Singularity-Free Superstar as an Alternative to Black Hole and Gravastar

Singularity-free superstar is proposed as a model for the collapse of large stars and for GRBs, and as an alternative to black hole and gravastar. Similar to a superconductor, a superstar contains extreme force fields that have non-zero momentum and non-zero wavelength to prevent the inactivation of force field at absolute zero and singularity (infinite interacting energy) at infinite density, respectively, based on the uncertainty principle. Emerging only at an extremely low temperature above absolute zero or an extremely high density below infinite density, extreme force fields are shortrange, and located in between a particle and its ordinary force fields (electromagnetic, weak, strong, and gravitational forces) to prevent the inactivation of force fields at absolute zero and singularity (infinite interacting energy) at infinite density in ordinary force fields. Extreme force fields are manifested as the bonds among electrons in a superconductor and among atoms in a Bose-Einstein condensate. When the stellar core of a large star reaches the critical extreme density during the stellar collapse, the stellar core is transformed into the super matter core with extreme force fields and ordinary force fields without singularity. A pre-superstar contains the super matter core, the ordinary matter region, and the thin phase boundary between the super matter core and the ordinary matter region. The stellar collapse increases the super matter core by converting the in falling ordinary energy and matter from the ordinary matter region into the super matter, and decreases the ordinary matter region. Eventually, the stellar breakup occurs to detach the ordinary matter region and the phase boundary from the super matter core, resulting in GRB to account for the observed high amount of gamma rays and the observed complex light curves in GRBs. Unlike black holes and gravastars that lose information, singularity-free superstars that keep all information exist.

Charged Gravastar in a Dark Energy Universe

Here we constructed a charged gravastar model formed by an interior de Sitter spacetime, a charged dynamical infinitely thin shell with an equation of state and an exterior de Sitter-Reissner-Nordstrom spacetime. We find that the presence of the charge is crucial to the stability of these structures. It can as much favor the stability of a bounded excursion gravastar, and still converting it in a stable gravastar, as make disappear a stable gravastar, depending on the range of the charge considered. There is also formation of black holes and, above certain values, the presence of the charge allows the formation of naked singularity. This is an important example in which a naked singularity emerges as a consequence of unstabilities of a gravastar model, which reinforces that gravastar is not an alternative model to black hole.

Gravastar in the framework of symmetric teleparallel gravity

We present a novel gravastar model based on the Mazur-Mottola(2004)method with an isotropic matter distribution in f(Q)gravity.The gravastar,which is a hypothesized substitute for a black hole,is built using the Mazur-Mottola mechanism.This approach allows us to define the gravastar as having three stages.The first one is an inner region with negative pressure;the next region is a thin shell that is made up of ultrarelativistic stiff fluid,and we studied the proper length,energy,entropy,and surface energy density for this region.Additionally,we demonstrated the possible stability of our suggested thin shell gravastar model through the graphical study of the surface redshift.The exterior Schwarzschild geometry describes the outer region of the gravastar.In the context of f(Q)gravity,we discovered analytical solutions for the interior of gravastars that are free of any type of singularity and the event horizon.

Thin-shell gravastar model in f(Q,T)gravity

In the last few decades,gravastars have been proposed as an alternative to black holes.The stability of a gravastar has been examined in many modified theories of gravity along with Einstein's GR.The f(Q,T)gravity,a successfully modified theory of gravity for describing the current accelerated expansion of the universe,has been used in this study to examine gravastar in different aspects.According to Mazur and Mottola[Proc.Natl.Acad.Sci.101,9545(2004);Gravitational condensate stars:An alternative to black holes,I12-011,(2002)],a gravastar has three regions with three different equations of state.In this study,we examined the interior of a gravastar by consid-ering p=-ρ EoS to describe the dark sector for the interior region.The next region is a thin shell of ultrarelativistic stiff fluid,in which we investigated several physical properties,including proper length,energy,entropy,and surface energy density.Additionally,we examined the surface redshift and speed of sound to check the potential stability of our proposed thin-shell gravastar model.Furthermore,we used the entropy maximization technique to verify the stability of the gravastar model.A gravastar's outer region is a complete vacuum described by exterior Schwarzschild geometry.Finally,we presented a stable gravastar model,which is singularity-free and devoid of any incom-pleteness in classical black hole theory.

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-Nordstrom 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 We and the electric potential field Ф obey a particular relation in a simple form a（b-εФ）^2＋b1, where a, b and bl are arbitrary constants. The simplest ansatz of Guilfoyle＇s solution is implemented by the following assumption： that the total energy density 8πρm＋Q^2/r^4 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.