The Q U Bolometric Interferometer for Cosmology (QUBIC)

QUBIC is a novel ground-based instrument that aims to detect the very faint B-mode polarisation pattern in the Cosmic Microwave Background. Since gravitational waves are the only way to produce this type of polarisation at large angular scales, B-modes are a key prediction of Inflation, the leading theory of the very early Universe. No confirmed primordial B-mode detection has ever been made. QUBIC uses bolometric interferometry, a technique that can combine the sensitivity of an imager with the immunity to systematic effects of an interferometer. It will directly observe the sky through an array of entry horns whose signals will be superimposed using a quasi- optical beam combiner. Images of the resulting interference fringes will be formed on two focal planes tiled with TES bolometers. This paper focuses on the design and modelling of the QUBIC beam combiner, including the feedhorn array, optics and shielding. The modelling was carried out using a combination of ray-tracing, a paraxial diffraction analysis and physical optics, with both commercial and in-house software.