We consider the shaded regions to discuss the stability of gravastars by using radial perturbation at the equilibrium shell radius. We then observe thin-shell dynamics with massless as well as massive scalar field through equations of motion and Klein-Gordon equations. The respective scalar thin-shell represents collapse and expansion of the developed structure with the shell radius, velocity and effective potential. We conclude that stable regions of gravastar shell decrease and dynamical configuration (collapse and expansion) increase with cosmological constant.

Stability of charged thin-shell gravastars with quintessence

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.

Stability of gravastars with exterior regular black holes

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.

Locally isotropic gravastars with cylindrical spacetime

This paper is aimed to set up a thin-shell gravastar model and address its physically accepted features in the background of noncommutative geometry. For this purpose, we have considered the cylindrically symmetric interior metric matched with suitable noncommutative exterior geometry using Israel boundary conditions. The stability of this thin-shell as well as thermodynamical stability is then explored under linear perturbations around the throat. We have found the stable regions near the horizon with some specific values of the involved parameters.

Charged thin-shell gravastars in noncommutative geometry

In this paper we construct a charged thin-shell gravastar model within the context of noncommutative geometry. To do so, we choose the interior of the nonsingular de Sitter spacetime with an exterior charged noncommutative solution by cut-and-paste technique and apply the generalized junction conditions. We then investigate the stability of a charged thin-shell gravastar under linear perturbations around the static equilibrium solutions as well as the thermodynamical stability of the charged gravastar. We find the stability regions, by choosing appropriate parameter values, located sufficiently close to the event horizon.

Tidal deformability and I-Love-Q relations for gravastars with polytropic thin shells

The moment of inertia, the spin-induced quadrupole moment, and the tidal Love number of neutron-star and quark-star models are related through some relations which depend only mildly on the stellar equation of state. These "I-Love-Q" relations have important implications for astrophysics and gravitational-wave astronomy. An interesting problem is whether similar relations hold for other compact objects and how they approach the black-hole limit. To answer these questions, here we investigate the deformation properties of a large class of thin-shell gravastars, which are exotic compact objects that do not possess an event horizon nor a spacetime singularity. Working in a small-spin and small-tidal field expansion, we calculate the moment of inertia, the quadrupole moment, and the (quadrupolar electric) tidal Love number of gravastars with a polytropic thin shell. The I-Love-Q relations of a thin-shell gravastar are drastically different from those of an ordinary neutron star. The Love number and quadrupole moment are negative for less compact models and the I-Love-Q relations continuously approach the black-hole limit. We consider a variety of polytropic equations of state for the matter shell, and find no universality in the I-Love-Q relations. However, we cannot deny the possibility that, similarly to the neutron-star case, an approximate universality might emerge for a limited class of equations of state. Finally, we discuss how a measurement of the tidal deformability from the gravitational-wave detection of a compact-binary inspiral can be used to constrain exotic compact objects like gravastars.

I-Love-Q relations for gravastars and the approach to the black-hole limit

The multipole moments and the tidal Love numbers of neutron stars and quark stars satisfy certain relations which are almost insensitive to the star's internal structure. A natural question is whether the same relations hold for different compact objects and how they possibly approach the black-hole limit. Here we consider "gravastars", which are hypothetical compact objects sustained by their internal vacuum energy. Such solutions have been proposed as exotic alternatives to the black-hole paradigm because they can be as compact as black holes and exist in any mass range. By constructing slowly-rotating, thin-shell gravastars to quadratic order in the spin, we compute the moment of inertia $I$, the mass quadrupole moment $Q$, and the tidal Love number $\lambda$ in exact form. The $I$-$\lambda$-$Q$ relations of a gravastar are dramatically different from those of an ordinary compact star, but the black-hole limit is continuous, i.e. these quantities approach their Kerr counterparts when the compactness is maximum. Therefore, such relations can be used to discern a gravastar from an ordinary compact star, but not to break the degeneracy with the black-hole case. Based on these results, we conjecture that the full multipolar structure and the tidal deformability of a spinning, ultracompact gravastar are identical to those of a Kerr black hole. The approach to the black-hole limit is nonpolynomial, thus differing from the critical behavior recently found for strongly anisotropic neutron stars.

