Towards accurate measurements of primordial CMB B-modes: studies of instrumental systematic effects for the LiteBIRD mission

Over the past decades, observations of the Cosmic Microwave Background (CMB) have firmly established the ΛCDM model as the standard cosmological framework, enabling precise measurements of fundamental parameters and deep insights into the evolution of the Universe. Future experiments, such as the LiteBIRD satellite, aim to detect CMB polarization with unprecedented sensitivity, offering a unique opportunity to probe the physics of the early Universe and to address some of the most fundamental open questions in cosmology, such as the origin of the tiny initial inhomogeneities that later grew into the large-scale structures we observe today. One of the leading explanations for these initial conditions is cosmic inflation, a period of extremely rapid expansion in the early Universe. A key prediction of inflationary theories is the existence of a stochastic background of primordial gravitational waves, expected to leave a distinct signature in the CMB in the form of B-mode polarization. This signal is encoded in the tensor-to-scalar ratio, r, which LiteBIRD aims to measure with an accuracy of δr < 10−3. To reach this goal, the mission will employ Half-Wave Plates (HWPs) to modulate the incoming polarization and highly sensitive transition-edge sensor (TES) bolometers to detect it with the required precision. This thesis contributes to the LiteBIRD mission development by studying the impact of a few instrumental systematics on the experiment capability to constrain r. In this context, I have developed a dedicated simulation framework suitable for testing different instrumental configurations. This thesis highlights the importance of modeling coupled systematics, taking a further step forward in the data analysis pipeline of future CMB experiments. We start by introducing the scientific context and the physics of the CMB (Chapter 1), before presenting the LiteBIRD mission and some of its key design elements in Chapter 2. Here, I also describe the redesign phase that the instrument is undergoing. Chapter 3 outlines the data analysis pipeline of a CMB experiment, taking LiteBIRD as a reference case. In particular, Section 3.2.2 presents the LiteBIRD Simulation Framework, which I have contributed to and is further described in Tomasi et al. (2025). Chapter 4 then discusses specific systematic effects related to HWP and TES bolometers, presenting both their working principles and non-idealities, and describing how these have been modeled in time-ordered data (TOD) simulations to assess their impact on cosmological parameter estimation. By propagating the contamination through the steps of end-to-end simulations, we derive instrumental requirements for the level of knowledge of the detectors’ non-linearity to achieve the mission’s expected sensitivity. The results of this analysis have been published in Micheli et al. (2024). Similar studies have also been conducted on alternative designs of the instrument. Complementing the full-TOD approach, Chapter 5 introduces a fast map-based algorithm to simulate such effects directly at the map level. Lastly, in Chapter 6 we propose some mitigation techniques which could be adopted to meet the requirements derived in this thesis.