(adsbygoogle = window.adsbygoogle || []).push({}); QG Paper of the Year---2007

In the poll please ignore the "(blue)" and "(green)" that got in by mistake. I tried but was unable to edit them out.

To me, for QG paper of the year, looks like a clear choice between Marseille and Utrecht. Both have made major progress this year, which can be summed up by pointing to one by Rovelli's team and two by Loll's.

I've color-coded for identification.

[EDIT in the poll choices, I stated the colors wrong at first and said blue/green by mistake. I can't edit the poll questions, so please ignore the error.]

Rovelli's (red) Marseille team essentially fullfilled the initial spinfoam program goals.

http://arxiv.org/abs/0711.0146 (8 pages)

LQG vertex with finite Immirzi parameter

1. they handled the Lorentz case (not just the simpler Euclidian case)

2. they got the Immirzi parameter to arise in spinfoam, which it hadn't till now

3. they got discrete area spectrum like in LQG

4. they got spinfoam to be compatible with canonical LQG.

There's still more to do, as usual! But the year saw a dramatic advance on this front.

Here's their abstract: "We extend the definition of the 'flipped' loop-quantum-gravity vertex to the case of a finite Immirzi parameter. We cover the Euclidean as well as the Lorentzian case. We show that the resulting dynamics is defined on a Hilbert space isomorphic to the one of loop quantum gravity, and that the area operator has the same discrete spectrum as in loop quantum gravity. This includes the correct dependence on the Immirzi parameter, and, remarkably, holds in the Lorentzian case as well. The ad hoc flip of the symplectic structure that was initially required to derive the flipped vertex is not anymore needed for finite Immirzi parameter. These results establish a bridge between canonical loop quantum gravity and the spinfoam formalism in four dimensions."

Loll's (blue) Utrecht team is getting small quantum universes, with matter, to emerge in computer QG simulations. They keep finding more out about these universes taking shape in the computer. It is pretty remarkable and I'd better just let you look at their abstracts and the papers themselves.

http://arxiv.org/abs/0712.2485 (10 pages)

Planckian Birth of the Quantum de Sitter Universe

http://arxiv.org/abs/0711.0273 (21 pages)

The Emergence of Spacetime, or, Quantum Gravity on Your Desktop

"We show that the quantum universe emerging from a nonperturbative, Lorentzian sum-over-geometries can be described with high accuracy by a four-dimensional de Sitter spacetime. By a scaling analysis involving Newton's constant, we establish that the linear size of the quantum universes under study is in between 17 and 28 Planck lengths. Somewhat surprisingly, the measured quantum fluctuations around the de Sitter universe in this regime are to good approximation still describable semiclassically. The numerical evidence presented comes from a regularization of quantum gravity in terms of causal dynamical triangulations."

"Is there an approach to quantum gravity which is conceptually simple, relies on very few fundamental physical principles and ingredients, emphasizes geometric (as opposed to algebraic) properties, comes with a definite numerical approximation scheme, and produces robust results, which go beyond showing mere internal consistency of the formalism? The answer is a resounding yes: it is the attempt to construct a nonperturbative theory of quantum gravity, valid on all scales, with the technique of so-called Causal Dynamical Triangulations. Despite its conceptual simplicity, the results obtained up to now are far from trivial. Most remarkable at this stage is perhaps the fully dynamical emergence of a classical background (and solution to the Einstein equations) from a nonperturbative sum over geometries, without putting in any preferred geometric background at the outset. In addition, there is concrete evidence for the presence of a fractal spacetime foam on Planckian distance scales. The availability of a computational framework provides built-in reality checks of the approach, whose importance can hardly be overestimated."

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# QG Paper of the Year-2007

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