  Deviations of the cosmic microwave background (CMB) frequency spectrum from a
pure blackbody tell an exciting story about the thermal history of our
Universe. In this paper we illustrate how well future CMB measurements might
decipher this tale, envisioning a PIXIE-like spectrometer, which could improve
the distortion constraints obtained with COBE/FIRAS some 20 years ago by at
least three orders of magnitude. This opens a large discovery space, offering
deep insights to particle and early-universe physics, opportunities that no
longer should be left unexplored. Specifically, we consider scenarios with
annihilating and decaying relic particles, as well as signatures from the
dissipation of primordial small-scale power. PIXIE can potentially rule out
different early-universe scenarios, and moreover will allow unambiguous
detections in many of the considered cases, as we demonstrate here. We also
discuss slightly more futuristic experiments, with several times improved
sensitivities, to highlight the large potential of this new window to the
pre-recombination universe.

Cosmology and Nongalactic Astrophysics