The Cosmology Large Angular Scale Surveyor (CLASS) is an experiment to measure the signature of a gravitational-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 . 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, τ .Recently, the BICEP2 experiment announced the detection of B-mode polarization at of 40-200, 5 but it is unclear whether this signal is cosmological or Galactic in nature. These results have generated strong interest in complementary experiments and have highlighted the importance of multi-frequency observations for foreground subtraction. A measurement of B-modes in the CMB would constitute important evidence for inflation and a measurement of the energy scale at which inflation occured. The tensor-to-scalar ratios, r ≤ 0.1, being probed correspond to E ∼ 10 16 GeV, near grand-unified-theory (GUT) energy scales. The gravitational waves from inflation are our only probe of the physics at such enormous energies and at such early times, just 10 −35 seconds after the Big Bang. They would also provide the first firm evidence for the existence of quantum-gravitational effects. 6 Detecting primordial gravitational waves requires greater frequency coverage to definitively rule out Galactic foreground contamination, as well as a measurement of the B-mode signal over a wider range of angular scales to verify the full shape of the B-mode power spectrum.A number of experiments are searching for B-mode polarization. Notably, the Planck satellite has mapped the entire sky in nine frequency bands from 30 to 857 GHz, allowing measurement of CMB polarization over a broad range of angular scales with the ability to remove Galactic foreground contamination; however, it is yet to be seen whether Planck will have the ability to constrain this signal. In this paper we present the Cosmology Large Angular Scale Surveyor (CLASS), which is leading the effort to map the CMB polarization at large angular scales from the ground. CLASS will observe in four frequency bands centered on 38, 93, 148, and 217 GHz. CLASS is uniquely poised to measure inflationary gravitational waves through its ability to measure CMB polarization at the largest angular scales, a...

CMB-S4—the next-generation ground-based cosmic microwave background (CMB) experiment—is set to significantly advance the sensitivity of CMB measurements and enhance our understanding of the origin and evolution of the universe. Among the science cases pursued with CMB-S4, the quest for detecting primordial gravitational waves is a central driver of the experimental design. This work details the development of a forecasting framework that includes a power-spectrum-based semianalytic projection tool, targeted explicitly toward optimizing constraints on the tensor-to-scalar ratio, r, in the presence of Galactic foregrounds and gravitational lensing of the CMB. This framework is unique in its direct use of information from the achieved performance of current Stage 2–3 CMB experiments to robustly forecast the science reach of upcoming CMB-polarization endeavors. The methodology allows for rapid iteration over experimental configurations and offers a flexible way to optimize the design of future experiments, given a desired scientific goal. To form a closed-loop process, we couple this semianalytic tool with map-based validation studies, which allow for the injection of additional complexity and verification of our forecasts with several independent analysis methods. We document multiple rounds of forecasts for CMB-S4 using this process and the resulting establishment of the current reference design of the primordial gravitational-wave component of the Stage-4 experiment, optimized to achieve our science goals of detecting primordial gravitational waves for r > 0.003 at greater than 5σ, or in the absence of a detection, of reaching an upper limit of r < 0.001 at 95% CL.

The Cosmology Large Angular Scale Surveyor (CLASS) is mapping the polarization of the cosmic microwave background (CMB) at large angular scales (2 < ℓ ≲ 200) in search of a primordial gravitational wave B-mode signal down to a tensor-to-scalar ratio of r ≈ 0.01. The same data set will provide a near sample-variance-limited measurement of the optical depth to reionization. Between 2016 June and 2018 March, CLASS completed the largest ground-based Q-band CMB survey to date, covering over 31,000 square-degrees (75% of the sky), with an instantaneous array noise-equivalent temperature sensitivity of . We demonstrate that the detector optical loading (1.6 pW) and noise-equivalent power (19 ) match the expected noise model dominated by photon bunching noise. We derive a 13.1 ± 0.3 K pW−1 calibration to antenna temperature based on Moon observations, which translates to an optical efficiency of 0.48 ± 0.02 and a 27 K system noise temperature. Finally, we report a Tau A flux density of 308 ± 11 Jy at 38.4 ± 0.2 GHz, consistent with the Wilkinson Microwave Anisotropy Probe Tau A time-dependent spectral flux density model.

The Cosmology Large Angular Scale Surveyor (CLASS) is a telescope array that observes the cosmic microwave background (CMB) over 75% of the sky from the Atacama Desert, Chile, at frequency bands centered near 40, 90, 150, and 220 GHz. CLASS measures the large angular scale (1 • θ 90 • ) CMB polarization to constrain the tensor-to-scalar ratio at the r ∼ 0.01 level and the optical depth to last scattering to the sample variance limit. This paper presents the optical characterization of the 40 GHz telescope during its first observation era, from September 2016 to February 2018. High signal-to-noise observations of the Moon establish the pointing and beam calibration. The telescope boresight pointing variation is < 0.023 • (< 1.6% of the beam's full width at half maximum (FWHM)). We estimate beam parameters per detector and in aggregate, as in the CMB survey maps. The aggregate beam has a FWHM of 1.579 • ± .001 • and a solid angle of 838 ± 6 µsr, consistent with physical optics simulations. The corresponding beam window function has sub-percent error per multipole at < 200. An extended 90 • beam map reveals no significant far sidelobes. The observed Moon polarization shows that the instrument polarization angles are consistent with the optical model and that the temperature-topolarization leakage fraction is < 10 −4 (95% C.L.). We find that the Moon-based results are consistent with measurements of M42, RCW 38, and Tau A from CLASS's CMB survey data. In particular, Tau A measurements establish degree-level precision for instrument polarization angles.