Cosmology Large Angular Scale Surveyor

The Cosmology Large Angular Scale Surveyor (CLASS) is an array of microwave telescopes which has been observing since 2016 from a high-altitude site in the Atacama Desert of Chile as part of the Parque Astronómico de Atacama. The CLASS experiment aims to improve our understanding of cosmic dawn when the first stars turned on and to test the theory of cosmic inflation by making precise measurements of the polarization of the Cosmic Microwave Background (CMB) over 65% of the sky at multiple frequencies in the microwave region of the electromagnetic spectrum.

To date, CLASS has produced maps of a majority of the sky at frequencies of 40 and 90 GHz (7.5 mm and 3.3 mm wavelength, respectively), constraints on circular polarization in the CMB, a detection of circular polarization from the atmosphere, and measurements of the disk-averaged microwave brightness temperature of Venus.

CLASS is addressing two primary science goals. The first is to improve our understanding of "cosmic dawn," when the first stars lit up the universe. Ultraviolet (UV) radiation from these stars stripped electrons from atoms in a process called reionization. The freed electrons scatter CMB light, imparting a polarization that CLASS measures. Constraints on reionization from the CMB are critical in reconciling astronomers' understanding of reionization with results from the James Webb Space Telescope that indicate that galaxies may have formed earlier than previously thought. In this way CLASS can improve our knowledge of when and how cosmic dawn occurred. A better understanding of cosmic dawn will also help other experiments measure the sum of the masses of the three known neutrino types using the gravitational lensing of the CMB.

A second science goal of CLASS is to test the theory of inflation. In physical cosmology, cosmic inflation is the leading theory of the very early universe; however, observational evidence for inflation is still inconclusive. Inflationary models generically predict that a gravitational-wave background (GWB) would have been produced along with the density perturbations that seed large-scale structure. Such an inflationary GWB would leave an imprint on both the temperature and polarization of the CMB. In particular it would leave a distinctive and unique pattern of polarization, called a B-mode pattern, in the CMB polarization. A measurement of B-mode polarization in the CMB would be important confirmation of inflation and would provide a rare glimpse into physics at ultra-high energies.

CLASS is also furthering our understand of our own Milky Way Galaxy and searching for evidence of exotic new physics through constraining circular polarization in the CMB and large-scale anomalies. (See the Low multipoles and other anomalies section of the cosmic microwave background article for more information on the latter.)

The CLASS instrument is designed to survey 65% of the sky at millimeter wavelengths, in the microwave portion of the electromagnetic spectrum, from a ground-based observatory with a resolution of about 1° — approximately twice the angular size of the sun and moon as viewed from Earth. The CLASS array consists of two altazimuth mounts that allow the telescopes to be pointed to observe different patches of sky. The four CLASS telescopes observe at a range of frequencies to separate emission from our galaxy from that of the CMB. One telescope observes at 40 GHz (7.5 mm wavelength); one telescope observes at 90 GHz (3.3 mm wavelength) with a second 90 GHz telescope planned in the future; and the fourth telescope observes in two frequency bands centered at 150 GHz (2 mm wavelength) and 220 GHz (1.4 mm wavelength). Two separate telescopes, observing at different frequencies, are housed on each mount. The 90 GHz telescope detector array was upgraded in 2022 to significantly increase sensitivity. In 2024 the variable-delay polarization modulator (VPM, see below for more details) for the CLASS 90 GHz telescope was replaced with a rotating reflective half-wave plate (HWP) to concentrate on improved sensitivity for linear polarization.

The CLASS instrument is specifically designed to measure polarization. As an electromagnetic wave, light consists of oscillating electric and magnetic fields. These fields can have both an amplitude, or intensity, and a preferred direction in which they oscillate, or polarization. The polarized signal that CLASS will attempt to measure is incredibly small. It is expected to be only a few parts-per-billion change in the polarization of the already-cold 2.725 K CMB. To measure such a small signal, CLASS employs focal plane arrays with large numbers of feedhorn-coupled, transition-edge-sensor bolometers cooled to just 0.1 °C above absolute zero by cryogenic helium refrigerators. This low temperature reduces the intrinsic thermal noise of the detectors.

The other unique aspect of the CLASS telescopes is the use of a VPM to allow a precise and stable measurement of polarization. The VPM modulates, or turns on and off, the polarized light going to the detector at a known frequency, approximately 10 Hz, while leaving unpolarized light unchanged. This allows for a clear separation of the tiny polarization of the CMB from the much larger unpolarized atmosphere by "locking in" to the 10 Hz signal. The VPM also modulates circular polarization out of phase with linear polarization, giving CLASS sensitivity to circular polarization. There are many potential scenarios that could generate circular polarization in the early universe, and CLASS has now put very strong limits on these theories.