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Does dark energy come from quantum vacuum?

French researchers, including IRAP Toulouse, offer a physical origin of dark energy. This would be the gravitational action of the quantum vacuum in an extra dimension of space. Considered for a long time in physics, the magnitude of the gravitational action of the quantum vacuum was nevertheless estimated to values ranging well beyond those authorized by the observations on a cosmological scale: some 10120 times the current density of the universe. This has led cosmologists to look for other mechanisms to explain the accelerating expansion of the universe, as quintessence or modifications of general relativity.

The recent work of the French researchers have shown that in the presence of a compact extra dimension (that is to say, looped on itself), the gravitational quantum vacuum could produce a contribution to the density of the universe by a mechanism similar to the Casimir effect in quantum electrodynamics. This contribution behaves exactly like a cosmological constant and can take a value consistent with the observations if the size of the extra dimension is of a few tens of micrometers. In a rather fascinating way, laboratory experiments designed to measure the behavior of gravity at sub-millimeter scales could test a result of this proposal.

The results recently obtained by the Planck mission came strengthen our knowledge of the composition of the universe and the characteristics of its history. THus, the material in the form of atoms is only slightly less than 5% of the total density, while another material of unknown nature, called non-baryonic, is a little over 25%. The universe is also subject to a large-scale repulsive force attributed to dark energy which represents 70% of the density of the Universe, which causes an acceleration of its expansion!

This result, first revealed in the years 1998-1999 by the Hubble diagram of distant supernovae, has since been largely confirmed not only by new and more extensive samples of distant supernovae, such as that provided by the SNSL at the CFH, but also with additional information relating to the distribution of galaxies on large scales or through the distribution of dark matter analyzed by gravitational lensing effects. Now firmly established, the reality of the phenomenon of acceleration gave rise to the attribution of the Nobel Prize in Physics to its discoverers in 2011. The origin of this acceleration is still a mystery which gives rise to a large confusion among the researchers. Thefirst goal of the EUCLID project selected by the European Space Agency (ESA) in June 2012 is to better characterize the source of this cosmic acceleration. Two possible explanations are the subject of numerous studies in recent years: the first one considers the existence of a new element in the universe, which could be in the form of a scalar field which behaves like a fluid with negative pressure ; general relativity can then lead to a reversal of the sign of the gravitational force, thus becoming repulsive. A second approach, maybe more radical, is to assume that the theory of general relativity has to undergo a change whose consequences exist on cosmological scales only.

In this landscape of fundamental physics a second gray area exists: the role of gravity in quantum vacuum. For almost a century physicists have noted that the vacuum in quantum mechanics carries an a priori non-zero energy. This gap may therefore contribute to the density of the universe. But the feedback that we can try to do when based on the scales of particle physics lead to catastrophically too large values, which led to think that this contribution must actually equals zero.

In this new work the French researchers have revisited this issue in the case that space would own additional dimensions. This suggestion made in the 30s by Kaluza and Klein is experiencing a renewed interest because it is at the heart of the theory of strings1, which leads in particular to the theory of "branes"2 : particle physics is confined to the usual four-dimensional spacetime, but gravity can propagate in extra dimensions. Assuming that this gravitational field, confined in a compact extra dimension is quantified, it can be the source of a Casimir effect well-known in the electromagnetic area3 and measured by laboratory experiments. This Casimir effect results in a real, measurable force and is interpreted as the manifestation of a change in the structure of the quantum vacuum. In the cosmological context, this effect leads to the appearance of a gravitational contribution of the vacuum which behaves exactly like a cosmological constant! Identifying this cosmological constant at the origin of the current acceleration of the expansion is possible if the size of the extra dimension is of a few tens of micrometers. Consequently, the law of gravitation no more follows the Newton's law at scales below these few tens of microns, thus providing a way to test this proposal. However, this measurement of the law of gravitation on scales of a few tens of micrometers is already within the reach of current experiments. Thus, even a relatively modest improvement of the current limits should allow a progress in our understanding of the origin of dark energy.

Notes :

1  The string theory is a theory that aims at a unified description of gravity and of other interactions. Elementary entities are no longer particles but vibrating strings in a space with more dimensions than the four known.

2  In cosmology, our four-dimensional space-time is the "brane" where the usual forces like electromagnetism perform, while other interactions can propagate in extra dimensions.

3  The Casimir effect results in an attractive force between two conducting plates, due to the change in the quantum vacuum between them.

Bibliographic source  :
"Can Dark Energy emerge from quantum effects in compact extra dimension ?" by A. Dupays et al., Astronomy & Astrophysics, 06/2013, http://www.aanda.org/articles/aa/abs/2013/06/aa21060-13/aa21060-13.html

Contact : Alain Blanchard, IRAP, alain.blanchard@irap.omp.eu

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