Future Steps in Cosmology using Spectral Distortions of the Cosmic
  Microwave Background
  Since the measurements of COBE/FIRAS in the mid-90's we know that the energy
spectrum of the cosmic microwave background (CMB) is extremely close to that of
a perfect blackbody at an average temperature T0~2.726K. However, a number of
early-universe processes are expected to create CMB spectral distortions -
departures of the average CMB energy spectrum from a blackbody - at a level
that is within reach of present-day technology. This provides strong motivation
to study the physics of CMB spectral distortions and ask what these small
signals might be able to tell us about the Universe we live in. In this
lecture, I will give a broad-brush overview of recent theoretical and
experimental developments, explaining why future spectroscopic measurements of
the CMB will open an unexplored new window to early-universe and particle
physics. I will give an introduction about the different types of distortions,
how they evolve and thermalize and highlight some of the physical processes
that can cause them. I hope to be able to convince you that CMB spectral
distortions could open an exciting new path forward in CMB cosmology, which is
complementary to planned and ongoing searches for primordial B-mode
polarization signals. Spectral distortions should thus be considered very
seriously as part of the activities in the next decades.
