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The future of cosmology? A case for CMB spectral distortions

par Lucca, Matteo
Président du jury Compère, Geoffrey
Promoteur Hambye, Thomas
Publication Non publié, 2023-07-10

Thèse de doctorat

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Résumé :

This thesis treats the topic of Cosmic Microwave Background (CMB) Spectral Distortions (SDs). In brief, CMB SDs are any deviation from a pure black body shape of the CMB energy spectrum. They can be generated by any heating, number changing and photon mixing process taking place in the history of the universe. As such, CMB SDs can probe the inflationary, expansion and thermal evolution of the universe. In particular, they are expected to be produced within the ΛCDM model as well by the presence of beyond-ΛCDM physics like in the case of e.g., features in the primordial power spectrum, primordial gravitational waves, non-standard dark matter properties, primordial black holes, primordial magnetic fields and many models attempting to solve the Hubble tension, among others.The currently missing observation of this rich probe of the universe makes of it an ideal target for future observational campaigns, as recently highlighted by ESA’s long-term science program in the context of the Voyage 2050 initiative. In fact, the ΛCDM signal guarantees a discovery and the sensitivity to such a wide variety of new physics opens the door to an enormous uncharted territory. With up-coming ground-based and balloon experiments expected to start collecting data in the next 1-3 years, this is an exciting time to do physics with CMB SDs.In light of these considerations, the thesis opens by reviewing the topic of CMB SDs. The theory of SDs, the many sources, the experimental status and its future perspectives are overviewed in the manuscript in a pedagogical and illustrative fashion, aimed at waking the interest of the broader community in regard to this fascinating and promising topic.This introductory premises sets the stage for the presentation of the first main contribution of the thesis to the field of CMB SDs: their implementation in the cosmological Boltzmann solver CLASS and the parameter inference code MontePython. The CLASS+MontePython pipeline is publicly available, fast, it includes all sources of SDs within ΛCDM and many others beyond that, and allows to consistently account for any observing CMB SD experimental setup. As such, it is unique and fully opens the door to the cosmological exploration of CMB SDs.By means of this newly developed numerical tools, the second main contribution of the thesis consists in showcasing the versatility and competitiveness (in terms of constraining power) of CMB SDs for all aforementioned cosmological models as well as for a number of different mission designs and data set combinations. Among others, the results presented in the thesis highlight the fact that CMB SDs could impose the by far most stringent constraints (in the relevant regions of parameter space) on features in the primordial power spectrum as well as on many dark matter and primordial black holes properties. Also, they would be able to play a significant role even within the ΛCDM model.Finally, the manuscript is disseminated with (20) follow-up ideas that naturally extend the work carried out so far, also highlighting how rich of unexplored possibilities the field of CMB SDs still is. The hope is that these suggestions will become a propeller for further interesting developments.