Key Open Problems in Black Hole Physics and Gravitation Theory Today (A personal perspective!) Pankaj S. Joshi

I. What is the Final Fate of a Massive Star? II. The Stability and Genericity of Gravitational Collapse Outcomes III. Quantum Aspects & Can One throw Matter into a Black Hole to Spin it Up? IV. Do Naked Singularities restore Predictability In the Universe? V. Future Perspectives Outline of the Talk

One of the Most Important Key Issues in Relativistic Astrophysics and Cosmology What Happens when a Massive Star dies? Chandrasekhar's work: Star collapse and Stable Configuration Limit Continual Collapse for Massive Stars What is the Final End state of such a Continual Collapse?

Dark Energy and Exploding Stars The Observations on Accelerating Universe are intimately related to the Measurements on Exploding Stars, far away in Cosmos. This is linked to a great mystery in Cosmology Today, which is the possible Presence of a Dark Energy in the Universe. In Supernovae, the core collapses in less than a Second, causing a Massive Explosion. A shock wave then blows off the outer layers of the star. The Supernova shines brighter than the Entire galaxy for a short time...

Kip Thorne's book

Recall the earlier Chandra comments in 1935!

Spacetime Singularities # General Relativity predicts that under reasonable physical conditions the gravitationally collapsing massive star must terminate into a Spacetime Singularity. # The densities, curvatures, and all physical quantities must go to infinity closer to the Singularity # Are such Singularities of Collapse visible to external observers in the Universe? THIS IS ONE OF THE MOST IMPORTANT ISSUES IN BLACK HOLE PHYSICS TODAY

Oppenheimer-Snyder-Datt Gravitational Collapse Scenario Use General Relativity/Homogeneous Density Spherical Dust cloud with No Rotation or Pressures Dynamical Collapse/Formation of Trapped Surfaces and Event Horizon/Collapse to a Spacetime Singularity/Formation of a Black Hole Region in the Spacetime Collapse settles eventually to a Final Schwarzschild Geometry

(Kip Thorne)

1960s: Resurgence of Interest A & A DEVELOPMENTS Discovery of Quasars, Radio Galaxies, and Of several High Energy Phenomena in the Universe No known physics explains such High Energy Observations!!

Result Very Many Developments in the Physics & Astrophysics of Black Holes J Wheeler/R Penrose/S Hawking/K Thorne... Investigations in Classical & Quantum Aspects Of Black Holes Interesting Thermodynamic Analogies Astrophysical Applications

COSMIC CENSORSHIP HYPOTHESIS # Will a generic star go the Oppenheimer-Snyder- Datt way only, and make a Black Hole only? Because, real stars are Inhomogeneous, have Internal pressure forces... # This is an Unanswered Q. Therefore the Hypothesis by Penrose: Any Star Collapse will make a black hole only, hiding the Singularity, the ultra-dense regions, behind an Event Horizon THIS HAS BEEN ONE OF THE MOST FUNDAMENTAL QUESTION IN GRAVITATION THEORY TODAY

The Information given by Singularity Theorems on the Existence Aspect is rather general What we need to know specifically is: What happens When a Massive Star collapses under its own gravity. In particular, we need to know on the Visibility or otherwise of the Super-Ultra-dense regions, or the Singularity that forms in such a Gravitational Collapse TOWARDS THIS PURPOSE, EXPLICIT MODELS NEED TO BE WORKED OUT, IN THE ABSENCE OF ANY GENERAL PROOF

Way Out: Back to Basics STUDY GRAVITATIONAL COLLAPSE AS OPPENHEIMER AND SNYDER DID! Over past decade and a half, Numerous Collapse studies carried out... CONCLUSION: Black Holes and Naked Singularity Final States develop as The Collapse Outcome

Stability and Genericity of Gravitational Collapse Final Outcomes

Gravitational Collapse in a Comoving Coordinate System

Introducing a scaling function R=rv, and F(r,v) = r^3 M(r,v), it is possible to integrate the Einstein equations to get the Equation of Motion of the Collapsing Fluid, the key point is to understand the structure of trapped surfaces. While doing so, one must ensure that the Collapse develops from a Regular initial Data. The Trapped Surfaces and Spacetime Singularity Develop then as the collapse evolves, and the main task then is to examine the Nature of the Singularity, namely whether it is Covered within Horizon, or Visible to External Observers. (PSJ & DM, Phys.Rev.D, 2011)

The function t(v,r) gives time for a shell at r to reach a coordinate value v, the Spacetime singularity being at v=0 OSD Model is then given by,

For the OSD model, {g_00} is necessarily Unity. We now allow it to be non-zero, this amounts to introducing small non-zero tangential pressures (with M=M_0 only).

The Singularity Curve and Apparent Horizon are given by

Quantum Aspects of the Black Hole Story & Can One throw Matter into a Black Hole to Spin it up?

Can we Spin up a Black Hole? Recent Big Debate on the Issue A black hole always Accretes Matter and Particles, Charged as well as Spinning. How they affect the Black Hole? Recent Data on possible Black Hole candidates measure their Masses and Spin. In many cases, the Spin is Dangerously close to Unity!! Thus a Kerr Naked Singularity could be a possible configuration for these objects...

Predictability, Cauchy Horizons and all that... Particle Collisions near the Cauchy Horizon (MP & PSJ, 2010) The Center of mass energy shown to be arbitrarily high near CH... IS PLANCK SCALE PHYSICS VISIBLE at the Cauchy Horizon? Does this phenomena convert Cauchy Horizon into a Singularity, thus recovering the Predictability of the Spacetime?

Physically Realistic Collapse implies Naked Singularity always? Examples: Dust Collapse, Radiation Collapse... Why Do Naked Singularities Develop in Gravitational Collapse? How Trapped Surfaces are Naturally Delayed due to Inhomogeneities

Future Perspectives and Possible Scenarios Where Do We go from here?

A `Hypernova', or a Final Complete disintegration of the Massive Star in late collapse? Advantages: (i) You need not then worry to Prove censorship hypothesis (ii) Infinite density singularity avoided (iii) Observational tests possible, look at and Examine carefully the Burstlike Phenomena In the Universe (iv) Black hole paradoxes avoided

Role of Classical & Quantum Physics * General Relativity: Evolve the Collapse to Very Late Stages--keeping the Causal Communication with the Outside Universe open... *Quantum Physics: Study the Quantum Gravity Effects in such Ultra-Strong-Gravity Regions SOME BIG GURUS HAVE ENDORSED AND EVEN ADVOCATED THIS AS A GOOD POSSIBILITY!!

******* GR implies Existence of Strong Gravity Regions, where Both Quantum Gravity and General Relativity come into their Own. ************** As R Wald pointed out: `If censorship fails, then in a literal sense, we would come face-to-face with the laws of Quantum Gravity whenever gravitational collapse to a naked singularity occurs in distant regions of our Universe..' AN EXCITING POSSIBILITY!!