GRAVITATIONAL COLLAPSE, BLACK HOLES AND NAKED SINGULARITIES *** Life and Death of Massive Collapsing Stars Pankaj S Joshi

Plan of the talk * Collapsing Stars—An Introduction * Recent Developments on Gravitational Collapse Studies * Can We Distinguish Black Holes and Naked Singularities through Observational Signatures? * Black Hole Paradoxes, Collapse & Quantum Gravity Plan of the talk

One of the Most Important Key Issues in 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?

Gravitational Collapse Massive Stars burn Much Faster & They are far More Luminous e.g a 10 Solar Masses Star does not endure More than Few Million Years Therefore the Question of Final Fate of such Massive Stars is of central importance in Astronomy and Astrophysics Today…

USE GENERAL RELATIVITY Oppenheimer-Snyder-Datt Gravitational Collapse Scenario Homogeneous Density Spherical Dust cloud; 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

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!! ASSUMING THAT ALL STAR COLLAPSES BEHAVE THE SAME WAY AS THE SIMPLE OSD MODEL, THE BLACK HOLE PHYSICS DEVELOPED

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

RECENT DEVELOPMENTS IN GENERAL RELATIVISTIC GRAVITATIONAL COLLAPSE

WHAT IS THE FINAL FATE OF A MASSIVE COLLAPSING STAR? We made this Picture and Scenario, based on the Oppenheimer-Snyder Collapse Calculation However, the Most Fundamental Question in Gravitation Theory remained very much Unanswered WHAT IS THE FINAL FATE OF A MASSIVE COLLAPSING STAR?

COSMIC CENSORSHIP HYPOTHESIS Will a generic star go the Oppenheimer-Snyder-Datt way only, making a Black Hole always? Because, real stars are Inhomogeneous, have Internal pressure forces... An Unanswered Q despite Huge Efforts… The Penrose Conjecture: Any Star Collapse makes black hole only, hiding the Singularity, the ultra-dense regions, behind an Event Horizon of Gravity

Collapse and Singularities What We know General Relativity implies that under reasonable physical conditions collapsing massive star must terminate into a Spacetime Singularity, where densities, curvatures, and all physical quantities diverge What We Do Not Know Whether Event Horizons also Always Form to Cover and Hide the Singularities ARE SINGULARITIES OF COLLAPSE VISIBLE TO FARAWAY OBSERVERS IN UNIVERSE? THIS IS THE MOST IMPORTANT OPEN ISSUE IN BLACK HOLE PHYSICS TODAY

The Main Point is, Back to Basics is the Only Way Out !! STUDY GRAVITATIONAL COLLAPSE AS OPPENHEIMER AND SNYDER DID… This is a Complex Task in General Relativity But, Over past decade and a half, Very many Collapse studies have been carried out... CONCLUSION: Black Holes and Naked Singularity Final States develop as The Collapse Outcome

NO! Take for example, the same model but with GRAVITATIONAL COLLAPSE => BLACK HOLE ONLY? NO! Take for example, the same model but with with density peaked at centre, and decreasing away from centre. Here, Then we have, The apparent horizon is then given by, The effect of inhomogeneity is immediately seen…

* Collapse with different Equations of State VISIBLE SINGULARITIES-STRUCTURE & IMPLICATIONS Work has taken place on many aspects of Collapse and Nature of Singularities, such as: * Collapse with different Equations of State * Spherical Collapse analyzed extensively from the perspective of Black Hole and Naked Singularity Formation * Few Non-spherical Collapse Models also available * Self-similar and Scalar Field Collapse-Critical phenomena * Numerical Models for Star Collapse developing well… * Quantum Gravity Issues * Genericity and Stability aspects * Gravitational Waves from Collapsing Matter Clouds… * Observational Implications of Naked Singularities THE POINT IS, IF NAKED SINGULARITIES DEVELOP AS COLLAPSE FINAL STATES, THEN SEVERAL ISSUES ARISE ON THEIR NATURE AND STRUCTURE, AND THEIR POSSIBLE THEORETICAL AS WELL AS OBSERVATIONAL IMPLICATIONS…

Gravitational Collapse

The Singularity Curve and Apparent Horizon are given by

BLACK HOLES AND NAKED SINGULARITIES OBSERVATIONALLY DISTINGUISHING BLACK HOLES AND NAKED SINGULARITIES OBSERVATIONALLY **** ASTROPHYSICAL IMPLICATIONS?

At Present, Researchers consider Three Main Ways * Accretion Disk properties around Black Holes and Naked singularities * High Energy Particle Collisions and their Physical Consequences * Gravitational Lensing around these objects

Accretion disk Properties * A study of accretion disk properties around black holes and naked singularities initiated (Malafarina et al, Schuik, Harko, Bambi et al) *Several interesting physical differences have emerged in the Accretion disks, which turn out to be rather different from each other. * This could allow distinguishing these objects, and may have other Astrophysical Implications too.

Joshi, Malafarina and Narayan (2013)

High Energy Particle Collisions near Black holes and Singularities * Silk et al pioneered a study of particles colliding with very high energies in black hole geometries * The Basic Motivation: Terrestrial Labs limited; Can We have Astrophysical Particle Accelerators? * While Black holes require various Fine Tunings, Naked Singularity geometries help avoid these. * Harada, Kimura, Nakao, Patil, PSJ et al examined many spacetimes such as Kerr, Reissner-Nordstrom and others...

Patil et al (2011)

Gravitational Lensing Studies * Can we use Graviational lensing to Distinguish the Black Holes and Naked singularities? * Studies in this direction (Boza, Ellis &Virbhadra, Sahu, Narasimha, Patil, PSJ et al) show the two objects differ critically from each other for their Lensing Signatures in terms of Formation of Images, Einstein Rings etc. * The key difference is due to the Structure of Photon Spheres in each of these cases

Black Hole Paradoxes * The Causality or Teleological Paradox with the Event Horizon * The Information Paradox—Firewalls & so on * Central singularity and Stability Issues See e.g. A. Almheiri et al (2013); S. Hawking (2014); Ramesh Narayan and PSJ (2014)

-QUANTUM GRAVITY LAB?- If Naked Singularities, which are Hypothetical Astrophysical Objects, did form in Collapse, Can we then Observe and See the Quantum Gravity Effects taking place in their vicinity? In other words, Does Nature produce a Quantum Gravity Lab whenever a Massive Star Collapses??

AN EXCITING POSSIBILITY!! ******* The Key Point is: GR implies Existence of Strong Gravity Regions, where Both Quantum Gravity and General Relativity come into their Own & These could be Visible to External observers in Universe ************** As 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!!

Conclusion & Current Scenario * The theory of Black Hole and Naked singularity formation as Collapse Final States in GR appears to be on Reasonable and Firm Footing now... * These may be Observationally Distinguishable. * If Naked Singularity formed in nature, What Will be the Astrophysical and Observational Consequences and Implications? This is a topic of current Interest…