MG14 - Talk detail

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Cosmic censorship is the one of the outstanding unsolved problem in general relativity. In this paper we study gravitational lensing in the strong field limit from the perspective of cosmic censorship to investigate whether or not naked singularities if at all they exist in nature can be distinguished from black holes. Specifically we consider both black hole and naked singularity spacetimes admitting photon spheres as well as naked singularity spacetimes without a photon sphere that were recently shown to be the end state of gravitational collapse. In the presence of the photon sphere gravitational lensing signature of naked singularities is identical to that of Schwarzschild black hole with infinitely many relativistic images and Einstein rings, all of them located beyond a certain critical angle from optic axis and the inner relativistic images all clumped together. However, in the absence of the photon sphere, which is the case for a wide range of parameter values in the naked singular spacetimes, we show that we get finitely many relativistic images and Einstein rings spaced reasonably apart from one another, some of which can be formed inside the critical angle for the corresponding Schwarzschild black hole. We also study the time delay between successive relativistic images. We show that the differential time delay between the relativistic images for naked singularities without a photon sphere progressively decreases as opposed to that for black holes and naked singularities with a photon sphere, where it is known to be roughly constant. This study suggests that the observation of relativistic images, rings and time delay would allow us to unravel the existence of the naked singularity in the absence of the photon sphere.

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In this talk we describe the process of ultra-high energy collisions between particles around near-extremal Kerr black holes and over-spinning Kerr geometry and make a critical comparison between the two. Fine-tuning of the geodesic parameters of the colliding particles is necessary to achieve large centre of mass energy in case of the black hole, while no such fine-tuning is necessary in case of the over-spinning Kerr geometry. We show that the time required for the trans-Planckian collisions of proton-like particles to occur around solar mass object is several orders of magnitude larger than the age of the universe in case of the Kerr black hole, while it is of the order of million years around over-spinning Kerr geometry. The self-gravity of the colliding particles sets an upper bound on the centre of mass energy of the colliding particles. We argue that the upper bound for the collision of proton-like particles around solar-mass object is less than Planck energy in case of the near-extremal black holes and it is much larger than Planck energy for the geometry above the extremality. We show that while most of the particles produced in the ultra-high collisions around black hole enter the event horizon, severely limiting the observability of this process, around half of the particle produced escape to infinity in case of the over-spinning geometry.

Pdf file

Session

Accepted

Order

Time

Talk

Title

Abstract

In this talk we describe the process of ultra-high energy collisions between particles around near-extremal Kerr black holes and over-spinning Kerr geometry and make a critical comparison between the two. Fine-tuning of the geodesic parameters of the colliding particles is necessary to achieve large centre of mass energy in case of the black hole, while no such fine-tuning is necessary in case of the over-spinning Kerr geometry. We show that the time required for the trans-Planckian collisions of proton-like particles to occur around solar mass object is several orders of magnitude larger than the age of the universe in case of the Kerr black hole, while it is of the order of million years around over-spinning Kerr geometry. The self-gravity of the colliding particles sets an upper bound on the centre of mass energy of the colliding particles. We argue that the upper bound for the collision of proton-like particles around solar-mass object is less than Planck energy in case of the near-extremal black holes and it is much larger than Planck energy for the geometry above the extremality. We show that while most of the particles produced in the ultra-high collisions around black hole enter the event horizon, severely limiting the observability of this process, around half of the particle produced escape to infinity in case of the over-spinning geometry.

Pdf file

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