The study of the fluctuations of the Cosmic Microwave Background (CMB), initiated by the COBE discovery (Nobel Prize 2006), have reached a mature state with the ESA Planck mission which established the standard, 6-parameter, cosmological model. The model succeeds in accommodating essentially all major observational results but postulates the presence of still unknown components: dark matter and dark energy. It is also consistent with the theory of the very early Universe called cosmic inflation. It remains now to understand the nature of the unknown components and further validate inflation as the theory of the early Universe eventually integrating it into a full theory of fundamental interactions. This requires further theoretical efforts but also a way to differentiate between the models. It is thus important to further characterize the seeds for these fluctuations, in particular by trying to measure the polarization of the CMB associated with presence of primordial gravitational waves.

Over the last 15 years Japan has built a strong involvement in a number of major CMB ground-based projects, QUIET, POLARBEAR and today GroundBIRD, Simons Array and Simons Observatory: in many of those there is also a significant involvement of CNRS/IN2P3. In particular, Japan has a leading role in the building and integration of several platforms for the Simons Observatory, which is a keystone of IN2P3 involvement in the ultimate ground experiment CMB-S4. Japan, through the development of instrumental components and testing facilities at Kavli IPMU and KEK, is also a lead partner on a space mission, LiteBIRD, to be launched late 2020s, that would complement the ground efforts. LiteBIRD is an international project in which the CMB community at CNRS/IN2P3 is strongly involved. The combination of the ground and space measurements is a particularly powerful approach in order to ensure a reliable detection and robust characterization of the primordial gravitational wave signal, and offers the best way forward in obtaining invaluable tests of the physics laws taking place at highest energy, 9-12 order of magnitude higher than those achievable at LHC.