[Paper Review] CMB Spectral Distortions: Status and Prospects

A. Kogut, Maximilian H. Abitbol+6 more|arXiv (Cornell University)|Jul 30, 2019

Cosmology and Gravitation Theories27 references49 citations

TL;DR

The paper advocates a space-based Fourier transform spectrometer approach (inspired by PIXIE) to measure CMB spectral distortions, analyzes foreground challenges, and outlines a multi-module mission concept achieving very tight constraints on y and μ distortions.

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.

Motivation & Objective

• Motivate the study of CMB spectral distortions as probes of the early universe and structure formation.
• Assess the sensitivity gains needed to detect minimal distortions and the role of foregrounds.
• Propose a practical mission concept using existing technology to reach the required sensitivity.
• Evaluate foreground subtraction strategies and instrumental design choices for robust distortion measurements.

Proposed method

• Describe Fourier transform spectroscopy as the core measurement technique for absolute CMB spectra.
• Derive detector noise and sky-averaged intensity relations, including NEP and Pν equations, to project sensitivity.
• Analyze synthesized channel construction, sampling, and apodization to optimize foreground subtraction.
• Assess how etendue, optical load, and channel width affect background-limited performance.
• Propose a PIXIE-like modular mission concept with multiple FTS units to separate CMB distortions from foregrounds.

Experimental results

Research questions

• RQ1 What sensitivity is required to detect the minimal CMB spectral distortions given realistic foregrounds?
• RQ2 How do foregrounds limit distortion measurements, and what priors or data are needed to mitigate them?
• RQ3 Can a modular, existing-technology space mission achieve statistically significant detections of y and μ distortions under worst-case foregrounds?

Key findings

• Three orders of magnitude improvement over FIRAS-like limits is achievable with modern detectors and cryogenics.
• Astrophysical foregrounds, not raw sensitivity, are the limiting factor for spectral distortions; without foreground constraints, sensitivity degrades by ~factor 30.
• A single module of the proposed MFM/LFM/HFM concept can reach |y|<6.6×10^-9 and |μ|<5.2×10^-8 (95% CL) in 4 years; adding modules improves to |y|<3.3×10^-9 and |μ|<1.9×10^-8 (95% CL).
• A full multi-module mission could detect primordial hydrogen/helium recombination lines at 2σ under pessimistic foregrounds and detect μ distortions at high significance if foregrounds are favorable.
• Even with foregrounds, PIXIE-like capability would detect the y-distortion and relativistic correction with high significance, while exploring beyond-ΛCDM physics via μ distortions.