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Cette ressource présente un dispositif pédagogique organisé autour d'une conférence donnée par un chercheur en cosmologie. L'activité poursuit différents objectifs : découvrir une discipline scientifique contemporaine : la cosmologie ; donner les moyens aux élèves de préciser leurs représentations et connaissances approximatives, voire de dépasser des représentations erronées concernant la connaissance scientifique ; vérifier la compréhension de la conférence et assoir les notions apportées ; promouvoir une synergie entre les enseignants de physique et de philosophie, pour autant que ce travail puisse servir de point de départ à une réflexion épistémologique sur les sciences de la nature, à mener en cours de philosophie.

Ref HAL: hal-00430271_v1
Ref Arxiv: 0809.3374
DOI: 10.1088/1475-7516/2009/02/034
WoS: 000263824100034
Ref. & Cit.: NASA ADS
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113 Citations
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We consider the linear growth of matter perturbations on low redshifts insome $f(R)$ dark energy (DE) models. We discuss the definition of dark energy(DE) in these models and show the differences with scalar-tensor DE models. Forthe $f(R)$ model recently proposed by Starobinsky we show that the growthparameter $\gamma_0\equiv \gamma(z=0)$ takes the value $\gamma_0\simeq 0.4$ for$\Omega_{m,0}=0.32$ and $\gamma_0\simeq 0.43$ for $\Omega_{m,0}=0.23$, allowingfor a clear distinction from $\Lambda$CDM. Though a scale-dependence appears inthe growth of perturbations on higher redshifts, we find no dispersion for$\gamma(z)$ on low redshifts up to $z\sim 0.3$, $\gamma(z)$ is alsoquasi-linear in this interval. At redshift $z=0.5$, the dispersion is stillsmall with $\Delta \gamma\simeq 0.01$. As for some scalar-tensor models, wefind here too a large value for $\gamma'_0\equiv \frac{d\gamma}{dz}(z=0)$,$\gamma'_0\simeq -0.25$ for $\Omega_{m,0}=0.32$ and $\gamma'_0\simeq -0.18$ for$\Omega_{m,0}=0.23$. These values are largely outside the range found for DEmodels in General Relativity (GR). This clear signature provides a powerfulconstraint on these models.

Ref HAL: hal-00430269_v1
Ref Arxiv: 0907.0393
Ref. & Cit.: NASA ADS
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We consider the linear growth of matter perturbations on low redshifts inmodified gravity Dark Energy (DE) models where G_eff(z,k) is explicitlyscale-dependent. Dispersion in the growth today will only appear for scales ofthe order the critical scale ~ \lambda_{c,0}, the range of the fifth-forcetoday. We generalize the constraint equation satisfied by the parameters\gamma_0(k) and \gamma'_0(k) \equiv \frac{d\gamma(z,k)}{dz}(z=0) to models withG_{eff,0}(k) \ne G. Measurement of \gamma_0(k) and \gamma'_0(k) on severalscales can provide information about \lambda_{c,0}. In the absence ofdispersion when \lambda_{c,0} is large compared to the probed scales,measurement of \gamma_0 and \gamma'_0 provides a consistency check independentof \lambda_{c,0}. This applies in particular to results obtained earlier for aviable f(R) model.

Commentaires: 8 pages, 5 figures

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Ref Arxiv: 0908.2669
DOI: 10.1103/PhysRevD.80.084044
WoS: 000271353700093
Ref. & Cit.: NASA ADS
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103 Citations
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We study the growth of matter density perturbations delta_m for a number ofviable f(R) gravity models that satisfy both cosmological and local gravityconstraints, where the Lagrangian density f is a function of the Ricci scalarR. If the parameter m=Rf_{,RR}/f_{,R} today is larger than the order of10^{-6}, linear perturbations relevant to the matter power spectrum evolve witha growth rate s=d (ln delta_m)/d (ln a) (a is the scale factor) that is largerthan in the LCDM model. We find the window in the free parameter space of ourmodels for which spatial dispersion of the growth index gamma_0= gamma(z=0) (zis the redshift) appears in the range of values 0.40< gamma_0<0.55, as well asthe region in parameter space for which there is essentially no dispersion andgamma_0 converges to values around 0.40

Ref HAL: hal-00398336_v1
Ref Arxiv: 0810.0087
Ref. & Cit.: NASA ADS
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According to the inflationary scenario of cosmology, all structure in the Universe can be traced back to primordial fluctuations during an accelerated (inflationary) phase of the very early Universe. A conceptual problem arises due to the fact that the primordial fluctuations are quantum, while the standard scenario of structure formation deals with classical fluctuations. In this essay we present a concise summary of the physics describing the quantum-to-classical transition. We first discuss the observational indistinguishability between classical and quantum correlation functions in the closed system approach (pragmatic view). We then present the open system approach with environment-induced decoherence. We finally discuss the question of the fluctuations' entropy for which, in principle, the concrete mechanism leading to decoherence possesses observational relevance.

Commentaires: 12 pages, Revtex, invited contribution to a special issue of Advanced Science Letters, final version

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Ref Arxiv: 0802.4196
DOI: 10.1088/1475-7516/2008/05/018
WoS: 000257167500018
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87 Citations
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We consider asymptotically stable scalar-tensor dark energy (DE) models for which the equation of state parameter $w_{DE}$ tends to zero in the past. The viable models are of the phantom type today, however this phantomness is milder than in General Relativity if we take into account the varying gravitational constant when dealing with the SNIa data. We study further the growth of matter perturbations and we find a scaling behaviour on large redshifts which could provide an important constraint. In particular the growth of matter perturbations on large redshifts in our scalar-tensor models is close to the standard behaviour $\delta_m \propto a$, while it is substantially different for the best-fit model in General Relativity for the same parametrization of the background expansion. As for the growth of matter perturbations on small redshifts, we show that in these models the parameter $\gamma'_0\equiv \gamma'(z=0)$ can take absolute values much larger than in models inside General Relativity. Assuming a constant $\gamma$ when $\gamma'_0$ is large would lead to a poor fit of the growth function $f$. This provides another characteristic discriminative signature for these models.

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