New Bojowald paper-BB non-singularity continued

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marcus

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New Bojowald paper---BB non-singularity continued

classical GR breaks down at time zero because of infinite density and curvature

quantizing GR removes the singularity---when quantized the Friedmann equations go smoothly back in time, showing a bounce at time zero

Martin Bojowald was the first to get this result, today he posted a new paper:

"Quantum Gravity and the Big Bang"
http://arxiv.org/astro-ph/0309478 [Broken]
6 pages, 2 figures

Here's the abstract and a few sentences from the beginning:


Martin Bojowald (New Address: Max-Planck-Institut fuer Gravitationsphysik, Albert-Einstein-Institut, Am Muehlenberg 1, D-14476 Golm, Germany)

Abstract. Quantum gravity has matured over the last decade to a theory which can tell in a precise and explicit way how cosmological singularities of general relativity are removed. A branch of the universe “before” the classical big bang is obtained which is connected to ours by quantum evolution through a region around the singularity where the classical space-time dissolves. We discuss the basic mechanism as well as applications ranging to new phenomenological scenarios of the early universe expansion, such as an inflationary period.

1 Introduction
When the big bang is approached, the volume becomes smaller and smaller and one enters a regime of large energy densities. Classically, those conditions will become so severe that a singularity is reached; the theory simply breaks down. For a long time, the expectation has been that somewhere along the way quantum gravity takes over and introduces new effects, e.g. a discrete structure, which prevent the singularity to develop. This presumably happens at scales the size of the Planck length lP , i.e. when the universe has about a volume lP3.

Since at the classical singularity space itself becomes singular and gravitational interactions are huge, such a quantum theory of gravity must be background independent and non-perturbative. A theory satisfying these conditions is in fact available in the form of loop quantum gravity/quantum geometry (see [1, 2] for reviews). One of its early successes was the derivation of discrete spectra of geometric operators like area and volume [3, 4, 5]. Thus, the spatial geometry is discrete in a precise sense. Furthermore, matter Hamiltonians exist as well-defined operators in the theory which implies that ultraviolet divergences are cured in the fundamental formulation [6, 7].

Both properties must be expected to have important consequences for cosmology. The discreteness leads to a new basic formulation valid at small volume, and since gravity couples to the matter Hamiltonian, its source term is modified at small scales when the good ultraviolet behavior is taken into account. It is possible to introduce both effects into a cosmological model in a systematic way, which allows us to test the cosmological consequences of quantum gravity (reviewed in [8, 9])."

The first time this came out was Bojowald's
"Absence of a Singularity in Loop Quantum Cosmology"
http://arxiv.org/gr-qc/0102069 [Broken]
 
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marcus

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New predictions from General Relativity

General Relativity in its quantized form has reached the stage of making concrete predictions about the early universe (time zero behavior, inflation)

I guess it is important to notice that classical GR is a non-perturbative background independent theory and it has been
quantized as conservatively as possible----introducing no new structure (no extra dimensions etc etc)----adhering as closely as possible to classical GR (formulated in Ashtekar variables).

Furthermore in its LQC form the quantized GR cosmology has the correct classical limits. So this is still the same theory----the Friedmann equations basic to cosmology have been quantized and are beginning to talk.

So the old 1915 GR has started predicting again and is telling us new things. It is an important development. Here are some sentences from Bojowald's concluding paragraph

"With new developments in quantum geometry, quantum gravity has become a theory which can make concrete predictions about the very early stages of the universe. Results include possible solutions of old conceptual problems, as the singularity problem [14] and the problem of initial conditions [20], and also new phenomenological proposals which can be confronted with cosmological observations..."

This paper was delivered at the recent Quantum Gravity conference at Marseilles.

Bojowald appears have left Ashtekar's Penn State institute and to now be at the Max Planck Institute for Gravitation Physics at Berlin.

edit: Golm and Potsdam (where the University is) are suburbs of Berlin, close to each other. It is easier for me to think of that MPI as being at Berlin
 
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selfAdjoint

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Bojowald appears have left Ashtekar's Penn State institute and to now be at the Max Planck Institute for Gravition at Golm.
Didn't Sahlmann go the other direction?
 

marcus

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Originally posted by selfAdjoint
Didn't Sahlmann go the other direction?
Both Sahlmann and Bojowald trained in Europe, then took postdocs in States, now Bojowald has gone back and likely
Sahlmann will too.

this sounds odd to be talking about doesnt it?----little details like where the QGR "center of gravity" is-----and who goes where
but there may be some non-obvious reason that it is interesting all the same

joking aside, I think Sahlmann is only at Penn for a temporary postdoc exposure to the American school of QG and that in a year or two he will be back in Europe: at MPI Berlin, or Marseille

Its pretty clear that Europe is taking the lead in QGR
 
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