ABS (Atacama B-mode Search)
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| ABS (Atacama B-mode Search) | |
|---|---|
| Name | Atacama B-mode Search |
| Acronym | ABS |
| Established | 2009 |
| Location | Parque Astronómico de Atacama, Chile |
| Telescope | 145 GHz polarimeter |
| Collaborators | Princeton University; Johns Hopkins University; Cardiff University; Universidad de Chile |
| Mission | Measure degree-scale B-mode polarization in the cosmic microwave background |
ABS (Atacama B-mode Search)
The Atacama B-mode Search was a ground-based cosmology experiment deployed to measure degree-scale B-mode polarization in the cosmic microwave background, combining instrumentation development, site logistics, and data analysis to probe inflationary gravitational waves and lensing signals. The project integrated expertise from teams at Princeton, Johns Hopkins, Cardiff, and Universidad de Chile while operating from the Atacama plateau near ALMA, leveraging heritage from experiments such as BICEP, POLARBEAR, and SPTpol.
Overview
ABS was conceived to target large-angular-scale polarization anisotropies in the Cosmic Microwave Background using a cryogenic polarimeter optimized for 145 GHz, aiming to constrain primordial tensor-to-scalar ratio parameters motivated by Inflation (cosmology), Cosmological parameters, and models explored in the context of Planck (spacecraft), WMAP, and ground-based surveys like South Pole Telescope and Atacama Cosmology Telescope. The collaboration drew on detector technologies advanced at Princeton University and observational strategies informed by campaigns at BICEP2, POLARBEAR, and Keck Array to address foreground separation challenges highlighted by Planck Collaboration. ABS targeted degree-scale B-mode patterns associated with recombination-era signatures and gravitational lensing from large-scale structure studies tied to Lambda-CDM and alternative early-universe scenarios discussed in the literature of Alan Guth, Andrei Linde, and Paul Steinhardt.
Instrumentation and Design
The ABS instrument featured a cryogenic refracting telescope with a rotating half-wave plate and a focal plane of transition-edge-sensor bolometers developed through partnerships involving Princeton University, Johns Hopkins University, and industrial vendors collaborating with National Institute of Standards and Technology teams. Optical design choices referenced implementations used by BICEP1, BICEP2, and POLARBEAR, while polarization modulation strategies echoed techniques employed by EBEX and SPIDER (balloon-borne experiment). Detectors were read out with multiplexing electronics inspired by systems from NIST, SRON, and university labs; thermal engineering benefited from cryostat experience at University of Chicago and California Institute of Technology. The instrument housing, calibration targets, and sidelobe controls were designed taking cues from infrastructure at Atacama Large Millimeter Array, Subaru Telescope, and Very Large Telescope projects to minimize systematic errors and control beam systematics comparable to those confronted by Planck and WMAP teams.
Observations and Site
Observations were conducted from the high-altitude Atacama Desert plateau in northern Chile, near facilities such as ALMA and the Parque Astronómico de Atacama, chosen for atmospheric transparency and logistical connectivity to institutions like Universidad de Chile and international consortia linked to European Southern Observatory. The observing strategy focused on low-foreground fields overlapping with surveys by Planck Collaboration, Herschel Space Observatory mapped regions, and ground-based instruments including ACT and SPTpol to enable multi-frequency foreground characterization with complementary data from WISE and IRAS. Scheduling, operations, and permitting involved coordination with Chilean agencies, local communities, and site partners similarly necessary for projects such as ALMA, AURA, and ESO-supported installations.
Data Reduction and Analysis
ABS data reduction pipelines adapted timestream cleaning, mapmaking, and power-spectrum estimation methods refined by teams from BICEP2, Keck Array, POLARBEAR, and SPTpol, incorporating polarization angle calibration techniques used by Planck, beam characterization approaches from Herschel teams, and noise modeling practices taught in analyses by WMAP and COBE. Foreground separation integrated templates and models developed in conjunction with results from Planck Collaboration, WMAP, and Galactic studies by groups at Max Planck Institute for Astrophysics and Harvard-Smithsonian Center for Astrophysics, while null tests and jackknife analyses mirrored best practices from BICEP and POLARBEAR results. Statistical inference employed likelihood frameworks comparable to those used in Planck cosmological parameter estimation, Bayesian model comparison methods championed by researchers at Cambridge University and University of Oxford, and Monte Carlo simulations drawing on software ecosystems established by HEALPix contributors and numerical toolkits associated with NASA and ESA missions.
Scientific Results
ABS produced maps and constraints on degree-scale B-mode polarization and provided limits on the tensor-to-scalar ratio r that complemented results from Planck Collaboration, BICEP2/Keck Array, and SPTpol, contributing to joint analyses combining multi-experiment datasets. The experiment characterized polarized Galactic foregrounds in fields imaged by Herschel and Planck, informing component-separation efforts related to studies by Dame (astronomer)-style surveys and Galactic magnetic field research connected to teams at Max Planck Institute for Radio Astronomy, Jansky VLA groups, and Stephan Beck-adjacent projects. ABS’s systematics controls and null tests influenced interpretations of low-multipole polarization measurements relevant to inflationary model discrimination advanced by theorists like Alan Guth and Andrei Linde and observational comparisons with constraints from WMAP and Planck.
Collaboration and Funding
The collaboration consisted of researchers and engineers from institutions including Princeton University, Johns Hopkins University, Cardiff University, Universidad de Chile, and partners who had previously contributed to projects such as BICEP, POLARBEAR, and SPT. Funding and logistical support were provided through national research agencies and foundations analogous to National Science Foundation (United States), Science and Technology Facilities Council (United Kingdom), and Chilean funding bodies, with in-kind institutional contributions similar to arrangements used by ALMA, ESO, and AURA consortia. Collaborative governance, data-sharing policies, and publication practices followed norms established in large collaborations exemplified by Planck Collaboration and BICEP2/Keck Array.
Legacy and Impact on CMB Research
ABS’s technical developments in cryogenic polarization modulation, detector implementation, and systematic-control strategies influenced subsequent experiments and instrument designs at institutions working on Simons Observatory, CMB-S4, and upgrades to SPT-3G and ALMA-adjacent instrumentation. Its datasets and methodologies contributed to cross-experiment comparisons with Planck, BICEP/Keck, and ACT that continue to shape constraints on inflationary models developed by theorists at Princeton University and Institute for Advanced Study, and to inform Galactic foreground modeling efforts pursued by teams at Max Planck Institutes and Harvard. The project remains a reference in the community for lessons learned about site operations in the Atacama Desert, instrument calibration approaches used by EBEX and SPIDER, and collaborative frameworks similar to those of major observatory consortia.