The COSmic Monopole Observer (COSMO): in quest of spectral distortion of the CMB

The Cosmic Microwave Background (CMB) is a cornerstone of modern cosmology. While its anisotropies have been extensively studied, its spectral properties remain largely unexplored. Measuring spectral distortions is a challenging objective, since the deviation from the perfect blackbody spectrum is very small, and requires extremely sensitive instrumentation, operating in the millimetre and submillimetre bands, where atmospheric emission and transmission represent major challenges, and effective strategies to cope with the atmosphere must be implemented. This thesis presents some conceptual and technical development of the COSMO (COSmic Monopole Observatory) experiment, a ground-based Fourier Transform Spectrometer (FTS) designed for deployment at Dome C, Antarctica, an exceptional site for millimetre astronomy due to its extremely low precipitable water vapour and stable atmospheric conditions. COSMO introduces a dual-modulation strategy based on a rotating wedge mirror and a moving translation mirror in a Martin-Puplett interferometer, allowing an effective separation of the astrophysical signal from the atmospheric contaminant. To assess this approach, an end-to-end simulation framework was developed to generate synthetic time-ordered data (TOD) from a realistic input signal and instrument model. Using this tool, the modulation frequencies and observational parameters were optimised, showing that adequate spectral reconstruction and atmospheric signal suppression are achievable within realistic mechanical and sampling constraints. Complementary to the simulation effort, a cryogenic testing campaign was carried out to characterise two candidates dilution refrigerators, one of which will cool the COSMO focal plane to 150 mK. One system achieved the expected performance, while the other exhibited anomalies likely due to an incorrect 3 He–4He mixture charge, indicating the need for further investigation and optimisation.