Advanced ACTPol

Advanced ACTPol
Name	Advanced ACTPol
Mission type	Ground-based telescope upgrade
Operator	Atacama Cosmology Telescope collaboration
Location	Atacama Desert, Chile
Launch	Commissioned 2016

Advanced ACTPol

Advanced ACTPol is a receiver upgrade to the Atacama Cosmology Telescope that enhanced polarization sensitivity for measurements of the cosmic microwave background. The instrument improved measurements of temperature and polarization anisotropies, enabling tighter constraints on parameters such as the Hubble constant, neutrino masses, and inflationary models. The program involved an international collaboration of institutes and leveraged techniques from microwave detector development, cryogenics, and millimeter-wave optics.

Overview

Advanced ACTPol succeeded the original Atacama Cosmology Telescope focal-plane systems and built on heritage from instruments such as Planck (spacecraft), Wilkinson Microwave Anisotropy Probe, and experiments including BICEP2, POLARBEAR, SPTpol, and Keck Array. Its goals paralleled science objectives pursued by projects like Simons Observatory, CMB-S4, LiteBIRD, and ACT (project). The program aimed to map CMB polarization over wide sky areas to probe Lambda-CDM parameters, physics beyond Standard Model (physics), and signatures of primordial gravitational waves motivated by Inflation (cosmology), as well as to measure lensing consistent with studies from Dark Energy Survey, Euclid (spacecraft), and DESI.

Instrumentation and Design

The hardware integrated transition-edge sensors informed by developments at NIST, Caltech, Brookhaven National Laboratory, Argonne National Laboratory, and Lawrence Berkeley National Laboratory. Cryogenic techniques drew on experience from Jet Propulsion Laboratory, Fermi Gamma-ray Space Telescope, and Herschel Space Observatory teams. Frequency bands were chosen in coordination with millimeter facilities such as Atacama Large Millimeter Array, MUSTANG-2, and Planck Collaboration maps. Optics and polarization modulation referenced designs used by Max Planck Institute for Astrophysics, University of Pennsylvania, Princeton University, and Harvard University groups. Readout electronics built on time-domain and frequency-domain multiplexing innovations from SLAC National Accelerator Laboratory, Brookhaven, and NIST Boulder. The focal plane integrated technologies from MIT, University of Michigan, Cardiff University, and University of British Columbia collaborators to achieve low noise equivalent temperatures comparable to contemporaneous arrays on South Pole Telescope.

Observational Strategy and Survey Fields

Survey planning coordinated with observatories and surveys including Pan-STARRS, Sloan Digital Sky Survey, Hubble Space Telescope, and Subaru Telescope to choose overlap fields for cross-correlation. Fields targeted included deep southern stripes overlapping Dark Energy Survey footprints, regions used by BOSS and eBOSS, and legacy fields from COMAP and HERA. Scanning strategies used lessons from WMAP and Planck (spacecraft) to mitigate atmospheric noise akin to strategies at South Pole Station and Mauna Kea Observatories. Observations accounted for foregrounds traced by IRAS, WISE, Herschel Space Observatory, AKARI, and radio catalogs such as NVSS and FIRST.

Data Processing and Analysis Methods

Data reduction pipelines adapted map-making and component-separation techniques used in Planck Collaboration analyses and methods employed by BICEP2 teams, including time-ordered data filtering, beam characterization, and calibration against planets like Jupiter (planet) and Uranus (planet). Component separation leveraged cross-correlation with Herschel dust templates and radio source masks derived from ATCA, VLA, and ALMA catalogs. Lensing reconstruction borrowed algorithms developed by groups at Perimeter Institute, University of Toronto, and University of Chicago, and power-spectrum estimation incorporated Monte Carlo approaches used by SDSS analyses. Parameter inference relied on Markov chain Monte Carlo codes and tools from CosmoMC, CAMB, and collaborators at Institute for Advanced Study and Lawrence Livermore National Laboratory.

Scientific Results and Cosmological Constraints

Advanced ACTPol measurements refined estimates of the spectral index n_s and provided competitive constraints on the effective number of relativistic species N_eff, complementing limits from Planck (spacecraft) and laboratory bounds from KATRIN. Results impacted determinations of the Hubble constant H0 in dialogue with local distance-ladder results from teams around SH0ES and independent probes like Cepheid variables studies with Hubble Space Telescope. Neutrino mass sum limits informed particle-physics interpretations relevant to Super-Kamiokande and IceCube. Lensing maps enabled cross-correlation science with BOSS and cluster catalogs from ROSAT and eROSITA, while constraints on primordial non-Gaussianity connected to analyses by WMAP and theoretical work from Alan Guth and Andrei Linde communities. Polarization measurements provided limits on tensor-to-scalar ratio r that contributed to the global effort alongside BICEP/Keck and POLARBEAR constraints.

Collaborations and Funding

The collaboration comprised institutions including Princeton University, Cornell University, University of Pennsylvania, Yale University, University of Oxford, University of California, Berkeley, University of British Columbia, University of Toronto, Stanford University, Columbia University, Carnegie Institution for Science, McGill University, University of Chicago, University of Michigan, Brown University, University of Minnesota, and national labs such as Brookhaven National Laboratory, Lawrence Berkeley National Laboratory, and SLAC National Accelerator Laboratory. Funding came from agencies including National Science Foundation (United States), Department of Energy (United States), and international partners analogous to support seen for European Research Council-backed projects and national research councils.

Legacy and Future Developments

Advanced ACTPol influenced successor programs, informing instrument design for Advanced ACT, Simons Observatory, and CMB-S4 and contributing datasets used in synergy with surveys like LSST and missions such as Euclid (spacecraft) and Roman Space Telescope. The technology pathfinder role extended to detector and cryogenic advances adopted by future microwave astronomy projects at Atacama Large Millimeter Array adjunct experiments and informed community roadmaps discussed at meetings of American Astronomical Society and International Astronomical Union.

Category:Cosmic microwave background experiments