The search for primordial B-modes in the polarization of the cosmic microwave background with LSPE/SWIPE and LiteBIRD

The most ambitious challenge in Experimental Cosmology today is the preci- sion measurement of the polarized signal of the Cosmic Microwave Background (CMB). CMB was discovered in 1967 by Penzias and Wilson. It is a snapshot of the primordial universe and represents an essential source of information about all epochs of the universe. This experimental thesis concerns the study of polarization measurement techniques and the development of a new superconducting magnetic bearing to continuously rotate a cryogenic half-wave plate (HWP). The chapter 1 of this thesis focuses on the fundamentals of the cold dark matter model (ΛCDM) which is a parametrization of the Big Bang cosmo- logical model. It describes the constituents and the evolution of the universe. The ΛCDM model can be extended by adding cosmological inflation, a short period of exponential expansion in the very early universe. Inflation’s basic predictions regarding the universe large-scale geometry and structure have been borne out by cosmological measurements to date. Inflation makes an additional prediction as the existence of a background of gravitational waves, or tensor mode perturbations. At the recombination epoch, the inflationary gravitational waves (IGW) contribute to the anisotropy of the CMB in both total intensity and linear polarization, discussed deeply in the second part of the first chapter. The amplitude of tensors is conventionally parameterized by r, the tensor-to-scalar ratio at a fiducial scale, and its trace in the CMB polarization is a direct measure of the energy scale of inflation. Theoretical predictions of the value of r cover a very wide range. Conversely, a measurement of r can discriminate between models of inflation. The current upper limit is r < 0.06 at 95% confidence. The chapter 2 presents the Large-Scale Polarization Explorer (LSPE), an experiment composed of two instruments (the ground-based telescope STRIP and the balloon-borne counterpart SWIPE) which aims to measure the polarization of the CMB at large angular scale with a goal of r = 0.01. This thesis is mainly focused on the development of few important subsystems of SWIPE balloon. The detection of this tiny signal requires a very large array of polarization-sensitive detectors coupled to an imaging optical system, to obtain a wide field of view, thus maximizing the mapping speed. SWIPE will focus the incoming radiation on two large curved focal planes (at a temperature of 0.3 K) hosting 326 multi-mode pixels with Transition Edge Sensor (TES) thermistors, divided in the 3 frequency bands: 145GHz (30% bandwidth), 210GHz (20% bandwidth) and 240GHz (10% bandwidth). Chapter 3 describes the tests performed on the multi-mode pixel assembly. A custom cryogenic neoprene absorber was developed to reduce the background on the detector at a level similar to the one expected in flight, allowing to measure the main beam of the pixel assembly. The measured FWHM of the pixel assembly is 21°, slightly narrower than the expected one (24°), due to vignetting produced by the filters stack. Unfortunately this CMB polarization signal is well below the level of un- polarized foregrounds. This makes systematic errors due to temperature-to-polarization leakage particularly detrimental. Polarization modulators offer a solution to separate the polarized signal of interest from these unpolarized foregrounds. Many polarization modulation schemes exist, and a rapidly-rotating half-wave plate (HWP) is one of the most promising. The working principle of a polarimeter is discussed in chapter 4, where there is also an analysis of the main systematics introduced by a rotating HWP, particularly focused on HWP spurious signals and HWP wobbling. Chapter 5 is focused on the SWIPE polarization modulator unit which op- erates at 1.6 K to reduce the background on the detector produced by the HWP emission. On the other hand rotating an object at cryogenic temperature is not trivial, in particular because the dissipation becomes an issue. The technology adopted is based on a superconducting magnetic bearing (SMB) which can significantly reduce the friction. After introducing the basics of superconductivity, the baseline design is described. A large number of tests were performed on a room temperature mockup to optimize the motor configuration while room and cryogenic temperature tests were performed on the clamp mechanism (necessary to hold the bearing at room temperature and release it below the superconductive transition). The total heat load expected from the polarization modulator unit is < 25 mW. This value has to be confirmed during cryogenic test of the whole system which is not performed yet due to delays in the cryostat fabrication. The expected heat load from the polarization modulator represents less than 15% of the total heat load on the superfluid He reservoir, and is fully compatible with the operation of the instrument. Finally, chapter 6 presents LiteBIRD mission and the development of both polarization modulators of the medium and high frequency instruments. LiteBIRD is the next generation spacecraft [5], expected to be operative in ∼ 10 years, and will map CMB polarization 20 times deeper than Planck, with a total error of δr < 0.001, conservatively assuming equal contributions of statis- tical error, systematic error, and margin. The use of 3 continuously rotating HWPs (for the 3 telescopes of LiteBIRD) mitigates important systematic errors already observed in Planck data. Their development is more challenging than for SWIPE due to the spacecraft requirements and the Technology Readiness Level (TRL) required. A scaled baseline design and an optimized configuration are discussed. We find that the optimized one will meet the power budget with a 100% of margin.