نبذة مختصرة : Cosmology is today at a turning point of its history. The diversity and high precision of the multiple observations available have led to the emergence of a standard concordance model that describes very well the large scale structure of the Universe. Nevertheless, the unknown nature of almost 90 % of the energy content of the Universe, the so-called dark matter and energy, may be a sign that some assumptions of the standard model are inadequate. In this thesis, we explore, through the dynamical properties of scalar fields, two assumptions at the heart of cosmology: the equivalence and cosmological principles. The equivalence principle is studied thanks to scalar-tensor theories, so that General Relativity is embedded in a wider class of theories that respect the weak equivalence principle and allow to test its strong version. Both the dynamical properties and cosmological consequences (Big Bang Nucleosynthesis, Cosmic Microwave Background, structure formation) of these theories are discussed. As for the cosmological principle, it is reformulated and precisely redefined to lead to new cosmological models different from the standard one, and called averaged inhomogeneous cosmologies. These models provide a self consistent description of structures on small scales and of an effective homogeneous model for the large scales; this may be a way to explain dark energy through structure formation. These models can also been put in correspondence with the appearance of scalar fields in the standard model. ; La cosmologie est actuellement à un tournant de l'histoire de son développement. L'abondance de données observationnelles diverses et précises a permis de développer et de confirmer un modèle standard de concordance pour décrire les grandes échelles de l'Univers. Fort de ce succès, il devient crucial d'interroger les fondements de ce modèle standard, afin, notamment, d'éclaircir l'origine d'environ 90 % du contenu énergétique de l'Univers, qualifié de matière et d'énergie sombres. En lien avec le problème de l'énergie ...
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