[Paper Review] New Horizons in Cosmology with Spectral Distortions of the Cosmic Microwave Background

Jens Chluba, Maximilian H. Abitbol+28 more|arXiv (Cornell University)|Sep 4, 2019

Cosmology and Gravitation Theories351 references161 citations

TL;DR

This paper advocates for a dedicated space mission to measure cosmic microwave background (CMB) spectral distortions across the full sky at high sensitivity, enabling precise tests of inflation, dark matter, and particle physics beyond the standard model. By deploying cooled, absolutely calibrated Fourier transform spectrometers with sub-millijansky sensitivity, the mission would detect primordial and late-time spectral distortions, transforming COBE/FIRAS's upper limits into definitive detections.

ABSTRACT

Voyage 2050 White Paper highlighting the unique science opportunities using spectral distortions of the cosmic microwave background (CMB). CMB spectral distortions probe many processes throughout the history of the Universe. Precision spectroscopy, possible with existing technology, would provide key tests for processes expected within the cosmological standard model and open an enormous discovery space to new physics. This offers unique scientific opportunities for furthering our understanding of inflation, recombination, reionization and structure formation as well as dark matter and particle physics. A dedicated experimental approach could open this new window to the early Universe in the decades to come, allowing us to turn the long-standing upper distortion limits obtained with COBE/FIRAS some 25 years ago into clear detections of the expected standard distortion signals.

Motivation & Objective

• To advance cosmology beyond thermal equilibrium by measuring CMB spectral distortions as a new probe of fundamental physics.
• To test the cosmological standard model (CSM) and detect new physics through primordial and late-time spectral distortions.
• To address key open questions in inflation, dark matter, and recombination by observing energy spectrum deviations from blackbody.
• To enable high-precision measurements of CMB distortions with a pathfinder and L-class mission concept.
• To overcome foreground and technological challenges through advanced spectrometer design and calibration techniques.

Proposed method

• Deploy a constellation of cooled (≈0.1 K), absolutely calibrated Fourier Transform Spectrometers (FTS) for wideband (10 GHz to THz) spectral coverage.
• Achieve all-sky spectral sensitivity at 0.1–0.5 Jy/sr to detect faint spectral distortions across large angular scales (≥1°).
• Use a multi-frequency, multi-beam FTS array to enable absolute calibration and minimize systematics.
• Leverage the heritage of COBE/FIRAS while improving sensitivity by orders of magnitude through cryogenic cooling and advanced detector technology.
• Integrate with existing and future surveys via synergies in CMB, large-scale structure, and line intensity mapping.
• Apply theoretical modeling of thermal history, recombination, and non-thermal processes to predict and interpret distortion amplitudes.

Experimental results

Research questions

• RQ1 Can primordial spectral distortions from the epoch of recombination (z ≳ 10³) be detected with current technology?
• RQ2 What constraints can CMB spectral distortions place on inflationary physics, particularly non-Gaussianity and the primordial power spectrum?
• RQ3 How do spectral distortions probe the nature of dark matter, including primordial black holes and axion-like particles?
• RQ4 To what extent can spectral distortions reveal energy injection from late-time processes such as reionization and feedback?
• RQ5 What is the detectable range of spectral distortion amplitudes, and how do they constrain new physics beyond the standard model?

Key findings

• The paper identifies multiple CSM-predicted spectral distortions—µ- and y-type—that are detectable with current technology and would extend the reach of cosmology by orders of magnitude in physical scale.
• A pathfinder mission with M-class capability would detect primordial distortions at modest significance (e.g., μ ≈ 10⁻⁶–10⁻⁷), while an L-class mission would achieve high-precision measurements of all expected CSM signals.
• The proposed FTS-based spectrometer design achieves sub-millijansky sensitivity (0.1–0.5 Jy/sr) and absolute calibration, enabling robust detection of spectral deviations from blackbody.
• Detection of µ-distortions from recombination would constrain the optical depth and energy injection history with unprecedented accuracy.
• The absence of expected distortions would signal new physics, such as modified recombination or non-thermal processes in the early universe.
• Synergies with future surveys in CMB, 21 cm, and line intensity mapping would enhance the science return, enabling cross-validation of spectral distortion signals.