Archive ouverte UNIGE | last documents for author 'Lukas Hollenstein'https://archive-ouverte.unige.ch/Latest objects deposited in the Archive ouverte UNIGE for author 'Lukas Hollenstein'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 +0200