The CMB is the most perfect blackbody ever measured, and that perfection is a ledger every cosmology must balance. COBE/FIRAS established in the 1990s that any Compton-y distortion, the spectral signature of energy injected into the photon bath after thermalization becomes inefficient, is smaller than 1.5 x 10^-5, with the chemical-potential mu-distortion bounded below 9 x 10^-5: numbers so constraining that they retired whole families of early-universe models on contact. Every exotic process between redshift 50,000 and recombination, decaying particles, dissipating turbulence, nonstandard energy release, must deposit its heat inside these margins or be excluded. The limits are thirty years old, unimproved, and still the binding constraint on the thermal history.
The unmeasured territory below the FIRAS bounds is where the next discrimination lives. ΛCDM itself predicts distortions within reach of a next-generation mission: y near 2 x 10^-6 from reionization and structure-era heating, mu near 2 x 10^-8 from Silk damping of the primordial spectrum, signals a PIXIE-class spectrometer would detect at high significance. Any alternative origin story faces the ledger twice: its early energetics must hide below FIRAS today, and its specific distortion predictions will be exposed the moment the spectral-distortion window opens. A framework whose origin event injected energy in the wrong epoch or amount dies by spectroscopy.
The standing is a dormant precision frontier: FIRAS still rules, PIXIE-class missions remain proposed rather than funded, and the y-distortion sky is the cleanest untapped test of everything any model claims happened to the photon bath after the first month.