CMB Spectral Distortions: Status and Prospects

TL;DR

This paper reviews the current status and future prospects of detecting spectral distortions in the cosmic microwave background, which can reveal detailed information about the universe's thermal history and early processes.

Contribution

It proposes a technological upgrade and observational program capable of detecting minimal spectral distortions, advancing our understanding of cosmology and astrophysics.

Findings

Current limits are at 50 ppm, with potential for 1000x sensitivity improvement.

Predicted distortions are within reach of proposed upgraded technology.

Foreground understanding is crucial for accurate detection.

Abstract

Departures of the energy spectrum of the cosmic microwave background (CMB) from a perfect blackbody probe a fundamental property of the universe -- its thermal history. Current upper limits, dating back some 25 years, limit such spectral distortions to 50 parts per million and provide a foundation for the Hot Big Bang model of the early universe. Modern upgrades to the 1980's-era technology behind these limits enable three orders of magnitude or greater improvement in sensitivity. The standard cosmological model provides compelling targets at this sensitivity, spanning cosmic history from the decay of primordial density perturbations to the role of baryonic feedback in structure formation. Fully utilizing this sensitivity requires concurrent improvements in our understanding of competing astrophysical foregrounds. We outline a program using proven technologies capable of detecting the minimal predicted distortions even for worst-case foreground scenarios.