Challenges and prospects of CMB cosmology: from inflation to galaxy clusters

Over the last 50 years, cosmology has evolved from a philosophy to a precision science. This has been made possible by several discoveries and measurements, the first of which is the Cosmic Microwave Background (CMB). The detection and subsequent observations of the CMB have confirmed the Big Bang theory and the ΛCDM model of the universe, constraining several of its cosmological parameters. However, current observations have not yet exhausted the scientific potential of this observable. In this thesis, I describe two open problems of observational cosmology that can be addressed with observations of the CMB, I present the current and future instrumentation used to make these measurements, and I develop calibration pipelines and requirements with a particular focus on detectors’ gain. After an introduction (Chapter 1) to the discovery, characteristics, and properties of the CMB, Chapter 2 discusses the LiteBIRD experiment. LiteBIRD is an upcoming satellite mission that will observe the polarization of the CMB on large angular scales with unprecedented sensitivity. The main scientific goal of the mission is to observe the B-Modes of the CMB and measuring the tensor-to-scalar ratio with an accuracy of δr = 10−3 . Such an achievement would enable researchers to place strong constraints on the amplitude of gravitational waves at the time of recombination and on the physics of inflation. Chapter 3 presents a minimum variance gain calibration approach for space missions such as LiteBIRD that aim to observe the CMB on large angular scales. The proposed methodology is not intended to be the definitive calibration pipeline for the instrument but rather to be a general approach based on well-founded assumptions. This allows to set instrumental requirements compatible with a variety of calibration pipelines that can be benchmarked and tested against each other. This calibration method is first discussed from a theoretical standpoint and then used to derive the optimal size of each tod chunk for the gain calibration of LiteBIRD. Finally, instrumental requirements are derived for gain stability, 1/f noise, and the maximum acceptable thermal fluctuations of the focal plane. Chapter 4 introduces the problem of the missing baryons and how this open question could be solved by measuring the imprint left on the CMB at small angular scales by the Sunyaev Zeldovich effect. It then introduces Mistral, a new cryogenic camera installed at the focus of the Sardinia Radio Telescope whose main objective is to make such observations and detect the presence of a Worm-Hot Intergalactic Medium (WHIM). Chapter 5 describes the activities carried out during the development and commissioning phases of the instrument. It first describes the housekeeping software and the hardware it manages; it then reports the activities and performances of the instrument during commissioning. The cryostat that houses the detector array and the procedures for operating it have been optimized; the instrument can achieve an operating temperature for the detector array of 203 mK and maintain it for over 20 h. This achievement was made particularly challenging by the need to place the cryostat in the Gregorian room of SRT; this forces the pulse tube cryocooler to work outside the tilt range for which it is designed and imposes the need to operate it using 110 m He lines. Finally, the responsivity of the detectors to thermal fluctuations of the focal plane was studied, both in terms of additional noise introduced into the data and in terms of induced gain variations of the detectors.