CMB B-mode Analysis

Generic inflationary models predict the existence of a background of primordial gravitational waves which would leave an imprint of "B-mode" polarization in the CMB. One of my important works is to analyze the CMB B-mode data from the BICEP/Keck telescopes and transform them into constraints on inflation models. These telescopes scanning over a sky patch of ≈ 600 square degrees yield multiple-frequency data which can 1) target the degree-scale inflationary B-modes at the recombination bump; and 2) separate the polarization foreground from Galactic thermal dust emission and Galactic synchrotron emission, since their spectral behaviors are different from that of CMB.

In particular, we use the BICEP2, Keck Array and BICEP3 95/150/220 GHz data up to the 2018 observing season to make polarization maps, compute B-mode power spectra and conduct the multi-component multi-spectral likelihood analysis. This "BK18" analysis delivers unprecedented discrimination power between inflation models. As shown in the accompanying figure, two once-popular classes of inflation models (with cosine law and monomial power law potential) are now strongly disfavored by the observation.

Telescope Mount

The BICEP/Keck collaboration has been building a larger telescope BICEP Array to replace the Keck Array telescope for tighter constraints on inflation models. BICEP Array (BA) will have four BICEP3-style receivers to host more detectors, and will expand the frequency coverage to 30/40/95/150/220/270 GHz to constrain the synchrotron and dust foreground dominating at two extremes of the microwave spectrum.

Our research group at the University of Minnesota is responsible for making a new telescope mount that can bear the weight of all new receivers and conduct scanning motion. We have worked on the assembly, electrical and mechanical tests of this telescope mount. The video records a partially assembled BA mount scanning at the Physics department's high bay, with a pulse-tube cooled BA receiver inside. It was subsequently disassembled and shipped to the South Pole.

Telescope Deployment

Another important part of my research is to install these telescopes at their exotic observation site. In the austral summer of 2019, the BA1 30/40 GHz receiver and the BA mount were deployed to the Amundsen-Scott South Pole station to exploit the exceptionally stable and transparent atmosphere for microwave frequency observation. We launched an entire summer season campaign for the Keck Array mount demolition, BA mount installation, BA1 receiver assembly and full system integration.

The attached photo displays the completely enclosed BA mount with BA1 receiver as well as three adapted Keck receivers at the end of the season. The first light BA1 maps were then generated by using merely 12 hours of data. This first light image shows faint but characteristic CMB temperature anisotropies in the BICEP/Keck sky patch, demonstrating that BA became fully functional. With three-year data coadded in 2023, BA1 Q/U maps show the first robust detection of cosmological polarization signals at 40 GHz from the same sky patch. More information for this deployment campaign is available in the BICEP Array construction blog.