Gravitational Collapse of Radiating Dyon Solution and Cosmic Censorship Hypothesis

We investigate the possibility of cosmic censorship violation in the gravitational collapse of radiating dyon solution. It is shown that the final outcome of the collapse depends sensitively on the electric and magnetic charge parameters. The graphs of the outer apparent horizon, inner Cauchy horizon for different values of parameters are drawn. It is found that the electric and magnetic components push the apparent horizon towards the retarded time-coordinate axis, which in turn reduces the radius of the apparent horizon in Valdya spacetime. Also, we extend the earlier work of Chamorro and Virbhadra [Pramana, J. Phys. 45 （1995） 181].

Non-Spherical Gravitational Collapse of Strange Quark Matter

We study the non-spherical gravitational collapse of the strange quark null fluid. The interesting feature which emerges is that the non-spherical collapse of charged strange quark matter leads to a naked singularity whereas the gravitational collapse of neutral quark matter proceeds to form a black hole. We extend the earlier work of Harko and Cheng [Phys. Lett. A 266 （2000） 249] to the non-spherical case.

The Reissner-Nordström black hole surrounded by quintessence may not be destroyed

In the study of weak cosmic censorship conjectures(WCCC),some research finds that the Reissner-Nordström black hole might be destroyed by a test particle with particular mass and charge under some conditions,which means that the naked singularity of the black hole could be observed.This is not allowed in WCCC.We have never observed such naked singularities which should not exist in theory,so we need to find a proper way to protect the black hole from being destroyed by such particles.In this paper,we study a Reissner-Nordström black hole that is surrounded by quintessence(RN-Q)and find that the black hole would be stable and safe because of the effective potential barrier induced by the quintessence term.This result may also show in a sense that the quintessence might have more potential value.

Tidal force effects and bound orbits in null naked singularity spacetime

Naked singularities form during the gravitational collapse of inhomogeneous matter clouds.The final nature of the singularity depends on the initial conditions of the matter properties and types of matter profiles.These naked singularities can also be divided into two types:null-like and timelike singularities.The spacelike singularity of the Schwarzschild black hole can be distinguished from the null and timelike naked singularity spacetimes.In light of this,we investigate the precession of timelike bound orbits in the null naked singularity spacetime,as well as tidal force effects and geodesic deviation features.As a result,we find that the orbital precession of the timelike bound orbits in null naked singularity spacetime could be distinguished from the Schwarzschild precession case.The radial component of the tidal force has an intriguing profile,whereas the angular component has a profile that is comparable to that of a Schwarzschild black hole scenario.The geodesic deviation equation is then solved numerically,yielding results that resemble Schwarzschild black holes.These characteristic features can then be used to discern among these singularities.

The ring-shaped shadow of a rotating naked singularity with a complete photon sphere

We investigate the shadows of the Konoplya-Zhidenko naked singularity.In the spacetime of the Konoplya-Zhidenko naked singularity,not only an unstable retrograde light ring(LR)but also an unstable prograde LR exists,leading to the formation of a complete photon sphere(PS).Due to the absence of an event horizon,a dark discshaped shadow does not appear;instead,a ring-shaped shadow is observed.The ring-shaped shadow appears as an infinite number of relativistic Einstein rings in the image of the naked singularity.For some parameter values,only the unstable retrograde LR exists,resulting in an incomplete unstable PS and thus giving rise to an arc-shaped shadow for the Konoplya-Zhidenko naked singularity.The shadow of the Konoplya-Zhidenko naked singularity gradually shifts to the right as the rotation parameter a increases and gradually becomes smaller as the deformation parameter |η| increases.Moreover,stable LRs and stable photon spherical orbits can exist in the Konoplya-Zhidenko naked singularity spacetime,but they have no effect on the image of the naked singularity.This study demonstrates that a rotating naked singularity can exhibit not only an arc-shaped shadow but also a ring-shaped shadow.

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.

A NEW CLASS OF SOLUTIONS OF VACUUM EINSTEIN'S FIELD EQUATIONS WITH COSMOLOGICAL CONSTANT

In this paper, a new class of solutions of the vacuum Einstein＇s field equa- tions with cosmological constant is obtained. This class of solutions possesses the naked physical singularity. The norm of the Riemann curvature tensor of this class of solutions takes infinity at some points and the solutions do not have any event horizon around the singularity.