# Extraction des paramètres cosmologiques et des propriétés de l'énergie noire

Abstract : With the unexpected discovery of the presently accelerated expansion of the universe, one decade ago, modern cosmology entered what one could call the era of high precision cosmology'. The current observational constraints indicate a world model of a flat geometry, where most of the current universe's energy content is made up by an unknown component, often depicted dark energy', contributing about seventy percent. In this model, ordinary baryonic matter and radiation contribute only roughly five percent, and dark matter accounts for about twenty-five percent. The measured properties of the dark energy component being consistent with a Cosmological Constant, $Lambda$, this cosmological standard model is referred to as the $Lambda$-Cold-Dark-Matter' ($Lambda$CDM') model. Despite its overall success, this model suffers from various problems. The existence of a Cosmological Constant raises fundamental questions about the theoretical explanation of its physical nature, and some authors also discuss the coincidence problem (`why now?'). Attempts to describe it as the energy contribution from the vaccum as following from Quantum Field Theory failed quantitatively. In consequence, a large number of alternative models have been developped to describe the dark energy component: modified gravity, additional dimensions, Quintessence models. Also, astrophysical effects have been considered to mimick an accelerated expansion. The basics of the $Lambda$CDM model and the various attempts of explaining dark energy are outlined in this thesis. Another major problem of the model comes from the dependencies of the fit results on a number of a priori assumptions and parameterization effects. Today, combined analysises of the various cosmological probes are performed to extract the parameters of the model, which can be as many as up to twenty, depending on the model. Different model assumptions (flat geometry, constant dark energy equation of state, assumptions on CMB physics like sound speed and initial power spectrum,...) are being applied, which may dangerously alter the fit results. Due to a wrong model assumption or a bad parameterization of the real physics, one might end up measuring with high precision something which is not there. We show, that indeed due to the high precision of modern cosmological measurements, purely kinematic approaches to distance measurements no longer yield valid fit results except for accidental special cases, and that a fit of the exact (integral) redshift-distance relation is necessary. The arising problem of degeneracy between the principal world-model parameters is discussed. The main results of this work are then presented, concerning the use of the CPL parameterization of dark energy when coping with the dynamics of tracker solutions of Quintessence models, and the risk of introducing biases on the parameters due to the possibly prohibited extrapolation to arbitrary high redshifts of the SN type Ia magnitude calibration relation, which is obtained in the low-redshift regime. Whereas the risks of applying CPL shows up to be small for a wide range of dynamical tracker models, we find classes of SN magnitude evolution that induce substantial risks of misinterpreting the fit results even in a combined analysis of future high precision data.
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Submitted on : Monday, December 20, 2010 - 4:15:03 PM
Last modification on : Tuesday, October 19, 2021 - 10:59:14 PM
Long-term archiving on: : Monday, March 21, 2011 - 3:37:57 AM

### Identifiers

• HAL Id : tel-00473183, version 4

### Citation

Sebastian Linden. Extraction des paramètres cosmologiques et des propriétés de l'énergie noire. Cosmologie et astrophysique extra-galactique [astro-ph.CO]. Université de Provence - Aix-Marseille I, 2010. Français. ⟨tel-00473183v4⟩

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