BICEP2/Keck Array

BICEP2/Keck Array
Name	BICEP2/Keck Array
Established	2006
Location	South Pole
Type	Polarimeter
Affiliations	Caltech, Jet Propulsion Laboratory, Harvard University, Stanford University, University of Chicago

BICEP2/Keck Array BICEP2/Keck Array was a series of ground-based microwave polarimeters deployed at the South Pole to measure polarized anisotropy in the cosmic microwave background. It aimed to detect primordial gravitational waves and constrain models associated with inflationary cosmology, operating in concert with multiple observatories and institutions to map polarization at degree angular scales. The project interacted with parallel efforts and legacy datasets from experiments such as WMAP, Planck, and the Atacama Cosmology Telescope.

Overview

BICEP2/Keck Array built on a lineage of polarimeters including DASI, CBI, and QUaD, integrating techniques developed at Caltech, University of Chicago, Harvard–Smithsonian Center for Astrophysics, and Jet Propulsion Laboratory. The program targeted B-mode polarization predicted by inflationary scenarios developed by researchers influenced by work from Alan Guth, Andrei Linde, and Paul Steinhardt. The collaboration involved teams affiliated with Stanford University, University of Minnesota, Cardiff University, Imperial College London, and other institutions active in observational cosmology. Data products were compared with maps and analyses from Planck and cross-checked against surveys by South Pole Telescope and the Keck Observatory (instrument names distinct from the Keck Array).

Instrumentation and Design

The instrument suite used refracting telescope optics with large-aperture cryostats and focal planes populated by antenna-coupled bolometers developed from detector technology pioneered at NASA, Jet Propulsion Laboratory, and groups at Caltech and University of California, Berkeley. The Keck Array comprised multiple cryostats modeled after the BICEP2 design, enabling frequency diversity with receivers tuned to bands near 95 GHz, 150 GHz, and 220 GHz to disentangle cosmological signal from foregrounds observed by Planck and the IRAS. Polarization modulation relied on careful control of instrument polarization angles and baffling techniques informed by methodologies used by POLARBEAR, SPTpol, and the BLAST. Cryogenic refrigeration systems shared engineering heritage with systems in use at South Pole Telescope and satellite missions supported by NASA and ESA.

Observations and Data Analysis

Observations occurred during austral winters, leveraging the dry, stable atmosphere near Amundsen–Scott South Pole Station to achieve long integration times comparable to campaigns by Planck and ground arrays like Atacama Cosmology Telescope and South Pole Telescope. Data reduction pipelines incorporated time-domain filtering, mapmaking algorithms inspired by pipelines from WMAP, foreground component separation approaches used by Planck Collaboration, and null tests developed alongside analyses performed for POLARBEAR and QUIET. Cross-correlation with external datasets—such as those from Planck, Herschel Space Observatory, and radio surveys undertaken by Very Large Array teams—was used to assess Galactic dust and synchrotron contamination by leveraging templates from IRAS and COBE legacy data.

Scientific Results and Interpretation

Initial maps revealed polarization patterns analyzed in the language of E-modes and B-modes, concepts central to studies by Zaldarriaga–Seljak formalisms and work by Wayne Hu and Martin White. Early results claimed an excess in degree-scale B-mode power that, if cosmological, would imply tensor-to-scalar ratio constraints relevant to inflationary models of Andrei Linde and Alan Guth. Subsequent joint analyses with the Planck Collaboration refined estimates of the tensor-to-scalar ratio r and placed tighter limits informed by foreground modeling techniques akin to those used in Planck2015 and later releases. The combined datasets constrained certain classes of single-field slow-roll inflation and informed theoretical work by researchers such as Liddle and Lyth and model builders in high-energy cosmology.

Systematic Errors and Controversies

The initial interpretation of B-mode excess prompted extensive scrutiny, particularly concerning polarized Galactic dust emission characterized in surveys by Planck and earlier models from IRAS and COBE. Debates invoked methodologies and statistical procedures also discussed in contexts involving Hankel transforms and power-spectrum estimation approaches used by WMAP and Planck. Follow-up joint publications with Planck collaborators revised the significance of the signal, attributing much of the measured power to polarized dust rather than primordial gravitational waves, echoing controversies seen in other high-profile cosmological claims and prompting methodological refinements similar to those after results from HST Key Project and reevaluations like those following Supernova Cosmology Project analyses.

Collaborations and Operational History

The project was a collaboration among a broad set of institutions, with leadership and contributors drawn from Caltech, Harvard University, Stanford University, University of Chicago, University of Minnesota, Cardiff University, Imperial College London, Jet Propulsion Laboratory, and others. Operations at Amundsen–Scott South Pole Station involved logistical coordination with United States Antarctic Program and engineering support reminiscent of deployments by the South Pole Telescope team and logistic chains used by National Science Foundation-supported Antarctic programs. The Keck Array and BICEP instruments influenced subsequent CMB polarization experiments, informing design choices for projects such as Simons Observatory, CMB-S4, and successor efforts by teams affiliated with SLAC National Accelerator Laboratory and Fermilab.

Category:Cosmic microwave background experiments