Singularities and Quantum Gravity

[marcus]

This just out:

http://arxiv.org/abs/gr-qc/0702144
Singularities and Quantum Gravity
Martin Bojowald
41 pages, lecture course at the XIIth Brazilian School on Cosmology and Gravitation, September 2006
IGPG-07/2-4, NSF-KITP-07-19

"Although there is general agreement that a removal of classical gravitational singularities is not only a crucial conceptual test of any approach to quantum gravity but also a prerequisite for any fundamental theory, the precise criteria for non-singular behavior are often unclear or controversial. Often, only special types of singularities such as the curvature singularities found in isotropic cosmological models are discussed and it is far from clear what this implies for the very general singularities that arise according to the singularity theorems of general relativity. In these lectures we present an overview of the current status of singularities in classical and quantum gravity, starting with a review and interpretation of the classical singularity theorems. This suggests possible routes for quantum gravity to evade the devastating conclusion of the theorems by different means, including modified dynamics or modified geometrical structures underlying quantum gravity. The latter is most clearly present in canonical quantizations which are discussed in more detail. Finally, the results are used to propose a general scheme of singularity removal, quantum hyperbolicity, to show cases where it is realized and to derive intuitive semiclassical pictures of cosmological bounces."

 

[marcus]

Bojowald evidently now has appointments both at Ashtekar's institute at Penn State (the main place for non-string, or background-independent, QG in the United States) and also at David Gross' KITP at Santa Barbara (a center for string researchers).

It looked as if things were shaping up this way judging from the atmosphere and talks at the 3-week KITP workshop in January, that Bojowald helped organize, called The Quantum Nature of Spacetime Singularities. Videos from that workshop are online and we've discussed some of them here at PF.

 

[jal]

Hi!
Another step forward.
Martin Bojowald is encouraging people to develop different simple models in the hope of discovering some partial answers that could be applied to a final model.

As we saw, symmetric models allow explicit investigations of many properties expected for singularities. It is important to keep a wide view of all types of characteristic models since any given model by itself may be too special as one can see it in isotropy. Still, there may always be properties not seen so far which may become relevant.
Generalizations to less symmetric models have thus presented many non-trivial tests of the whole framework which so far were all passed successfully.
A further test is the independence of the mechanism from details of the matter Hamiltonian, implying that we are dealing with a pure quantum geometry effect. Even curvature couplings, which can arise for non-minimally coupled scalar fields, do not change the mechanism [93] although at first the classical structure seems to be quite different from that in the absence of curvature couplings. The deep quantum picture crucially relies on spatial discreteness and dynamical equations for a wave function.

Sounds like he’s on the same wavelength as Baez.
Martin Bojowald proposes a model … “quantum hyperbolicity”.

NOTE: p. 31
FIGURE 15. Effective curvature potential in a Bianchi IX model
THERE ARE 12 “nodes”
The problem, DYNAMICS, …. is to have those 12 “nodes” do their dancing in 3d and figure out which of those configurations mean something and how they relate to the triads (12).
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I'm going to post further thoughts in my blog
jal

 

[wolram]

In theories without singularities is origonal information retained, or is there
some scope for morphology?