It's important to be able to test LQG with observations, most likely using new data on the microwave Background (CMB) collected by instruments such as the Planck spacecraft.(adsbygoogle = window.adsbygoogle || []).push({});

It turned out to be impossible to derive a prediction of dispersion (energy dependent speed of light) at least so far. So that particular avenue of testing does not seem to work. However there have been a number of papers recently which discuss how the spectrum of gravity waves imprinted in the CMB could be tilted in a calculable way by LQG.

These papers have been coming from researchers in several different countries, many of them exploring LQG phenomenology for the first time. The newcomer's names are often not familiar to me. I have made a more or less arbitrary choice of one of these papers, so as to have an example to consider, and, if we choose, discuss.

The lead author Aurelien Barrau (CERN) has an excellent publication track record and has earlier done research in string and stringy cosmology.

http://arXiv.org/abs/0902.0145

Cosmological footprints of loop quantum gravity

J. Grain, A. Barrau

Accepted byPhysical Review Letters, 7 pages, 2 figures

(Submitted on 2 Feb 2009)

"The primordial spectrum of cosmological tensor perturbations is considered as a possible probe of quantum gravity effects. Together with string theory, loop quantum gravity is one of the most promising frameworks to study quantum effects in the early universe. We show that the associated holonomy correction should modify the potential seen by gravitational waves during the inflationary amplification. The resulting power spectrum should exhibit a characteristic tilt. This opens a new window for cosmological tests of quantum gravity."

The journal reference is Phys.Rev.Lett.102:081301,2009

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# Cosmological footprints of LQG (Aurelien Barrau and Julien Grain)

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