CLASS: The Cosmology Large Angular Scale Surveyor

TL;DR

CLASS is a multi-frequency telescope array designed to measure the polarization of the CMB over 70% of the sky, aiming to detect primordial gravitational waves and improve cosmological parameter constraints.

Contribution

It introduces a high-altitude, multi-frequency survey with broad sky coverage and systematic control to measure B-mode polarization and constrain inflationary models.

Findings

Projected sensitivity to tensor-to-scalar ratio r=0.01

Potential to measure optical depth τ with cosmic variance limit

Ability to distinguish Galactic foregrounds from CMB signals

Abstract

The Cosmology Large Angular Scale Surveyor (CLASS) is an experiment to measure the signature of a gravita-tional-wave background from inflation in the polarization of the cosmic microwave background (CMB). CLASS is a multi-frequency array of four telescopes operating from a high-altitude site in the Atacama Desert in Chile. CLASS will survey 70\% of the sky in four frequency bands centered at 38, 93, 148, and 217 GHz, which are chosen to straddle the Galactic-foreground minimum while avoiding strong atmospheric emission lines. This broad frequency coverage ensures that CLASS can distinguish Galactic emission from the CMB. The sky fraction of the CLASS survey will allow the full shape of the primordial B-mode power spectrum to be characterized, including the signal from reionization at low $\ell$. Its unique combination of large sky coverage, control of systematic errors, and high sensitivity will allow CLASS to measure or place upper limits on the tensor-to-scalar ratio at a level of $r=0.01$ and make a cosmic-variance-limited measurement of the optical depth to the surface of last scattering, $\tau$.