1 - Dark energy, gravitation and the Copernican principle

Summary

Cosmological models and their hypotheses

1.1.1 Introduction

The progress of physical cosmology during the past ten years has led to a “standard” cosmological model in agreement with all available data. Its parameters are measured with increasing precision but it requires the introduction of a dark sector, including both dark matter and dark energy, attracting the attention of both observers and theoreticians.

Among all the observational conclusions, the existence of a recent acceleration phase of the cosmic expansion has become more and more robust. The quest for the understanding of its physical origin is however just starting (Peebles and Ratra, 2003; Peter and Uzan, 2005; Copeland et al., 2006; Uzan, 2007).Models and speculations are flourishing and we may wonder to what extent the observations of our local universe may reveal the physical nature of the dark energy. In particular, there exist limitations to this quest intrinsic to cosmology, related to the fact that most observations are located on our past light-cone (Ellis, 1975), and to finite volume effects (Bernardeau and Uzan, 2004) that can make many physically acceptable possibilities indistinguishable in practice.

This text discusses the relations between the cosmic acceleration and the theory of gravitation and more generally with the hypotheses underlying the construction of our cosmological model, such as the validity of general relativity on astrophysical scales and the Copernican principle. We hope to illustrate that cosmological data now have the potential to test these hypotheses, which go beyond the measurements of the parameters.