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Key Open Problems in Black Hole
Physics and Gravitation Theory Today
(A personal perspective!)
Pankaj S. Joshi
Outline of the Talk
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
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-SnyderDatt 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!!