Evidence for the Fifth Element: Astrophysical status of Dark Energy

Abstract: Evidence for an accelerated expansion of the universe has been provided ten years ago by the Hubble diagram of distant type Ia supernovae. This represents one of the major modern revolutions for fundamental physics and cosmology. It is yet unclear whether the explanation of the fact that gravity becomes repulsive on large scales should be found within general relativity or within a new theory of gravitation. Existing evidences for this acceleration all come from astrophysical observations. In this seminar I will critically examine the present situation of the astrophysical observations and the possible limitation in their interpretation. The main various observational probes will be presented and discussed. I will show that, even when scrutinized with skeptical eyes, the evidence for an accelerating universe is rather robust. Investigation of its very origin appears as the most fascinating challenge of modern physics.