Applications of black hole perturbation theory

Black holes (BHs) have many faces. Within the gauge/string duality they provide useful insights on strongly coupled quantum field theories and on quantum gravity. Furthermore, probing the strong curvature regime of any gravity theory, BHs carry the imprint of possible strong curvature corrections to General Relativity. Finally, beside their unique theoretical properties, compelling experimental evidence suggests that astrophysical BHs are very common objects in the universe. Here we discuss some applications of BH perturbation theory in various contexts. As applications in theoretical physics, we study perturbations of dilatonic BHs in Einstein-Maxwell theory and the holographic properties of the dual field theory via the Anti de Sitter/Condensed Matter duality. As applications in astrophysics, we discuss how the characteristic oscillations of BHs in string-inspired theories of gravity can provide observable signatures of deviations from Einstein’s theory. We study two well-motivated effective theories: Dynamical Chern-Simons gravity and Einstein-Dilatonic-Gauss-Bonnet gravity. We conclude by discussing the BH paradigm. Motivated by the lack of a definitive answer on the existence of astrophysical BHs, we study some viable alternatives, generically called “BH mimickers”. We focus on two representative cases: static thin-shell gravastars and superspinars, discussing their stability, gravitational-wave signature and viability as astrophysical objects.

Generic thin-shell gravastars

We construct generic spherically symmetric thin-shell gravastars by using the cut-and-paste procedure. We take considerable effort to make the analysis as general and unified as practicable; investigating both the internal physics of the transition layer and its interaction with “external forces” arising due to interactions between the transition layer and the bulk spacetime. Furthermore, we discuss both the dynamic and static situations. In particular, we consider “bounded excursion” dynamical configurations, and probe the stability of static configurations. For gravastars there is always a particularly compelling configuration in which the surface energy density is zero, while surface tension is nonzero.

Gravitational wave signatures of the absence of an event horizon. II. Extreme mass ratio inspirals in the spacetime of a thin-shell gravastar

We study gravitational-wave emission from the quasi-circular, extreme mass ratio inspiral of compact objects of mass m0 into massive objects of mass M>>m0 whose external metric is identical to the Schwarzschild metric, except for the absence of an event horizon. To be specific we consider one of the simplest realizations of such an object: a nonrotating thin-shell gravastar. The power radiated in gravitational waves during the inspiral shows distinctive peaks corresponding to the excitation of the polar oscillation modes of the gravastar. For ultra-compact gravastars the frequency of these peaks depends mildly on the gravastar compactness. For masses M~10^6Msun the peaks typically lie within the optimal sensitivity bandwidth of LISA, potentially providing a unique signature of the horizonless nature of the central object. For relatively modest values of the gravastar compactness the radiated power has even more peculiar features, carrying the signature of the microscopic properties of the physical surface replacing the event horizon.

Gravitational-wave signature of a thin-shell gravastar

We discuss how the emission of gravitational waves by ultra-compact objects can be qualitatively different depending on the presence or absence of an event horizon. Our case study are nonrotating thin-shell gravastars. The model has an infinitely thin shell with finite tension separating a de Sitter interior and a Schwarzschild exterior. The shell can be located arbitrarily close to the would-be event horizon, so a gravastar might seem indistinguishable from a black hole when tests are only performed on its external metric. We discuss gravitational perturbations of thin-shell gravastars and show that the spectrum of axial and polar quasinormal modes is completely different from that of a Schwarzschild black hole, even in the limit when the surface redshift becomes infinite. Furthermore, we study gravitational wave emission from the quasi-circular extreme mass ratio inspiral of compact objects of mass m0 into massive thin-shell gravastars of mass M ≫ m0. The power radiated in gravitational waves during the inspiral shows distinctive peaks corresponding to the excitation of the polar oscillation modes of the gravastar. The frequency of these peaks usually depends on the equation of state, but for ultra-compact gravastars it typically lies within the optimal sensitivity bandwidth of LISA, providing a very specific signature of the horizonless nature of the central object.

Gravitational wave signatures of the absence of an event horizon: Nonradial oscillations of a thin-shell gravastar

Gravitational waves from compact objects provide information about their structure, probing deep into strong-gravity regions. Here we illustrate how the presence or absence of an event horizon can produce qualitative differences in the gravitational waves emitted by ultra-compact objects. In order to set up a straw-man ultra-compact object with no event horizon, but which is otherwise almost identical to a black hole, we consider a nonrotating thin-shell model inspired by Mazur and Mottola's gravastar, which has a Schwarzschild exterior, a de Sitter interior and an infinitely thin shell with finite tension separating the two regions. As viewed from the external space-time, the shell can be located arbitrarily close to the Schwarzschild radius, so a gravastar might seem indistinguishable from a black hole when tests are only performed on its external metric. We study the linearized dynamics of the system, and in particular the junction conditions connecting internal and external gravitational perturbations. As a first application of the formalism we compute polar and axial oscillation modes of a thin-shell gravastar. We show that the quasinormal mode spectrum is completely different from that of a black hole, even in the limit when the surface redshift becomes infinite. Polar QNMs depend on the equation of state of matter on the shell and can be used to distinguish between different gravastar models. Our calculations suggest that low-compactness gravastars could be unstable when the sound speed on the shell vs/c>0.92.