This award supports new observations with the Cosmology Large Angular Scale Surveyor (CLASS). The main science goals of CLASS are (1) to measure when the first stars lit up the universe and (2) to detect and characterize primordial gravitational waves created at the start of the universe. The first goal is important for understanding how galaxies formed and also improves our ability to measure the mass of fundamental particles called neutrinos, the study of which is a frontier in physics. The second goal will tell us what happened at the earliest moments of the universe and connects gravity with quantum mechanics, a long-term quest in physics. To accomplish these goals, telescopes with new technologies have been developed over the past decade and deployed high in the Chilean Andes, in the Atacama Desert. This is a premier site for observations of the Cosmic Microwave Background (CMB), which is the afterglow of the hot early universe. CLASS probes the largest-angular-scale (more than 10 degrees across the sky) variations in the CMB polarization to reach its science goals. Broader societal impacts include rigorous training of students and postdocs in the US and Chile. CLASS reaches the public through public events at universities and local schools, Quarknet high-school teacher training, and development of high-school-level cosmology modules. Publications and data will be publicly distributed. Transformational data on the early universe will be of enormous interest to physicists, astronomers, students, and the general public.
CLASS is unique among current CMB surveys in targeting CMB polarization corresponding to angular multipoles down to ell of order 2. At these scales, scattering of CMB photons by the ionized intergalactic medium (post reionization) alters the CMB polarization angular spectra (both E and B-modes). This distinctive low-ell signature in the angular power spectrum is targeted by CLASS to measure the optical depth to reionization at the cosmic-variance limit and the tensor-to-scalar ratio at the 0.01 level. CLASS uses four telescopes to measure the CMB polarization over 70% of the sky. The telescope array observes at four microwave frequencies (40, 90, 150, and 220 GHz) to differentiate between the CMB signal and the microwave emission from our Galaxy. The key technology enabling the low-ell measurement through low-frequency noise reduction is rapid polarization amplitude modulation with a Variable-delay Polarization Modulator. Additional technology developments include novel microwave optics, cryogenic transition-edge sensors, and a custom high-performance data pipeline. The initial 5-year CLASS survey (funded under a previous MSIP), was begun in 2016. This grant allows the team to capitalize on developments in instrumentation, survey strategy, and data analysis pipeline from the first five years, and acts as a bridge to future, enhanced measurements.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
