Centre national de la recherche scientifique Commissariat à l'énergie atomique et aux énergies alternatives
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The nature of Gravity

by Yannis Karyotakis - 21 March 2012

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When dark energy is made to correspond to the energy of the vacuum our present theoretical foundations become an embarrassment in trying to wed gravity with quantum mechanics and urge us to seek a new paradigm. The ENIGMASS project will address these topics both at the theoretical and the experimental level especially that our members have been contributing to important aspects on the nature of gravity. One of the ultimate goals of understanding gravity would be to build a Quantum Theory of Gravity. To some extent, one can argue that there is no other option: black hole centres and the Big Bang are singular. Those singularities are not pathologies of space-time itself but of our theory of space time. General relativity has to be extended in its ultra-violet limit. We are involved in the phenomenology of quantum gravity and plan to reinforce this axis. We are also involved in Loop Quantum Gravity, which is the major attempt to provide a non-perturbative and background-independent quantization of general relativity. It is also the main challenger to string theory. It does not provide unification but does require neither extra-dimensions nor supersymmetry. We have obtained innovative results on the black hole structure and the early universe in this framework. On the experimental side, probes of the actual formulation of quantum gravity are Lorentz invariance violation and cosmological footprints as pointed out by a member of ENIGMASS. On the first point, physicists from HESS and CTA at LAPP are developing new methods to analyse the data and improve the already impressive limits recently published by gamma-ray experiments. About the second point, the LPSC has pioneered the study of new signatures in the Cosmological Microwave Background (CMB), especially through B-mode spectra. This is being pushed further through interactions with experimentalists from Planck in Grenoble. We have also shown that strong links are to be expected with inflation. Within the Labex framework, we plan to develop a combined analysis of the CMB and gamma-rays that would greatly enhance the discovery or exclusion power of those approaches.

From a totally complementary perspective, ENIGMASS is lucky to host physicists heavily involved in GRANIT. The existence of quantum states of matter in the gravitational field was demonstrated recently by a series of experiments with ultra-cold neutrons (UCN) carried out at ILL. GRANIT is a follow-up project based on a second-generation UCN gravitational spectrometer with ultra-high energy resolution. It will provide more accurate studies of these quantum states as well as measurements of the resonant transitions between them. The GRANIT spectrometer will be a unique tool for carrying out a wide range of investigations in particle physics, on the foundations of quantum mechanics, in surface physics, as well as for development of experimental techniques and their applications. Most exciting as concerns the notion of mass and gravity is the fact that GRANIT could be a test on the nature and interpretation of gravity as in the context of the fascinating suggestion of E. Verlinde that gravity is not a fundamental theory but an emergent phenomenon.