CMB surveys provide, for free, blindly selected samples of extragalactic
radio sources at much higher frequencies than traditional radio surveys.
Next-generation, ground-based CMB experiments with arcmin resolution at mm
wavelengths will provide samples of thousands radio sources allowing the
investigation of the evolutionary properties of blazar populations, the study
of the earliest and latest stages of radio activity, the discovery of rare
phenomena and of new transient sources and events. Space-borne experiments will
extend to sub-mm wavelengths the determinations of the SEDs of many hundreds of
blazars, in temperature and in polarization, allowing us to investigate the
flow and the structure of relativistic jets close to their base, and the
electron acceleration mechanisms. A real breakthrough will be achieved in the
caracterization of the polarization properties. The first direct counts in
polarization will be obtained, enabling a solid assessment of the
extra-galactic source contamination of CMB maps and allowing us to understand
structure and intensity of magnetic fields, particle densities and structures
of emitting regions close to the base of the jet.Comment: Science white paper submitted to the Astro2020 US Decadal Surve