BICEP/Keck Array

The data release page and papers are located at: bicepkeck.org

The primary goal of BICEP/Keck Array telescopes was to measure the very faint polarization of the cosmic microwave background (CMB). The CMB is a nearly perfect, uniform black body at 2.7 K, with degree-scale temperature anisotropy of about 0.1 mK and polarization on the order of microkelvin. This radiation was emitted 380,000 years after the Big Bang, at the time of recombination, when the Universe first became transparent to light. The temperature anisotropy and polarization of the CMB are some of the most powerful ways of understanding the early Universe. Cosmologists believe the Universe experienced a rapid period of cosmic inflation during its first fraction of a second, exponentially expanding from a dense, hot subatomic volume. Many models of inflation predict that this rapid acceleration would have generated gravitational waves that would remain energetic enough 380,000 years later to leave an imprint on the CMB. BICEP/Keck Array telescopes targeted this imprint by measuring the pure-curl component of the CMB polarization on degree angular scales, which is largely free of contamination from sources other than primordial gravitational waves.

BICEP2 detected this B-mode polarization in its three year data set with a strong significance.  Interpreting this as primordial gravitational waves, this places a constraint on the tensor to scalar ratio at r=0.20+0.07/-0.06.  This is strong evidence for cosmic inflation, as well as the first direct image of gravitational waves, and direct empirical evidence of quantized gravity. However, the joint analysis with Planck satellite data followed and showed that the galactic dust seems to contribute much of this B-mode polarization measured by BICEP2. BICEP2 observed at a single frequency band. Multi-frequency observation with the series of BICEP/Keck Array telescopes will help distinguish the origin of the observed B-mode polarization.

BICEP2 maps achieved the unprecedented sensitivity of 85 nK per square degree pixel over an effective area of 400 deg^2.  The approach taken in the BICEP/BICEP2/Keck Array/BICEP3 family of telescopes is aimed at increasing the sensitivity of the telescope at the degree angular scales by using cold, refractive optics. All of the optics are cooled to 4K, reducing the optical loading and the noise levels, but limiting the beam size to 0.22 deg.

BICEP2 built upon the success of BICEP1, replacing horn-coupled bolometers with antenna-coupled TES arrays fabricated at JPL. This change allows an increase in the number of pixels, and an improvement in mapping speed of a factor of 10. BICEP2 was deployed in November, 2009, took data through December, 2012 from the Amundsen-Scott South Pole Station, which offers excellent environmental conditions for microwave astronomy. BICEP3 was deployed in the austral summer of 2014-2015 and has been observing at the south pole since. Keck Array consists of 5 receivers similar to BICEP2, with different configuration of observing frequencies, and have been observing since 2011.

KIPAC is a participant in BICEP/Keck Array telescope series through the group of Chao-Lin Kuo. The group also actively works on the next generation telescope, the BICEP Array, which is observing at the South Pole from 2019 (expected). Our collaborators on BICEP/Keck Array are Caltech, JPL, Harvard, Chicago, Minnesota, Berkeley, San Diego, British Columbia, and Toronto.

Gravitational waves from inflation generate a faint but distinctive twisting pattern in the polarization of the CMB, known as a "curl" or B-mode pattern. For the density fluctuations that generate most of the polarization of the CMB, this part of the primordial pattern is exactly zero. Shown here is the actual B-mode pattern observed with the BICEP2 telescope, with the line segments showing the polarization from different spots on the sky. The red and blue shading shows the degree of clockwise and anti-clockwise twisting of this B-mode pattern. Gravitational waves from inflation generate a faint but distinctive twisting pattern in the polarization of the CMB, known as a "curl" or B-mode pattern. For the density fluctuations that generate most of the polarization of the CMB, this part of the primordial pattern is exactly zero. Shown here is the actual B-mode pattern observed with the BICEP2 telescope, with the line segments showing the polarization from different spots on the sky. The red and blue shading shows the degree of clockwise and anti-clockwise twisting of this B-mode pattern.

BICEP2 calibration measurements at the South Pole in February 2010. The BICEP2 beam functions were measured by mapping a microwave source mounted above the nearby MAPO building, using a far-field flat mirror. Photo by J. Brevik.

The BICEP2 telescope at twilight, which occurs only twice a year at the South Pole. The MAPO observatory (home of the Keck Array telescope) and the South Pole station can be seen in the background. Photo by Steffen Richter.