Polarization of the Cosmic Microwave Background

The detection of primordial B-mode polarization in the Cosmic Microwave Background (CMB) is the subject of a worldwide effort due to its importance as a confirmation of the Inflationary model of Cosmology. A clear detection of polarization B-modes in the CMB at degree angular scales, distinguished from B-modes incurred by foreground effects, is evidence of primordial gravitational waves expected during the inflationary phase in the earliest moments of the Universe.

Bolometric Interferometry

The Q and U Bolometric Interferometer (QUBIC) is designed with particular attention to the limitation and control of systematic effects. QUBIC uses the technique of interferometry which leads to the possibility of doing “self-calibration”, a procedure that ensures exquisite control of instrumental systematic effects. Furthermore, the resulting frequency-dependent beam profile on the sky leads to the possibility of doing spectral imaging.

Bolometric interferometry is the marriage of techniques bringing together the great sensitivity and large bandwith of bolometers and the instrumental control and high fidelity imaging of aperture synthesis. Using this innovative approach, any residual systematic effect in the data will be largely independent from those in other experiments, thus providing a uniquely powerful dataset in the context of the worldwide experimental effort.

An imaging interferometer measures “visibilities” which are the complex (amplitude and phase) correlations between each antenna pair (baseline). In radio astronomy, the visibilities are recorded directly. A “correlator” digitizes the signals and multiplies pairs of signals to produce a stream of complex numbers, each of which corresponds to the cross correlation product of an antenna-pair. Channelization of the bandpass permits signal processing of individual, very narrow bands, and for each channel the signal is nearly monochromatic. In radio astronomy, large bandwidths are achieved by adding more digital electronics.

A bolometric interferometer takes advantage of the high sensitivity and large bandwidth of bolometers while also benefitting from the calibration technique possible with an imaging interferometer. The spatial sampling of the sky is generated by placing a cluster of back-to-back horns that behave effectively as electromagnetic nozzles. This horn cluster creates the u-v sampling of the aperture plane equivalent to what is done by a distribution of antennas in a radio array. For the bolometric interferometer, instead of sampling the signals and computing the cross correlations between antenna pairs, the interference pattern is imaged.

An additional feature of bolometric interferometry is the possibility to do spectral imaging. The synthesized beam is predicted to vary in a well-understood way with frequency. In particular, the beam secondary lobes are closer to the central lobe for higher frequency of incident radiation while the central lobe remains in the same place across the spectral band. As a result, the Time Ordered Data (TOD) effectively samples different electromagnetic frequencies as the beam passes over the same point in the sky.

This spectral selectivity can be deconvolved in the data post processing. The spectral resolution improves with the number of baselines as the synthesized beam has finer secondary lobes with a larger cluster of horns. Spectral imaging is an innovative feature of bolometric interferometry which gives QUBIC an important advantage over other CMB imagers.