Spectral Distortions of the CMB as a Probe of Inflation, Recombination,
\n Structure Formation and Particle Physics

Following the pioneering observations with COBE in the early 1990s, studies
\nof the cosmic microwave background (CMB) have focused on temperature and
\npolarization anisotropies. CMB spectral distortions - tiny departures of the
\nCMB energy spectrum from that of a perfect blackbody - provide a second,
\nindependent probe of fundamental physics, with a reach deep into the primordial
\nUniverse. The theoretical foundation of spectral distortions has seen major
\nadvances in recent years, which highlight the immense potential of this
\nemerging field. Spectral distortions probe a fundamental property of the
\nUniverse - its thermal history - thereby providing additional insight into
\nprocesses within the cosmological standard model (CSM) as well as new physics
\nbeyond. Spectral distortions are an important tool for understanding inflation
\nand the nature of dark matter. They shed new light on the physics of
\nrecombination and reionization, both prominent stages in the evolution of our
\nUniverse, and furnish critical information on baryonic feedback processes, in
\naddition to probing primordial correlation functions at scales inaccessible to
\nother tracers. In principle the range of signals is vast: many orders of
\nmagnitude of discovery space could be explored by detailed observations of the
\nCMB energy spectrum. Several CSM signals are predicted and provide clear
\nexperimental targets, some of which are already observable with present-day
\ntechnology. Confirmation of these signals would extend the reach of the CSM by
\norders of magnitude in physical scale as the Universe evolves from the initial
\nstages to its present form. The absence of these signals would pose a huge
\ntheoretical challenge, immediately pointing to new physics.