Archive ouverte UNIGE | last documents for author 'Maud Jaccard'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Maud Jaccard'engZero-point quantum fluctuations in cosmologyhttps://archive-ouverte.unige.ch/unige:35679https://archive-ouverte.unige.ch/unige:35679We reexamine the classic problem of the renormalization of zero-point quantum fluctuations in a Friedmann-Robertson-Walker background. We discuss a number of issues that arise when regularizing the theory with a momentum-space cutoff, and show explicitly how introducing noncovariant counterterms allows to obtain covariant results for the renormalized vacuum energy-momentum tensor. We clarify some confusion in the literature concerning the equation of state of vacuum fluctuations. Further, we point out that the general structure of the effective action becomes richer if the theory contains a scalar field ϕ with mass m smaller than the Hubble parameter H(t). Such an ultralight particle cannot be integrated out completely to get the effective action. Apart from the volume term and the Einstein-Hilbert term, that are reabsorbed into renormalizations of the cosmological constant and Newton's constant, the effective action in general also has a term proportional to F(ϕ)R, for some function F(ϕ). As a result, vacuum fluctuations of ultralight scalar fields naturally lead to models where the dark energy density has the form ρDE(t) =ρx(t) + ρz(t), where ρX is the component that accelerates the Hubble expansion at late times and ρz(t)is an extra contribution proportional to H2(t). We perform a detailed comparison of such models with CMB, SNIa and BAO dataMon, 14 Apr 2014 16:05:45 +0200Early dark energy from zero-point quantum fluctuationshttps://archive-ouverte.unige.ch/unige:35678https://archive-ouverte.unige.ch/unige:35678We examine a cosmological model with a dark energy density of the form ρDE(t)=ρX(t)+ρZ(t)ρDE(t)=ρX(t)+ρZ(t), where ρXρX is the component that accelerates the Hubble expansion at late times and ρZ(t)ρZ(t) is an extra contribution proportional to H2(t)H2(t). This form of ρZ(t)ρZ(t) follows from the recent proposal that the contribution of zero-point fluctuations of quantum fields to the total energy density should be computed by subtracting the Minkowski-space result from that computed in the FRW space–time. We discuss theoretical arguments that support this subtraction. By definition, this eliminates the quartic divergence in the vacuum energy density responsible for the cosmological constant problem. We show that the remaining quadratic divergence can be reabsorbed into a redefinition of Newtonʼs constant only under the assumption that ∇μ〈0|Tμν|0〉=0∇μ〈0|Tμν|0〉=0, i.e. that the energy–momentum tensor of vacuum fluctuations is conserved in isolation. However in the presence of an ultra-light scalar field X with mX<H0mX<H0, as typical of some dark energy models, the gravity effective action depends both on the gravitational field and on the X field. In this case general covariance only requires ∇μ(TμνX+〈0|Tμν|0〉). If there is an exchange of energy between these two terms, there are potentially observable consequences. We construct an explicit model with an interaction between ρXρX and ρZρZ and we show that the total dark energy density ρDE(t)=ρX(t)+ρZ(t)ρDE(t)=ρX(t)+ρZ(t) always remains a finite fraction of the critical density at any time, providing a specific model of early dark energy. We discuss the implication of this result for the coincidence problem and we estimate the model parameters by means of a full likelihood analysis using current CMB, SNe Ia and BAO dataMon, 14 Apr 2014 16:05:02 +0200Nonlocal theory of massive gravityhttps://archive-ouverte.unige.ch/unige:35657https://archive-ouverte.unige.ch/unige:35657We construct a fully covariant theory of massive gravity which does not require the introduction of an external reference metric, and overcomes the usual problems of massive gravity theories (fatal ghosts instabilities, acausality and/or van Dam-Veltman-Zakharov discontinuity). The equations of motion of the theory are nonlocal but respect causality. The starting point is the quadratic action proposed in the context of the degravitation idea. We show that it is possible to extend it to a fully nonlinear covariant theory. This theory describes the 5 degrees of freedom of a massive graviton plus a scalar ghost. However, contrary to generic nonlinear extensions of Fierz-Pauli massive gravity, the ghost has the same mass m as the massive graviton, independently of the background, and smoothly goes into a nonradiative degree of freedom for m→0. As a consequence, for m∼H0 the vacuum instability induced by the ghost is irrelevant even over cosmological time scales. We finally show that an extension of the model degravitates a vacuum energy density of order M4Pl down to a value of order M2Plm2, which for m=O(H0) is of order of the observed value of the vacuum energy density.Mon, 14 Apr 2014 12:21:55 +0200Bardeen variables and hidden gauge symmetries in linearized massive gravityhttps://archive-ouverte.unige.ch/unige:35656https://archive-ouverte.unige.ch/unige:35656We give a detailed discussion of the use of the (3+1) decomposition and of Bardeen's variables in massive gravity linearized over a Minkowski as well as over a de Sitter background. In Minkowski space the Bardeen “potential” Φ, that in the massless case is a nonradiative degree of freedom, becomes radiative and describes the helicity-0 component of the massive graviton. Its dynamics is governed by a simple Klein-Gordon action, supplemented by a term (□Φ)2 if we do not make the Fierz-Pauli tuning of the mass term. In de Sitter the identification of the variable that describes the radiative degree of freedom in the scalar sector is more subtle, and even involves expressions nonlocal in time. The use of this new variable provides a simple and transparent derivation of the Higuchi bound and of the disappearance of the scalar degree of freedom at a special value of m2g/H2. The use of this formalism also allows us to uncover the existence of a hidden gauge symmetry of the massive theory, that becomes manifest only once the nondynamical components of the metric are integrated out, and that is present both in Minkowski and in de Sitter backgrounds.Mon, 14 Apr 2014 12:21:19 +0200Infrared modifications of general relativity and nonlocal massive gravityhttps://archive-ouverte.unige.ch/unige:35257https://archive-ouverte.unige.ch/unige:35257Dans cette thèse, nous examinons plusieurs problèmes potentiellement utiles à la compréhension de la nature de l'énergie noire, ce mystérieux composé qui accélère l'expansion cosmique. Après avoir considéré certains aspects techniques relatif au calcul de la densité d'énergie du vide des champs quantiques dans un contexte cosmologique, nous explorerons différents modèles qui modifient la relativité générale dans le régime infrarouge. Ceci sera réalisé soit par l'introduction d'un champ scalaire ultra-léger, en dotant le graviton d'une masse, ou encore en ajoutant des termes non-locaux dans les équations d'Einstein.Wed, 02 Apr 2014 09:25:19 +0200