Measurement of CMB B-mode polarisation with the LiteBIRD satellite mission: development of innovative data analysis techniques.


The Cosmic Microwave Background (CMB) radiation is a relic emission from 380 000 years after the Big-Bang at the time of decoupling between matter and radiation. The small CMB temperature and polarisation fluctuations, induced by quantum perturbation generated in the early Universe, contain precious information about the physics of the primordial Universe and its physical content. Several experiments, including the Planck satellite mission of ESA have measured those perturbations with high accuracy, leading to per cent precision on the determination of cosmological parameters. The objective of the community is to measure the CMB B-mode polarization pattern (negative parity), which are the imprint of primordial gravitational waves (or tensor modes) generated during the inflation period.  This signal, undetected today, provide a unique probe of new physics such as quantum gravity driving this very early phase of the Universe. The LiteBIRD mission of the Japanese space agency (JAXA) is currently in phase A-2 of JAXA and of the French space agency (CNES). The main goal of the mission is to measure the primordial B-mode at large angular scale of the sky with an accuracy on the tensor-to-scalar ratio r of less than 10-3.
The measurement of B-mode polarization is complex because of the presence of polarised astrophysical foreground emissions with an amplitude higher than the CMB. It also requires an unprecedented control of systematic effects originating from the instrument. Several component separation techniques have been developed in the community based on diverse assumptions on foregrounds. There is a whole class of methods, so called ‘blind methods’, that are currently being studied in the context of B-mode polarisation measurements, while they have been proven successful for CMB temperature estimation with Planck. The SMICA method, developed at APC is one of the most promising. The PhD student will adapt the SMICA method for the treatment of CMB polarisation with LiteBIRD. This will require incorporating new models based on nowadays best knowledge of foregrounds. In a second phase, he will also incorporate the treatment of instrumental effects focusing particularly on the beam transfer function, and half wave plate systematic effects (two of the most important effects), fitting well-chosen parameters after modelling. The work will involve the development of instrumental modelling and data simulations to incorporate the systematic effects in the timestreams and propagating those into the maps after map-making. The impact on the recovery of inflation parameters will be carefully assessed.
Those studies will be performed within a large consortium including researchers in Europe and Japan. The student will benefit from the CMB-Inflate European funding financing long visits in Japan as this thesis project fit with the objective of the CMB-Inflate project.


Guillaume Patanchon






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