CMB Spectral Distortions: Status and Prospects
  Departures of the energy spectrum of the cosmic microwave background (CMB)
from a perfect blackbody probe a fundamental property of the universe -- its
thermal history. Current upper limits, dating back some 25 years, limit such
spectral distortions to 50 parts per million and provide a foundation for the
Hot Big Bang model of the early universe. Modern upgrades to the 1980's-era
technology behind these limits enable three orders of magnitude or greater
improvement in sensitivity. The standard cosmological model provides compelling
targets at this sensitivity, spanning cosmic history from the decay of
primordial density perturbations to the role of baryonic feedback in structure
formation. Fully utilizing this sensitivity requires concurrent improvements in
our understanding of competing astrophysical foregrounds. We outline a program
using proven technologies capable of detecting the minimal predicted
distortions even for worst-case foreground scenarios.
