Current Status and Perspectives of Cosmic Microwave Background Observations

Measurements of the cosmic microwave background (CMB) radiation provide a unique opportunity for a direct study of the primordial cosmic plasma at redshift z ∼103. The angular power spectra of temperature and polarisation fluctuations are powerful observational objectives as they encode information on fundamental cosmological parameters and on the physics of the early universe. A large number of increasingly ambitious balloon‐borne and ground‐based experiments have been carried out following the first detection of CMB anisotropies by COBE‐DMR, probing the angular power spectrum up to high multipoles. The recent data from WMAP provide a new major step forward in measurements percision. The ESA mission “Planck Surveyor”, to be launched in 2007, is the third‐generation satellite devoted to CMB imaging. Planck is expected to extract the full cosmological information from temperature anisotropies and to open up new fronteers in the CMB field.