Tuesday, Apr 4th
**Parampreet Singh, LSU**
**Title: Transition times through the black hole bounce **
PDF of the talk (2M)
Audio+Slides [.mp4 18MB]

by Gaurav Khanna, University of Massachusetts Dartmouth

Loop quantum cosmology (LQC) is an application of loop quantum gravity
theory in the context of spacetimes with a high degree of symmetry (e.g. homogeneity, isotropy). One of the main successes of LQC is
the resolution of "singularities" that generically appear in the classical
theory. An example of this is the "big bang" singularity that causes a
complete breakdown of general relativity (GR) in the very early universe.
Models studied within the framework of LQC replace this "big bang" with a
"big bounce" and do not suffer a singular breakdown like in the classical
theory.

It is, therefore, natural to consider applying similar techniques to
study black holes; after all, these solutions of GR are also plagued with
a central singularity. In addition, it is plausible that a LQC model
may shed some light on long-standing issues in black hole physics, i.e.,
information loss, Hawking evaporation, firewalls, etc.

Now, if one restricts the model only to the Schwarzschild black hole
interior region, the spacetime can actually be considered as a homogeneous,
anisotropic cosmology (the Kantowski-Sachs spacetime). This allows
techniques of LQC to be readily applied to the black hole case. In fact,
a good deal of study has been performed in this direction by Ashtekar,
Bojowald, Modesto and many others for over a decade. While these models
are able to resolve the central black hole singularity and include
important improvements over previous versions, they still have a number of
issues.

Recently, Singh and Corichi (2016), proposed a new LQC model for the black
hole interior that attempts to address these issues. In this talk, Singh
describes some of the resulting phenomenology that emerges from that improved
model.

The main emphasis of this talk is on the following questions:

(1) Is the "bounce" in the context of a black hole LQC model, i.e.,
transition from a black hole to a white hole, symmetric? Isotropic and
homogeneous models in LQC have generally exhibited symmetric bounces. But,
that is not expected to hold in the context of more general models.

(2) Does quantum gravity play a role only once during the bounce process?

(3) What quantitative statements can be made about the time-scales of this
process; and what are the full implications of those details?

(4) Do all black holes, independent of size, exhibit very similar characteristics?

Based on detailed numerical calculations that Singh reviews in his
presentation, he uncovers the following features from this model:

(1) The bounce is indeed not symmetric; for example, the sizes of the
parent black hole and the offspring white hole are widely different. Other
details on this asymmetry appear below.

(2) Two distinct quantum regimes appear in this model, with very different
associated time-scales.

(3) In terms of the proper time of an observer, the time spent in the
quantum white hole geometry is much larger than in the quantum black hole.
And, in particular, the time for the observer to reach the white hole horizon
is exceedingly large. This also implies that the formation of the white
hole interior geometry happens a lot quicker than the formation of its horizon.

(4) The relation of the bounce time with the black hole mass, does depend
on whether the black hole is large or small.

On the potential implications of such details on some of the important open
questions in black hole physics, Singh speculates:

(1) For large black holes, the time to develop a white hole (horizon)
is much larger than the Hawking evaporation time. This may suggest that
for an external observer, a black hole would disappear long before the
white hole appears!

(2) For small black holes, the time to form a white hole is smaller than
Hawking time, i.e., small black holes explode before they can evaporate!

These could have some interesting implications for the various proposed
black hole evaporation paradigms. Given the concreteness of the results
Singh presents, they are also likely to be relevant to the many previous
phenomenological studies on black hole to white hole transitions including
Planck stars.

The two main limitations of Singh's results are: (1) the current model
ignores the black hole exterior entirely; and (2) the conclusions rely on
effective dynamics, and not the full quantum evolution. These may be
addressed in future work.