http://arxiv.org/hep-th/0407115 [Broken](adsbygoogle = window.adsbygoogle || []).push({});

Loop Quantum Gravity and the Cyclic Universe

Martin Bojowald, Roy Maartens, Parampreet Singh

6 pages, 4 figures

Notice that Roy Maartens has published over 20 stringy-type papers, mostly on "brane-world" cosmology and brane-related matters.

He now seems to have developed an interest in seeing what Loop Gravity predicts: not only about inflation (an earlier Maartens paper) but also about alternative scenarios like a hypothetical brane-collision. Apparently there used to be a concern that colliding branes might result in a singularity. Loop Gravity (which seems useful for curing singularities) has now been applied to the hypothetical brane scenario and has dispelled that possibility.

Abstract:

"Loop quantum gravity introduces strong non-perturbative modifications to the dynamical equations in the semi-classical regime, which are responsible for various novel effects, including resolution of the classical singularity in a Friedman universe. Here we investigate the modifications for the case of a cyclic universe potential, assuming that we can apply the four-dimensional loop quantum formalism within the effective four-dimensional theory of the cyclic scenario. We find that loop quantum effects can dramatically alter the near-collision dynamics of the cyclic scenario. In the kinetic-dominated collapse era, the scalar field is effectively frozen by loop quantum friction, so that the branes approach collision and bounce back without actual collision."

----exerpts from the introduction---

"...A bold attempt to tackle these problems is the ekpyrotic/cyclic scenario[1], which invokes ideas from M-theory to construct an alternative to the standard inflationary paradigm. A crucial issue for the cyclic scenario is how to process the cosmological dynamics and perturbations through time[2]..."

"Loop quantum gravity is a four-dimensional nonperturbative candidate theory of quantization of space-time whose successes include prediction of a discrete spectrum for geometrical operators [3], matter Hamiltonians that are free from ultraviolet divergences [4] and derivation of the Bekenstein-Hawking entropyformula[5].

Recently, loop quantum gravity has been applied tocosmology (for reviews see[6, 7]), leading to a resolution of cosmological singularities[8] and a new view on initial conditions [9].

In general, singularity avoidance entails a breakdown of smooth classical spacetime structure and the quantum geometric discretization of spacetime. Loop quantum cosmology derives a difference equation for the wave function whose evolution does not stop where the classical singularity would be.

The system then continues to a new branch after which a semiclassical description may be used[10]."

I point out this paper partly because it breaks the "two camps" paradigm and represents crossover on the part of the authors.

Personally I am not so captivated by the "colliding braneworlds" scenarios and think they are a bit on the pure speculation side. But here we have Roy Maartens, a recognized braneworld expert, collaborating with Martin Bojowald to apply Loop Gravity anaylysis to the braneworld scenario (purely hypothetical though it may be). It could be fruitful, this kind of crossing over and applying one thing to another---and anyway it breaks the fences. so bravo to them and also to Parampreet Singh

who I recall has published with Maarten before and is part of the growing

quantum gravity scene at Imperial College London

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# Loop Gravity cures brane collision singularity

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