THE PROBES AND SOURCES OF COSMIC REIONISATION

The reionisation of the all-pervading intergalactic medium (IGM) is a landmark event in the cosmic history of structure formation. Still, despite much recent progress, a coherent description of the thermal state and ionisation degree of the IGM, the repository of most of the baryons across the history of the Universe, remains elusive. Most of our understanding of IGM physics, and its implication for galaxy formation and metal enrichment, depends critically on the properties of the cosmic ionising background. Over the last 15 years detailed models of cosmic reionisation have been propsed and adopted in cosmological studies. Yet, many uncertainties still exist, and background models are now facing hard challenges, when confronted to new observations both in the local Universe and at high redshifts. In this contribution, I review our current understanding of the reionisation era, and the many problems still open.