Searching for Inflation Signals with the BICEP/Keck Telescopes

The inflation paradigm is now the leading candidate for a theory that describes the evolution of the early Universe. Nevertheless, generic inflationary models make a prediction that has not yet been observed — the existence of a background of primordial gravitational waves (PGWs). The magnitude of PGWs can be parameterized by the tensor-to-scalar ratio r, and they would leave an imprint of “B-mode polarization” in the Cosmic Microwave Background (CMB). Therefore, measuring the degree-scale B-mode polarization emerges as one of the most promising methods to detect or set limits on PGWs.

In the past decade, the BICEP/Keck collaboration has been operating a series of telescopes at the Amundsen-Scott South Pole Station for this science mission. These telescopes are compact refracting polarimeters mapping about 2% of the sky under the exceptionally stable and transparent atmosphere of the Geographical South Pole. They observe at a broad range of frequencies to account for the polarized foreground from Galactic synchrotron and thermal dust emission. My PhD research utilizes the data collected up to the 2018 observing season, in conjunction with the publicly available WMAP and Planck data. It particularly includes (1) the 3-year BICEP3 data which is the current deepest CMB polarization map at the foreground-minimum 95 GHz; and (2) the Keck 220 GHz map with a higher signal-to-noise ratio on the dust foreground than the Planck 353 GHz map. We fit the auto- and cross-spectra of these maps to a multicomponent likelihood model (ΛCDM+dust+synchrotron+noise+r) and find it to be an adequate description of the data at the current noise level. The likelihood analysis yields σ(r)=0.009. The inference of r from our baseline model is tightened to r=0.014+0.010-0.011 and r<0.036 at 95% confidence. They imply the BICEP/Keck B-mode data is the most powerful existing dataset for the constraints of PGWs, and it hence provides unprecedented discriminating power among classes of inflation theories in the r-n_s plane. We conclude that the natural inflation and monomial inflation models are now strongly disfavored by the data.