CMB: A, B, C, … , W and beyond (P!)

In this introductory note to the issue of the Comptes Rendus Physique, I describe the ABC of Cosmic Microwave Background (CMB) physics, which explains why high accuracy observations of the CMB spectrum and of its spatial structure are unique tools for the determination of the global cosmological parameters and for simultaneously constraining the physics of the early universe. I also briefly survey the many experiments, Archeops, Boomerang, COBE but also DASI, CBI, MAXIMA, … to name but a few, which have measured the anisotropies of the CMB and led to crucial advances in observational cosmology. The somewhat frantic series of new results has recently culminated with the outcome of the WMAP satellite, which confirmed earlier results, set new standards of accuracy, and suggested that the Universe may have re-ionised earlier than anticipated. Many more CMB experiments are currently taking data or being planned, with the Planck satellite on the 2007 Horizon poised to extract all the cosmological information in the temperature anisotropies, and foray deeply into polarisation. To cite this article: F.R. Bouchet, C. R. Physique 4 (2003).