Abstract : This thesis presents a work on the determination of the cosmological parameters, used to describe the energetic content of the Universe and its curvature, from the actual data : supernovae (UNION), cosmic microwave background (WMAP) and baryon acoustic oscillations (SDSS). Type Ia supernovae observation has shown for the first time the acceleration of the Universe expansion, which is often explained by a dark energy component with an unknown nature. In this context, we realized first a combined analysis of the three probes which shows data are compatible with the ¤CDM model. Then, using simulations, we examine in detail different assumptions often used to extract the cosmologcal parameters. In particular, because of the geometrical degeneracy, the flatness is often assumed. We study in detail the bias which can be introduced.We show that it is possible that a curved universe with a dynamical dark energy may be interpreted as a flat universe with a cosmological constant.