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| Cosmology Large Angular Scale Surveyor | |
|---|---|
| Name | Cosmology Large Angular Scale Surveyor |
| Abbreviation | CLASS |
| Type | Ground-based microwave telescope array |
| Established | 2016 |
| Location | Atacama Desert; South Pole |
| Operator | Princeton University; Cardiff University; NIST; Jet Propulsion Laboratory |
| Purpose | Measurement of cosmic microwave background polarization on large angular scales |
Cosmology Large Angular Scale Surveyor is a ground-based experimental program to measure polarization of the cosmic microwave background on angular scales sensitive to primordial gravitational waves and reionization. The project operates an array of cryogenic polarimeters at multiple sites to separate polarized foregrounds and characterize the large-angle E-mode and B-mode signals relevant to inflationary models. CLASS is part of a generation of experiments complementing satellite missions and interferometers in the effort to constrain parameters from early-Universe physics.
CLASS was conceived to target the low multipole regime of the cosmic microwave background with broad frequency coverage and rapid sky modulation. Motivated by theoretical work from researchers associated with Alan Guth, Andrei Linde, Viktor Mukhanov, and observational milestones achieved by missions like WMAP and Planck, CLASS emphasizes control of systematic errors on degree and larger scales. The instrument suite employs technologies developed at laboratories including National Institute of Standards and Technology, NASA Jet Propulsion Laboratory, and university groups at institutions such as Princeton University and Cardiff University. The collaboration engages with community efforts including the Simons Observatory, BICEP/Keck, and legacy datasets from COBE.
The CLASS instrument uses cryogenic bolometric polarimeters with polarization modulators and dichroic optics to observe at multiple bands designed to separate synchrotron and thermal dust. Detector development built on heritage from Transition-edge sensor work at NIST and bolometer arrays used by SPT and ACT. The telescope optics follow Gregorian or crossed-Dragone designs similar to those used by Planck HFI and LiteBIRD concepts, while fast polarization modulation shares techniques with instruments like POLARBEAR and QUIET. Frequency bands (e.g., ~40 GHz, ~90 GHz, ~150 GHz) map to foreground components characterized by studies from Fermi Gamma-ray Space Telescope analyses of cosmic-ray electrons and by surveys like Haslam 408 MHz survey and IRAS dust templates. Cryogenics rely on closed-cycle refrigerators and dilution refrigerators developed in collaboration with engineering teams at JPL and fabrication facilities at Princeton Plasma Physics Laboratory.
CLASS operates at high, dry sites to minimize atmospheric emission and achieve stable long-term observations. Primary deployments include the Atacama Desert plateau and facilities at the South Pole Station, co-located with infrastructures supporting BICEP/Keck and SPT experiments. The observing strategy combines constant-elevation scans, azimuthal modulation, and stepped half-wave plates inspired by scanning strategies used by WMAP and Planck to reduce 1/f noise and ground pickup. Field selection accounts for overlap with ancillary surveys such as SDSS, DES, and radio catalogs like the NVSS to enable cross-correlation and component separation. The collaboration adopted data-taking schedules informed by atmospheric characterization campaigns similar to those conducted by ALMA and APEX.
CLASS data processing pipelines implement timestream filtering, mapmaking, component separation, and power spectrum estimation, integrating algorithms developed in the context of HEALPix and maximum-likelihood map solvers used by Planck and WMAP. Systematics mitigation uses null tests pioneered in analyses from BICEP2 follow-ups and component separation techniques compared to those from SMICA and Commander. Foreground modeling employs templates and parametric fits referencing results from Planck Collaboration dust and synchrotron products, and cross-spectra with surveys such as WMAP and Haslam 408 MHz survey. Statistical inference uses Markov Chain Monte Carlo tools like those employed by the Cosmological Monte Carlo (CosmoMC) framework and model comparison techniques parallel to analyses from Polarbear and ACT teams.
CLASS aims to constrain the tensor-to-scalar ratio r at degree angular scales, measure the optical depth to reionization tau through large-angle E-mode polarization, and characterize polarized foregrounds across frequencies. These targets directly test inflationary scenarios proposed by theorists related to Guth and Linde and complement constraints from Planck, BICEP/Keck, and the Simons Observatory. Early CLASS results have reported maps and cross-spectra that improve low-ell polarization sensitivity and aid in refining tau estimates, contributing to joint analyses with datasets from Planck and WMAP. CLASS measurements feed into cosmological parameter chains alongside results from large-scale structure surveys like BOSS and DESI, impacting derived limits on parameters such as the tensor-to-scalar ratio and reionization history.
The CLASS collaboration comprises researchers from multiple universities and laboratories including Princeton University, Cardiff University, NIST, JPL, University of Manchester, and other institutions that have historically participated in CMB experiments like UC Berkeley and Harvard University. Governance follows structures resembling other large collaborations such as Planck Collaboration and BICEP/Keck with science working groups, instrument teams, and data analysis groups. Funding and logistical support have been provided through agencies and programs with histories of supporting astrophysics infrastructure, analogous to investments from NASA, NSF, and national research councils associated with participating institutions.
CLASS contributes legacy low-ell polarization maps that complement satellite data from Planck and missions planned like LiteBIRD and support ground-based programs including the Simons Observatory and CMB-S4. By refining measurements of tau and improving constraints on primordial B-modes, CLASS influences theoretical model selection in works related to inflationary cosmology and impacts joint cosmological analyses with large-scale structure surveys such as DESI and Euclid. The instrument and analysis techniques developed by CLASS have informed detector technologies and scanning strategies adopted across the next generation of CMB experiments.