Can Dark Energy emerge from quantum effects in compact extra dimension ?

The origin of the observed acceleration of the expansion of the universe is a major problem of modern cosmology and theoretical physics. Simple estimations of the contribution of vacuum to the density energy of the universe in quantum field theory are known to lead to catastrophic large values compared to observations. Such a contribution is therefore generally not regarded as a viable source for the acceleration of the expansion. In this letter we propose that the vacuum contribution actually provides a small positive value to the density energy of the universe. The underlying mechanism is a manifestation of the quantum nature of the gravitational field, through a Casimir-like effect from an additional compact dimension of space. A key ingredient is to assume that only modes with wavelength shorter than the Hubble length contribute to the vacuum. Such a contribution gives a positive energy density, has a Lorentz invariant equation of state in the usual 4D spacetime and hence can be interpreted as a cosmological constant. Its value agrees with observations for a radius of a 5th extra dimension given by $35\,\mu$m. This implies a modification of the gravitational inverse square law around this scale, close but below existing limits from experiments testing gravity at short range.Comment: To be published in A\&

Light scalars coupled to photons and non-newtonian forces

A particle $\phi$ coupling to two photons couples also to charged particles, like protons, through a loop. If the particle is a light scalar this induced coupling to protons leads to non-newtonian forces. We show that the experimental constraints on exotic, fifth-type forces lead to stringent constraints on the $\phi\gamma\gamma$ coupling.Comment: With respect to v1, we have extended the paper, also new authors have joined. Submitted to PR

Dark sectors of the Universe: A Euclid survey approach

In this paper we study the consequences of relaxing the hypothesis of the pressureless nature of the dark matter component when determining constraints on dark energy. To this aim we consider simple generalized dark matter models with constant equation of state parameter. We find that present-day low-redshift probes (type-Ia supernovae and baryonic acoustic oscillations) lead to a complete degeneracy between the dark energy and the dark matter sectors. However, adding the cosmic microwave background (CMB) high-redshift probe restores constraints similar to those on the standard $\Lambda$CDM model. We then examine the anticipated constraints from the galaxy clustering probe of the future Euclid survey on the same class of models, using a Fisher forecast estimation. We show that the Euclid survey allows us to break the degeneracy between the dark sectors, although the constraints on dark energy are much weaker than with standard dark matter. The use of CMB in combination allows us to restore the high precision on the dark energy sector constraints.Comment: 10 pages, 6 figure

Looking for Light Pseudoscalar Bosons in the Binary Pulsar System J0737-3039

We present numerical calculations of the photon-light-pseudoscalar-boson conversion in the recently discovered binary pulsar system J0737-3039. Light pseudoscalar bosons (LPBs) oscillate into photons in the presence of strong magnetic fields. In the context of this binary pulsar system, this phenomenon attenuates the light beam emitted by one of the pulsars, when the light ray goes through the magnetosphere of the companion pulsar. We show that such an effect is observable in the gamma-ray band since the binary pulsar is seen almost edge-on, depending on the value of the LPB mass and on the strenght of its two-photon coupling. Our results are surprising in that they show a very sharp and significant (up to 50%) transition probability in the gamma-ray ($>$ tens of MeV) domain. The observations can be performed by the upcoming NASA GLAST mission.Comment: to appear in Phys. Rev. Let

Proton Zemach radius from measurements of the hyperfine splitting of hydrogen and muonic hydrogen

While measurements of the hyperfine structure of hydrogen-like atoms are traditionally regarded as test of bound-state QED, we assume that theoretical QED predictions are accurate and discuss the information about the electromagnetic structure of protons that could be extracted from the experimental values of the ground state hyperfine splitting in hydrogen and muonic hydrogen. Using recent theoretical results on the proton polarizability effects and the experimental hydrogen hyperfine splitting we obtain for the Zemach radius of the proton the value 1.040(16) fm. We compare it to the various theoretical estimates the uncertainty of which is shown to be larger that 0.016 fm. This point of view gives quite convincing arguments in support of projects to measure the hyperfine splitting of muonic hydrogen.Comment: Submitted to Phys. Rev.

Transfert de charge muonique

J. M. Launay (rapporteur), D. Taqqu (rapporteur), O. Roncero, D. Bakalov, B. Lepetit, J. A. Beswick (président du jury), C. Rizzo.This work concerns muon transfer from muonic hydrogen to heavier atoms. Recently, a method of measurement of the hyperfine structure of ground-state muonic hydrogen based on the collision energy dependence of the muon transfer rate to oxygen has been proposed. This proposal is based on measurements which where performed at the Paul Scherrer Institut in the early ninetee's which indicate that the muon transfer from muonic hydrogen to oxygen increases by a factor of 4 going from thermal to 0.12 eV energies. The motivation of our calculations was to confirm this behaviour. To study the collision energy dependence of the muon transfer rate, we have used a time-independent close-coupling method. We have set up an hyperspherical elliptic formalism valid for nonzero total angular momentum which allows accurate computations of state-to-state reactive and charge exchange processes. We have applied this formalism to muon-tranfer process to oxygen and neon. The comparison with experimental results is in both cases excellent. Finally, the neon transfer rate dependence with energy suggests to use neon instead of oxygen to perform a measurement of the hyperfine structure of muonic hydrogen. The results of accurate calculations of the muon transfer rates from muonic protium and deuterium atoms to nitrogen, oxygen and neon are also reported. Very good agreement with measured rates is obtained and for the three systems, the isotopic effect is perfectly reproduced.Cette thèse traite du transfert du muon entre l'hydrogène muonique et d'autres atomes et molécules. Récemment, Adamczak et al. ont proposé une méthode de mesure de la structure hyperfine de l'état fondamental de l'hydrogène muonique basée sur la dépendance énergétique du taux de transfert muonique sur l'oxygène. Réalisées dans les années 90 au Paul Scherrer Institut, des expériences ont en effet indiqué que le taux de transfert sur l'oxygène semblait augmenter d'un facteur 4 entre des énergies de collision thermiques (0.04 eV) et épithermiques (0.12 eV). Nos calculs avaient pour première motivation de vérifier ce comportement. Pour étudier cette dépendance du taux de transfert, nous avons utilisé une méthode de résolution de l'équation de Schrödinger indépendante du temps du type close-coupling. Nous avons ainsi mis en oeuvre un formalisme utilisant les coordonnées hypersphériques elliptiques étendu pour traiter le cas d'un moment angulaire total différent de zéro. Nous avons utilisé ce formalisme pour calculer le processus de transfert sur l'oxygène et le néon. Dans ces deux cas, l'accord avec les résultats expérimentaux est excellent. Finalement, la dépendance énergétique du taux de transfert sur le néon suggère de préférer plutôt le néon à l'oxygène pour réaliser l'expérience de mesure de la structure hyperfine de l'hydrogène muonique. Les effets isotopiques (lorsque l'hydrogène muonique est remplacé par le deutérium muonique) sont aussi parfaitement reproduits et expliqués pour l'azote, l'oxygène et le néon