Cosmology Large Angular Scale Surveyor

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Cosmology Large Angular Scale Surveyor
CLASS telescope 2018-06-07.jpg
CLASS Q and W band telescopes at night
Alternative namesCLASS Edit this at Wikidata
Location(s)Cerro Toco, Atacama Desert, Chile Edit this at Wikidata
Coordinates22°58′S 67°47′W / 22.96°S 67.79°W / -22.96; -67.79Coordinates: 22°58′S 67°47′W / 22.96°S 67.79°W / -22.96; -67.79 Edit this at Wikidata
Altitude5,140 m (16,860 ft) Edit this at Wikidata
Wavelength1.4, 2.0, 3.3, 7.5 mm (214, 150, 91, 40 GHz)
Telescope styleCosmic microwave background experiment
Radio telescope Edit this on Wikidata Edit this at Wikidata
Cosmology Large Angular Scale Surveyor is located in Chile
Cosmology Large Angular Scale Surveyor
Location of Cosmology Large Angular Scale Surveyor

The Cosmology Large Angular Scale Surveyor (CLASS)[1][2][3][4][5] is an array of microwave telescopes at a high-altitude site in the Atacama Desert of Chile as part of the Parque Astronómico de Atacama.[6] The CLASS experiment aims to test the theory of cosmic inflation and distinguish between inflationary models of the very early universe 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.

Science goals[edit]

Overview of the CLASS site in 2016.

A primary 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;[7] 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 very 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.[8][9]

CLASS will also 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. 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.

Additional science goals for CLASS are to better understand our own Milky Way Galaxy and to search for evidence of exotic new physics through 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.)


CLASS 40 GHz camera, showing the feedhorns that couple light onto the transition-edge sensor bolometers at a temperature of 0.1 Kelvin.

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 will consist of two altazimuth mounts that will allow the telescopes to be pointed to observe different patches of sky. The four CLASS telescopes will observe at a range of frequencies to separate emission from our galaxy from that of the CMB. One telescope will observe at 40 GHz (7.5 mm wavelength); two telescopes will observe at 90 GHz (3.3 mm wavelength); and the fourth telescope will observe 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 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.[10] To measure such a small signal, CLASS will employ 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.[11][12][13]

The other unique aspect of the CLASS telescopes is the use of a variable-delay polarization modulator (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. Because no circular polarization is expected in the CMB, the VPM allows for a valuable check for systematic errors in the data by looking at the circular polarization signal, which should be consistent with zero.

Because water vapor in the atmosphere emits at microwave frequencies, CLASS will observe from a very dry and high-altitude site in the Andes Mountains on the edge of the Atacama Desert of Chile. Nearby sites have been chosen by other observatories for the same reason, including CBI, ASTE, Nanten, APEX, ALMA, ACT, and POLARBEAR.

Current Status[edit]

The CLASS 40 GHz telescope achieved first light on 8 May 2016 and began a roughly five-year survey in September 2016 after initial commissioning observations were complete. In early 2018, the first 90 GHz telescope was installed on the same mount as the 40 GHz telescope, achieving first light on 30 May 2018. Work is ongoing to deploy the remaining two telescopes.

See also[edit]


  1. ^ "CLASS: Cosmology Large Angular Scale Surveyor". The Johns Hopkins University. Retrieved 2015-08-12.
  2. ^ Essinger-Hileman, T. E.; et al. (2014). Holland, Wayne S; Zmuidzinas, Jonas, eds. "CLASS: the cosmology large angular scale surveyor". Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series. Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII. 9153: 91531I. arXiv:1408.4788. Bibcode:2014SPIE.9153E..1IE. doi:10.1117/12.2056701.
  3. ^ "Astrophysicist, team win stimulus grant to build telescope". The Johns Hopkins University. Retrieved 2014-01-15.
  4. ^ "ARRA to Help Build Telescope". Photonics Media. Retrieved 2014-01-15.
  5. ^ "Johns Hopkins astrophysics team builds telescope to study origins of the universe". The Johns Hopkins University. Retrieved 2014-05-27.
  6. ^ "Astronomy, Technology, Industry: Roadmap for the Fostering of Technology Development and Innovation in the Field of Astronomy in Chile" (PDF). Conicyt Ministry of Education, Government of Chile. Retrieved 2014-02-10.
  7. ^ Linde, A. (2014). "Inflationary Cosmology after Planck 2013". arXiv:1402.0526 [hep-th].
  8. ^ Boyle, Latham A.; Steinhardt, PJ; Turok, N (2006). "Inflationary predictions for scalar and tensor fluctuations reconsidered". Physical Review Letters. 96 (11): 111301. arXiv:astro-ph/0507455. Bibcode:2006PhRvL..96k1301B. doi:10.1103/PhysRevLett.96.111301. PMID 16605810.
  9. ^ Tegmark, Max (2005). "What does inflation really predict?". JCAP. 0504 (4): 001. arXiv:astro-ph/0410281. Bibcode:2005JCAP...04..001T. doi:10.1088/1475-7516/2005/04/001.
  10. ^ Mather, J. C. (January 1994). "Measurement of the cosmic microwave background spectrum by the COBE FIRAS instrument". The Astrophysical Journal. 420: 439–444. Bibcode:1994ApJ...420..439M. doi:10.1086/173574.
  11. ^ Eimer, J. R.; Bennett, C. L.; Chuss, D. T.; Marriage, T.; Wollack, E. W.; Zeng, L. (2012). Holland, Wayne S, ed. "The cosmology large angular scale surveyor (CLASS): 40 GHz optical design". Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series. Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VI. 8452: 845220. arXiv:1211.0041. Bibcode:2012SPIE.8452E..20E. doi:10.1117/12.925464.
  12. ^ Eimer, J. R. (2013). The Cosmology Large Angular Scale Surveyor (CLASS): In Search of the Energy Scale of Inflation (Ph.D.). Johns Hopkins University.
  13. ^ Appel J. W.; et al. (2014). Holland, Wayne S; Zmuidzinas, Jonas, eds. "The cosmology large angular scale surveyor (CLASS): 38-GHz detector array of bolometric polarimeters". Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series. Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy VII. 9153: 91531J. arXiv:1408.4789. Bibcode:2014SPIE.9153E..1JA. doi:10.1117/12.2056530.