The Cosmic Microwave Background (CMB) radiation is a relic emission from 380 000 years after the Big-Bang at the time of decoupling between matter and radiation. The small CMB temperature and polarisation fluctuations, induced by quantum perturbation generated in the early Universe, contain precious information about the physics of the primordial Universe and its physical content. Several experiments, including the Planck satellite mission of ESA have measured those perturbations with high accuracy, leading to per cent precision on the determination of cosmological parameters.

Measurements of the Cosmic Microwave Background (CMB) have transformed cosmology into a precision science, and they continue to deliver new insights into the birth and evolution of our Universe. One of the major hurdles in extracting these insights, however, come from "foreground" signals emitted from intervening matter. These spurious emissions can mask and even mimic the primordial CMB, introducing errors into our conclusions.

Rayleigh scattering of the Cosmic Microwave Background (CMB) is a less studied yet potentially powerful probe of the recombination history. Scattering of CMB photons off neutral species right after recombination presents a distinctive $\nu^4$ scaling with frequency as well as a strong correlation with the primary CMB. These unique features should facilitate its detection by the next generation of CMB experiments.

The Cosmic Microwave Background (CMB), relic radiation from the hot Big-Bang, carries a wealth of information about physical processes at work in the very early universe, at energies far beyond the reach of man-made particle accelerators. Next generation observatories are poised to further constrain physics beyond the Standard Model of particles and fields, the nature of Dark Matter and Dark Energy, the existence of primordial Gravitational Waves, and the formation of structures.