POLARBEAR

POLARBEAR. It was a ground-based cosmic microwave background (CMB) polarization experiment designed to measure the faint B-mode polarization patterns imprinted on the CMB by primordial gravitational waves from cosmic inflation and by gravitational lensing. The project, led by an international collaboration, operated a high-sensitivity telescope in the Atacama Desert of Chile to observe from one of the driest and highest astronomical sites on Earth. Its primary scientific aim was to probe the physics of the very early universe, particularly testing models of inflation, and to constrain the sum of neutrino masses through measurements of gravitational lensing of the CMB.

Overview

POLARBEAR was installed on the Huan Tran Telescope (HTT) platform at the Atacama Submillimeter Telescope Experiment (ASTE) site in the Chilean Andes. The experiment's initial deployment and first light occurred in early 2012, with subsequent observing campaigns conducted over several years. The instrument featured a refractive telescope coupled to an array of over 1,500 transition-edge sensor bolometers, cooled to a fraction of a degree above absolute zero, which provided exceptional sensitivity to the polarization of the CMB at 95 GHz. Key institutional partners included the University of California, Berkeley, the University of California, San Diego, the University of Colorado Boulder, the Kavli Institute for the Physics and Mathematics of the Universe in Japan, and several French laboratories under the Centre national de la recherche scientifique.

Scientific goals and design

The core scientific objective was the detection and characterization of the B-mode polarization signal. This signal has two potential origins: the faint imprint of primordial gravitational waves generated during inflation (often parameterized by the tensor-to-scalar ratio, r), and the distortion of the dominant E-mode polarization by intervening large-scale structure, known as gravitational lensing. POLARBEAR was specifically designed to have the sensitivity, angular resolution, and control of systematic errors necessary to separate these two components. Its design emphasized high-density focal plane arrays, continuous rotation of the entire telescope to mitigate instrumental polarization, and careful site selection at the Llano de Chajnantor Observatory to minimize atmospheric water vapor interference. The collaboration also focused on characterizing polarized foreground emission from sources like the Milky Way and extragalactic radio sources.

Observations and results

POLARBEAR made deep observations of three separate fields on the sky, covering a total area of approximately 30 square degrees. In 2014, the collaboration reported the first direct measurement of a non-zero B-mode polarization signal at sub-degree angular scales, which was confidently attributed to gravitational lensing of the CMB by the large-scale structure of the universe, a finding consistent with the Lambda-CDM model. This result provided a powerful independent probe of the distribution of matter, including dark matter. Subsequent analyses placed increasingly stringent upper limits on the amplitude of the primordial B-mode signal, helping to constrain models of inflation. The data also yielded precise measurements of the CMB's E-mode polarization power spectrum and contributed to constraints on the effective number of relativistic species and the sum of neutrino masses.

Collaboration and funding

The POLARBEAR project was a major international collaboration involving scientists and engineers from North America, Japan, and Europe. Leading U.S. institutions included the University of California, Berkeley, Lawrence Berkeley National Laboratory, and the Jet Propulsion Laboratory. In Japan, the effort was spearheaded by the Kavli Institute for the Physics and Mathematics of the Universe and the High Energy Accelerator Research Organization. Significant contributions came from French groups at the Institut d'Astrophysique de Paris and the Institut de Planétologie et d'Astrophysique de Grenoble. Primary funding was provided by the National Science Foundation, the United States Department of Energy, the Japan Society for the Promotion of Science, and the European Research Council under the FP7 framework program.

Related experiments and legacy

POLARBEAR was the first stage of a larger experimental program. It was succeeded by the Simons Array, which incorporated three POLARBEAR-type receivers observing at multiple frequencies (95, 150, and 220 GHz) on the HTT to better separate CMB signals from galactic foregrounds. The technologies and analysis techniques developed for POLARBEAR directly informed and enabled next-generation CMB experiments, including the Simons Observatory in Chile and the planned CMB-S4 project. Its work on characterizing gravitational lensing B-modes is a critical precursor for experiments aiming to detect the even fainter primordial signal, contributing to the broader global effort involving projects like the BICEP/Keck Array at the South Pole and the Atacama Cosmology Telescope.

Category:Cosmic microwave background experiments Category:Astronomical surveys Category:Scientific projects in Chile