  Voyage 2050 White Paper highlighting the unique science opportunities using
spectral distortions of the cosmic microwave background (CMB). CMB spectral
distortions probe many processes throughout the history of the Universe.
Precision spectroscopy, possible with existing technology, would provide key
tests for processes expected within the cosmological standard model and open an
enormous discovery space to new physics. This offers unique scientific
opportunities for furthering our understanding of inflation, recombination,
reionization and structure formation as well as dark matter and particle
physics. A dedicated experimental approach could open this new window to the
early Universe in the decades to come, allowing us to turn the long-standing
upper distortion limits obtained with COBE/FIRAS some 25 years ago into clear
detections of the expected standard distortion signals.
