FOSSIL Proposal Targets 1000-Fold Gain in CMB Spectral Distortion Sensitivity Over COBE/FIRAS

The arXiv preprint presents a satellite concept with 130 spectral channels, cryogenic optics, and on-board calibration sources designed to separate mu-type, y-type, and relativistic y-type distortions from foregrounds. The design draws on heritage from Planck and LiteBIRD while adding absolute referencing to reach ~10^-9 level distortion sensitivity. No flight hardware exists yet; the paper is a feasibility study only.

These measurements would test single-field inflation models by bounding small-scale power, limit dark-matter annihilation cross-sections, and quantify the integrated thermal energy from galaxy feedback. Unlike PIXIE, which emphasized polarization, FOSSIL prioritizes absolute intensity calibration, addressing a gap left after FIRAS. The 2026 preprint timing coincides with renewed European interest in medium-class cosmology missions post-Euclid.

Key limitation is the absence of demonstrated end-to-end calibration at the required 10^-9 accuracy in flight-like conditions. A sub-orbital balloon pathfinder with similar detectors would strengthen the case before any agency down-selection. Next steps include detailed instrument modeling and cost estimates for an ESA or NASA medium-class opportunity.

The proposal correctly identifies that spectral distortions remain the only direct probe of energy release between recombination and reionization, yet it understates foreground modeling challenges at 100–300 GHz where CIB and Galactic dust overlap distortion signals.