Constraining interacting cosmological models with observational data

Résumé

In 1998, measurements of the luminosity distance of a handful of supernovae Type Ia hinted at an accelerated expansion of the present Universe, Since then, several observational data sets including CMB and LSS lent further support to the idea of a spatially flat universe dominated by a cosmological constant, Lambda, with 70% of the total energy density and Cold Dark Matter energy density 3% of it. Although the Lambda CDM model fits reasonably well all observational data it faces two serious theoretical problems. In the first place, a fine-tuning problem. The observed cosmological constant is about 121 times smaller than the value predicted by quantum field theory. This is why scalar fields were proposed as an alternative to the cosmological constant; they naturally arises in particle physics including string theory. Up to now, a wide variety of scalar fields dark energy models have been suggested. These include quintessence, phantoms, k-essence, tachyon, ghost condensates and dilatonic dark energy amongst many others. Quintessence scalar fields present energy scales compatible with the energy scale of particle physics. This may help alleviate the severe fine-tuning problem of the cosmological constant. In the second place, it appears rather unnatural that the densities of two components, cold dark matter and dark energy, that evolve so differently with expansion happen to be of the same order precisely today. This is the coincidence problem. In order to solve, or alleviate it, models featuring an interaction between dark energy and matter were advanced proposed.In Chapter 2 we studied the Interacting Quintessence Model of Chimento et al. The interaction is proposed from a phenomenological viewpoint. The dark energy decays into cold dark matter in such a way that their ratio remains constant during the late epoch of accelerated expansion, thus substantially alleviating the coincidence problem.