Executive Summary

In this report the Cosmic Microwave Background (CMB) Working Group provides a roadmap and recommendations for the next decade of CMB space astronomy in Canada. Key Science: How did the Universe begin? Inflation is the leading theoretical paradigm for describing the first moments after the Big Bang. The 'smoking gun ' prediction of favored inflationary models is the existence of an inflationary gravity wave background that imprints a signature on the CMB as a faint curl-component to the polarization pattern on large angular scales. Its detection would be a direct probe of physics at the very highest energies. The primary objective for the decadal roadmap described in this document is a measurement of the CMB-polarization that will allow for the detection of the inflationary gravity wave signature, or an upper bound on its energy scale. These measurements will additionally provide tests of Einstein's gravity on large scales, information about new particles or forces, and possibly new insight on Dark Energy. They will enable a new window on the Universe, creating new knowledge that will continue Canada's tradition of international excellence in this field. Canadian Expertise: The CMB community in Canada is characterized by a high level of expertise in the Universities, with a strong commitment to training of graduate students. Canadians have established themselves as world leaders in detector electronics, data analysis and associated software, and th