Peering Beyond the Veil of Last Scattering: A View of the Universe with CMB Spectral Distortions
Abstract
The frequency spectrum of the cosmic microwave background is the most precise blackbody ever measured in nature, with deviations constrained at the level of almost one part per million from the COBE satellite. Nevertheless, departures away from a perfect blackbody are present in standard ΛCDM cosmology, lurking just beneath the surface of our current observational bounds. These spectral distortions provide invaluable information on our thermal history in both the post- and pre-recombination epochs, allowing us to peer beyond last scattering and into the primordial Universe. Here, we present an overview of the underlying physics responsible for generating CMB spectral distortions at all epochs. As an illustration of this rich physics, we review a comprehensive set of mechanisms capable of generating distortions both within and beyond the standard ΛCDM paradigm. We also discuss the information that can be gleaned by going beyond the monopole (sky-averaged) spectrum and exploiting the spatial information present in anisotropic spectral distortions. To supplement our discussion of the diverse science of spectral distortions, we provide an overview of the upcoming and proposed experimental landscape. We highlight that the combination of the TMS, COSMO, and BISOU experiments will provide the first discovery of a monopole y-type distortion within the coming decade. From space, the proposed FOSSIL experiment is forecasted to improve upon the original COBE/FIRAS measurement by roughly three orders of magnitude in sensitivity, bringing with it the detection of the ΛCDM μ-type distortion sourced by the dissipation of small scale acoustic modes in the pre-recombination plasma. With transformational measurements on the horizon, CMB spectral distortions offer a uniquely sensitive probe of the thermal history of the Universe at redshifts z ≲ 2 × 106.
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BibTeX
@misc{cyr2026peering,
title = {{Peering Beyond the Veil of Last Scattering: A View of the Universe with CMB Spectral Distortions}},
author = {Bryce Cyr and Nabila Aghanim and Ethan Baker and Elia Stefano Battistelli and Richard Battye and José Luis Bernal and Jens Chluba and Xavier Coulon and William Coulton and Paolo de Bernardis and Eleonora Di Valentino and Guillem Domènech and Sara Evangelista and Giulio Fabbian and Fabio Finelli and J. Colin Hill and Rishi Khatri and Alan Kogut and Guilaine Lagache and Hongwan Liu and Bruno Maffei and Fereshteh Majidi and Abhishek S Maniyar and C.J.A.P Martins and Silvia Masi and Jeff Mcmahon and Daniela Paoletti and Vivian Poulin and Wenzer Qin and Mathieu Remazeilles and José Alberto Rubiño-Martín and Alina Sabyr and Mayuri Sathyanarayana Rao and Carlos Sierra and Jack Singal and Tracy R Slatyer and Elsa M Teixeira and Leander Thiele and Léo Vacher and Duncan Watts},
howpublished = {HAL},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.16194},
eprint = {2609.16194},
archivePrefix = {arXiv},
url = {https://hal.science/hal-05770484v1},
}
RIS
TY - UNPB
TI - Peering Beyond the Veil of Last Scattering: A View of the Universe with CMB Spectral Distortions
AU - Bryce Cyr
AU - Nabila Aghanim
AU - Ethan Baker
AU - Elia Stefano Battistelli
AU - Richard Battye
AU - José Luis Bernal
AU - Jens Chluba
AU - Xavier Coulon
AU - William Coulton
AU - Paolo de Bernardis
AU - Eleonora Di Valentino
AU - Guillem Domènech
AU - Sara Evangelista
AU - Giulio Fabbian
AU - Fabio Finelli
AU - J. Colin Hill
AU - Rishi Khatri
AU - Alan Kogut
AU - Guilaine Lagache
AU - Hongwan Liu
AU - Bruno Maffei
AU - Fereshteh Majidi
AU - Abhishek S Maniyar
AU - C.J.A.P Martins
AU - Silvia Masi
AU - Jeff Mcmahon
AU - Daniela Paoletti
AU - Vivian Poulin
AU - Wenzer Qin
AU - Mathieu Remazeilles
AU - José Alberto Rubiño-Martín
AU - Alina Sabyr
AU - Mayuri Sathyanarayana Rao
AU - Carlos Sierra
AU - Jack Singal
AU - Tracy R Slatyer
AU - Elsa M Teixeira
AU - Leander Thiele
AU - Léo Vacher
AU - Duncan Watts
PY - 2026
DA - 2026/09/14
PB - HAL
DO - 10.48550/arxiv.2609.16194
UR - https://hal.science/hal-05770484v1
AB - The frequency spectrum of the cosmic microwave background is the most precise blackbody ever measured in nature, with deviations constrained at the level of almost one part per million from the COBE satellite. Nevertheless, departures away from a perfect blackbody are present in standard ΛCDM cosmology, lurking just beneath the surface of our current observational bounds. These spectral distortions provide invaluable information on our thermal history in both the post- and pre-recombination epochs, allowing us to peer beyond last scattering and into the primordial Universe. Here, we present an overview of the underlying physics responsible for generating CMB spectral distortions at all epochs. As an illustration of this rich physics, we review a comprehensive set of mechanisms capable of generating distortions both within and beyond the standard ΛCDM paradigm. We also discuss the information that can be gleaned by going beyond the monopole (sky-averaged) spectrum and exploiting the spatial information present in anisotropic spectral distortions. To supplement our discussion of the diverse science of spectral distortions, we provide an overview of the upcoming and proposed experimental landscape. We highlight that the combination of the TMS, COSMO, and BISOU experiments will provide the first discovery of a monopole y-type distortion within the coming decade. From space, the proposed FOSSIL experiment is forecasted to improve upon the original COBE/FIRAS measurement by roughly three orders of magnitude in sensitivity, bringing with it the detection of the ΛCDM μ-type distortion sourced by the dissipation of small scale acoustic modes in the pre-recombination plasma. With transformational measurements on the horizon, CMB spectral distortions offer a uniquely sensitive probe of the thermal history of the Universe at redshifts z ≲ 2 × 10^6.
ER -