Séminaires communs des Masters

"Astrophysique"

et

"Physique Subatomique et Astroparticules"

Primordial CMB B-modes and QUBIC, the QU Bolometric Interferometer for Cosmology

par Jean-Christophe HAMILTON (APC, Paris)
 

Résumé

One of the major challenges of modern cosmology is the detection of B-mode polarization anisotropies in the CMB. These originate from tensor fluctuations of the metric produced during the inflationary phase. Their detection would therefore constitute a major step towards understanding the primordial Universe. The expected level of these anisotropies is however so small that it requires a new generation of instruments with high sensitivity and extremely good control of systematic effects.

The BICEP2 Collaboration has announced in march 2014 the detection of this primordial B-mode polarization [1] but the Planck Satellite has recently released an article [2] expressing serious and motivated doubts about the primordial nature of the signal seen by BICEP2, pointing towards a likely contamination by the polarized emission of dusts in our Galaxy.

We are constructing the QUBIC instrument [3] based on the novel concept of bolometric interferometry, bringing together the sensitivity advantages of bolometric detectors with the systematics effects advantages of interferometry. The instrument will directly observe the sky through an array of entry horns whose signals will be combined together using an optical combiner. The whole set-up is located inside a cryostat. Polarization modulation will be achieved using a rotating half-wave plate and interference fringes will be imaged on two focal planes tiled with bolometers, one at 150 GHz and the second à 220 GHz allowing to control the possible contamination of the signal by galactic dust polarized emission.

We show that QUBIC can be considered as a synthetic imager, exactly similar to a usual imager but with a synthesized beam formed by the array of entry horns. Scanning the sky provides an additional modulation of the signal and improve the sky coverage shape. The usual techniques of map-making and power spectrum estimation can then be applied. We show that the sensitivity of such an instrument is comparable with that of an imager with the same number of horns. We anticipate a low level of beam-related systematics thanks to the fact that the synthesized beam is determined by the location of the primary horns. Other systematics should be under good control thanks to the selfcalibration technique [4], specific to our concept, that will permit the accurate determination of most of the systematics parameters.

References

[1] Detection of B-mode Polarization at Degree Angular Scales by BICEP2, The BICEP2 Collaboration, Phys. Rev. Lett. 112, 241101, 2014 (arXiv:1403.3985) 

[2] Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes (arXiv:1409.5738)

[3] The QUBIC Collaboration, Astroparticle Physics, Volume 34, Issue 9, p. 705-716., 2011 (arXiv:1010.0645)

[4] Bigot-Sazy, M.-A.; Charlassier, R.; Hamilton, J.-Ch.; Kaplan, J.; Zahariade, G., Astronomy & Astrophysics, Volume 550, id.A59, 11 pp., 2013 (arXiv:1209.4905)