The Latest Constraints on Inflationary B-modes from the BICEP/Keck
  Telescopes
  For the past decade, the BICEP/Keck collaboration has been operating a series
of telescopes at the Amundsen-Scott South Pole Station measuring degree-scale
$B$-mode polarization imprinted in the Cosmic Microwave Background (CMB) by
primordial gravitational waves (PGWs). These telescopes are compact refracting
polarimeters mapping about 2% of the sky, observing at a broad range of
frequencies to account for the polarized foreground from Galactic synchrotron
and thermal dust emission. Our latest publication "BK18" utilizes the data
collected up to the 2018 observing season, in conjunction with the publicly
available WMAP and Planck data, to constrain the tensor-to-scalar ratio $r$. 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 ($\Lambda$CDM+dust+synchrotron+noise+$r$) and
find it to be an adequate description of the data at the current noise level.
The likelihood analysis yields $\sigma(r)=0.009$. The inference of $r$ from our
baseline model is tightened to $r_{0.05}=0.014^{+0.010}_{-0.011}$ and
$r_{0.05}<0.036$ at 95% confidence, meaning that the BICEP/Keck $B$-mode data
is the most powerful existing dataset for the constraint of PGWs. The up-coming
BICEP Array telescope is projected to reach $\sigma(r) \lesssim 0.003$ using
data up to 2027.
