Introduction to “Exploring cosmic origins with CORE” Special Issue

More than 50 years after its discovery, the Cosmic Microwave Background still has not finished delivering all the unique information encoded in its tiny temperature and polarization signals. Polarization information in particular has only been exploited very partially. While the Planck satellite has measured practically all the information encoded in the primordial temperature anisotropies of the CMB, only about 10% of the polarization E modes have been measured with good accuracy. Lensing polarization B modes have just begun to be detected statistically, opening the path to the direct mapping of (dark) matter structures in the Hubble volume. Polarization B modes from primordial gravitational waves, generically predicted in most scenarios of cosmic inflation, still escape detection. This special issue discusses the scientific justification and goals, the mission and instru-ment design, and the feasibility of a fourth-generation space mission dedicated to Cosmic Microwave Background (CMB) polarization science.