Teasing bits of information out of the CMB energy spectrum

Departures of the cosmic microwave background (CMB) frequency spectrum from a blackbody-commonly referred to as spectral distortions-encode information about the thermal history of the early Universe (redshift z ≤ few × 10⁶). While the signal is usually characterized as μ-and y-type distortion, a smaller residual (non-y/non-μ) distortion can also be created at intermediate redshifts 10⁴ ≤z ≤3×10⁵. Here, we construct a new set of observables, μk, that describes the principal components of this residual distortion. The principal components are orthogonal to temperature shift, y-and μ-type distortion, and ranked by their detectability, thereby delivering a compression of all valuable information offered by the CMBspectrum. This method provides an efficient way of analysing the spectral distortion for given experimental settings, and can be applied to a wide range of energy-release scenarios. As an illustration, we discuss the analysis of the spectral distortion signatures caused by dissipation of small-scale acoustic waves and decaying/annihilating particles for a PIXIE-type experiments. We provide forecasts for the expected measurement uncertainties of model parameters and detections limits in each case. We furthermore show that a PIXIE-type experiments can in principle distinguish dissipative energy release from particle decays for a nearly scale-invariant primordial power spectrum with small running. Future CMBspectroscopy thus offers a unique probe of physical processes in the primordial Universe.