Teasing bits of information out of the CMB energy spectrum

TL;DR

This paper introduces a new set of observables, mu_k, to efficiently analyze and interpret spectral distortions in the CMB caused by early Universe energy releases, enhancing our understanding of primordial processes.

Contribution

The paper develops principal component-based observables for residual CMB spectral distortions, enabling improved analysis of energy-release scenarios with future experiments.

Findings

PIXIE-like experiments can detect small-scale acoustic wave dissipation.

Spectral distortions can distinguish between particle decay and dissipation.

Forecasts show potential to probe primordial power spectrum features.

Abstract

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 x 10^6). While the signal is usually characterized as mu- and y-type distortion, a smaller residual (non-y/non-mu) distortion can also be created at intermediate redshifts 10^4 < z < 3 x 10^5. Here, we construct a new set of observables, mu_k, that describes the principal components of this residual distortion. The principal components are orthogonal to temperature shift, y- and mu-type distortion, and ranked by their detectability, thereby delivering a compression of all valuable information offered by the CMB spectrum. This method provides an efficient way of analyzing 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 CMB spectroscopy thus offers a unique probe of physical processes in the primordial Universe.