Archive ouverte UNIGE | last documents for author 'Ermis Mitsou'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Ermis Mitsou'engApparent ghosts and spurious degrees of freedom in non-local theorieshttps://archive-ouverte.unige.ch/unige:55807https://archive-ouverte.unige.ch/unige:55807Recent work has shown that non-local modifications of the Einstein equations can have interesting cosmological consequences and can provide a dynamical origin for dark energy, consistent with existing data. At first sight these theories are plagued by ghosts. We show that these apparent ghost-like instabilities do not describe actual propagating degrees of freedom, and there is no issue of ghost-induced quantum vacuum decay.Mon, 27 Apr 2015 10:31:32 +0200Aspects of infrared non-local modifications of General Relativityhttps://archive-ouverte.unige.ch/unige:48192https://archive-ouverte.unige.ch/unige:48192Le sujet de recherche abordé dans cette thèse concerne le problème de l'énergie sombre en cosmologie, cette forme d'énergie encore non-identifiée qui domine actuellement la dynamique de notre univers. Nous considérons l'éventualité que ce soit une modification non-locale de la théorie de la Relativité Générale qui soit à l'origine de cet effet. Inspirés par la théorie de gravité massive, nous construisons des théories non-locales dans lesquelles la gravité peut avoir une masse mais où l'invariance sous difféomorphismes n'est pas brisée. Nous nous focalisons sur la cosmologie de ces théories et les confrontons à certaines contraintes observationnelles. Sur un plan plus théorique, nous nous attardons également sur les subtilités de la théorie des champs non-locale, en clarifiant certains malentendus sur la question de stabilité.Mon, 16 Mar 2015 10:25:44 +0100Cosmological dynamics and dark energy from nonlocal infrared modifications of gravityhttps://archive-ouverte.unige.ch/unige:40530https://archive-ouverte.unige.ch/unige:40530We study the cosmological dynamics of a recently proposed infrared modification of the Einstein equations, based on the introduction of a nonlocal term constructed with m2gμν□-1R, where m is a mass parameter. The theory generates automatically a dynamical dark energy component, that can reproduce the observed value of the dark energy density without introducing a cosmological constant. Fixing m so to reproduce the observed value ΩDE≃0.68, and writing wDE(a)= w0+(1-a)wa, the model provides a neat prediction for the equation of state parameters of dark energy, w0≃-1.042 and wa≃-0.020, and more generally provides a pure prediction for wDE as a function of redshift. We show that, because of some freedom in the definition of □-1, one can extend the construction so to define a more general family of nonlocal models. However, in a first approximation this turns out to be equivalent to adding an explicit cosmological constant term on top of the dynamical dark energy component. This leads to an extended model with two parameters, ΩΛ and m. Even in this extension the EOS parameter w0 is always on the phantom side, in the range -1.33 ≲w0≤-1, and there is a prediction for the relation between w0 and wa.Fri, 26 Sep 2014 09:31:08 +0200Zero-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 +0200