BICEP2

BICEP2
NASA/JPL-Caltech · Attribution · source
Name	BICEP2
Location	South Pole Station
Established	2006
Wavelength	Microwave (150 GHz)
Type	Polarimeter
Aperture	26 cm
Detectors	Transition-edge sensor bolometers
Status	Decommissioned (2012)

BICEP2 was a ground-based polarimeter experiment deployed at the South Pole Station to measure the polarization of the cosmic microwave background. It operated as part of a lineage of experiments including BICEP (experiment), Keck Array, and predecessors such as Degree Angular Scale Interferometer, aiming to probe signatures predicted by inflationary cosmology such as primordial B-mode polarization. The project involved collaborations among institutions like the Harvard–Smithsonian Center for Astrophysics, Caltech, and the Jet Propulsion Laboratory.

Overview

BICEP2 was conceived to map CMB polarization at arcminute to degree scales using sensitive bolometric detectors similar to those developed for experiments like Planck (spacecraft), WMAP, and South Pole Telescope. The instrument targeted the recombination-era signal predicted by single-field inflation models associated with theorists such as Alan Guth, Andrei Linde, and Paul Steinhardt. The collaboration included researchers from University of Chicago, Stanford University, University of California, Berkeley, and international partners including University of Cambridge and University of British Columbia.

Instrumentation and Design

The optical design was a compact refractor with a 26 cm aperture and an array of superconducting transition-edge sensor bolometers derived from technologies used by SPTpol, POLARBEAR, and ACT (Atacama Cosmology Telescope). Readout was accomplished with multiplexed SQUID amplifiers similar to systems deployed at NIST and Brookhaven National Laboratory. Cryogenics used a helium sorption refrigerator, with engineering contributions from groups at Jet Propulsion Laboratory and Caltech. The focal plane was optimized for a single frequency band centered near 150 GHz, informed by spectral characterizations from Planck (spacecraft) and WMAP dust and synchrotron maps.

Observations and Data Analysis

BICEP2 conducted deep integrations on a low-foreground patch of sky in the southern celestial hemisphere observed extensively by South Pole Telescope teams and overlapping surveys by Keck Array and Planck (spacecraft). Data reduction pipelines incorporated mapmaking algorithms comparable to those used by MAXIMA and BOOMERanG, including timestream filtering, pointing reconstruction with references to IRTF star cameras, and detector calibration cross-checks against measurements from Planck (spacecraft), WMAP, and radio source catalogs such as the AT20G survey. Statistical analysis employed maximum-likelihood estimators analogous to methods used in CosmoMC and parameter inference that referenced cosmological constraints from Lambda-CDM fits published by Planck Collaboration.

Initial Detection and Claims

In 2014 the team reported a detection of degree-scale B-mode polarization with a high signal-to-noise ratio in a peer-reviewed venue, invoking interpretations tied to primordial gravitational waves produced during inflation as described by Alan Guth and Andrei Linde. The claim referenced a tensor-to-scalar ratio parameter r ≈ 0.2, a value that, if primordial, would have profound implications for models associated with Chaotic inflation and particle-physics motivated constructions like those explored by Kehagias and Silverstein. The announcement generated rapid responses from observational groups including members of Planck Collaboration, Keck Array, and theorists such as Juan Maldacena and Alan H. Guth.

Subsequent Reanalysis and Results

Following the initial claim, joint analyses were performed combining BICEP2 data with foreground maps from Planck (spacecraft), polarized dust studies by teams at University of Oxford and Caltech, and microwave foreground models developed by groups associated with Fermi Gamma-ray Space Telescope data cross-correlations. The combined reanalysis showed that polarized thermal emission from interstellar dust in the Milky Way, documented in Planck Collaboration dust polarization maps, could account for a significant fraction of the observed B-mode signal, leading to revised upper limits on the tensor-to-scalar ratio. Subsequent joint papers with the Planck Collaboration and follow-up observations by the Keck Array and BICEP3 instrument tightened constraints on r, aligning results more closely with limits reported by Planck Collaboration cosmological parameter analyses.

Scientific Impact and Legacy

BICEP2 catalyzed intensive collaboration between experimentalists and data-modeling teams across institutions such as Harvard–Smithsonian Center for Astrophysics, Caltech, University of Chicago, and European Space Agency. The episode emphasized the importance of multi-frequency foreground characterization, influencing design choices in successor instruments like BICEP3, Keck Array, Simons Observatory, and CMB-S4. It stimulated theoretical reassessment of inflationary model priors among researchers including Andrei Linde, Alan Guth, and João Magueijo, and fostered improved data-sharing practices between ground-based projects and space missions such as Planck (spacecraft)]. The legacy includes technological advances in superconducting detectors, readout electronics, and analysis frameworks that benefit contemporary CMB polarization experiments.

Category:Cosmic microwave background experiments Category:South Pole Station