Cosmology Large Angular Scale Surveyor: Difference between revisions

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{{Short description|Microwave telescope array in Chile}}

{{Infobox telescope}}

The '''Cosmology Large Angular Scale Surveyor''' ('''CLASS''')<ref name=CLASS_website/><ref name=2014SPIE_EH/><ref name=JHUGazette-CLASS-Article/><ref name=Photonics-CLASS-Article/><ref name=JHUGazette-CLASS-Article-2014/> is an array of microwave telescopes atwhich has been observing since 2016 from a high-altitude site in the [[Atacama Desert]] of [[Chile]] as part of the Parque Astronómico de Atacama.<ref name=Conicyt-Astronomy-Roadmap-CLASS/> The CLASS experiment aims to improve our understanding of [[Reionization|cosmic dawn]] when the first stars turned on, and to test the theory of [[cosmic inflation]], and distinguish between inflationary models of the very early universe by making precise measurements of the [[polarization (waves)|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&nbsp;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.

== Science goals ==

[[File:CLASS site annotated 2019-12.svg|thumbnail|300px|left|Overview of the CLASS site in 2019.]]

ACLASS secondis addressing two primary science goalgoals. ofThe CLASSfirst 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 [[James Webb Space Telescope#Bright early galaxies|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.<ref name=":0">{{Cite journal |last1=Allison |first1=R. |last2=Caucal |first2=P. |last3=Calabrese |first3=E. |last4=Dunkley |first4=J. |last5=Louis |first5=T. |date=2015-12-23 |title=Towards a cosmological neutrino mass detection |journal=Physical Review D |language=en |volume=92 |issue=12 |pages=123535 |arxiv=1509.07471 |bibcode=2015PhRvD..92l3535A |doi=10.1103/PhysRevD.92.123535 |issn=1550-7998|bibcode=2015PhRvD..92l3535A|arxiv=1509.07471 |s2cid=53317662}}</ref>

CLASS has two primary science goals. The first is to test the theory of inflation. In [[physical cosmology]], [[cosmic inflation]] is the leading theory of the very early universe;<ref name=2014-Linde-Inflation-Overview/> however, observational evidence for inflation is still inconclusive. Inflationary models generically predict that a [[gravitational wave|gravitational-wave background]] (GWB) would have been produced along with the density perturbations that seed [[structure formation|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-modes|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.<ref name=Boyle-Inflation/><ref name=Tegmark-Inflation/>

CLASSA has two primarysecond science goals.goal Theof firstCLASS is to test the theory of inflation. In [[physical cosmology]], [[cosmic inflation]] is the leading theory of the very early universe;<ref name="2014-Linde-Inflation-Overview" /> however, observational evidence for inflation is still inconclusive. Inflationary models generically predict that a [[gravitational wave|gravitational-wave background]] (GWB) would have been produced along with the density perturbations that seed [[structure formation|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-modes|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.<ref name="Boyle-Inflation" /><ref name="Tegmark-Inflation" />

A second primary science goal of CLASS 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. 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.<ref name=":0">{{Cite journal|last1=Allison|first1=R.|last2=Caucal|first2=P.|last3=Calabrese|first3=E.|last4=Dunkley|first4=J.|last5=Louis|first5=T.|date=2015-12-23|title=Towards a cosmological neutrino mass detection|journal=Physical Review D|language=en|volume=92|issue=12|pages=123535|doi=10.1103/PhysRevD.92.123535|issn=1550-7998|bibcode=2015PhRvD..92l3535A|arxiv=1509.07471|s2cid=53317662}}</ref>

AdditionalCLASS scienceis goalsalso forfurthering CLASSour areunderstand to better understandof our own [[Milky Way Galaxy]] and to searchsearching for evidence of exotic new physics through constraining [[circular polarization]] in the CMB and large-scale anomalies. (See the [[cosmic microwave background#Low multipoles and other anomalies|Low multipoles and other anomalies]] section of the [[cosmic microwave background]] article for more information on the latter.)

== Instrument ==

[[File:CLASS Experiment 40 GHz Focal Plane.png|thumbnail|250px|left|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 consists of two [[altazimuth mount]]s 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 [[Milky Way|galaxy]] from that of the CMB. One telescope observes at 40&nbsp;[[Hertz|GHz]] (7.5&nbsp;mm wavelength); one telescope observes at 90&nbsp;GHz (3.3&nbsp;mm wavelength) with a second 90 GHz telescope planned in the future; and the fourth telescope observes in two frequency bands centered at 150&nbsp;GHz (2&nbsp;mm wavelength) and 220&nbsp;GHz (1.4&nbsp;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)<ref>{{Cite book |last1=Shi |first1=Rui |title=Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XII |last2=Brewer |first2=Michael |last3=Chan |first3=Carol |last4=Chuss |first4=David T. |last5=Couto |first5=Jullianna D. |last6=Eimer |first6=Joseph R. |last7=Karakla |first7=John |last8=Shukawa |first8=Koji |last9=Valle |first9=Deniz |date=2024-08-16 |publisher=SPIE |isbn=978-1-5106-7527-8 |editor-last=Zmuidzinas |editor-first=Jonas |pages=125 |chapter=Design and characterization of a 60-cm reflective half-wave plate for the CLASS 90 GHZ band telescope |doi=10.1117/12.3016346 |editor2-last=Gao |editor2-first=Jian-Rong |chapter-url=https://www.spiedigitallibrary.org/conference-proceedings-of-spie/13102/3016346/Design-and-characterization-of-a-60-cm-reflective-half-wave/10.1117/12.3016346.full |last10=Appel |first10=John W. |last11=Bennett |first11=Charles L. |last12=Dahal |first12=Sumit |last13=Essinger-Hileman |first13=Thomas |last14=Marriage |first14=Tobias T. |last15=Petroff |first15=Matthew A. |arxiv=2407.08912}}</ref> 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.<ref name=apj420_439/> To measure such a small signal, CLASS employs focal plane arrays with large numbers of [[horn antenna|feedhorn]]-coupled, [[transition-edge sensor|transition-edge-sensor]] [[bolometers]] cooled to just 0.1&nbsp;°C above absolute zero by [[Dilution refrigerator|cryogenic helium refrigerators]]. This low temperature reduces the intrinsic thermal noise of the detectors.<ref name=2012SPIE_Eimer/><ref name=2013Eimer_Thesis/><ref name=2014SPIE_Appel/>

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&nbsp;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 "[[lock-in amplifier|locking in]]" to the 10 [[Hertz|Hz]] signal. The VPM also modulates circular polarization out of phase with linear polarization, giving CLASS sensitivity to [[circular polarization]]. BecauseThere noare circularmany polarizationpotential isscenarios expectedthat incould thegenerate CMB,circular the VPM allows for a valuable check for [[systematic errors]]polarization in the dataearly by looking at the circular polarization signaluniverse, which should be consistent with zero. In 2024 the VPM for the CLASS 90 GHz telescope was replaced with a rotating reflective [[half-wave plate]] (HWP)<ref>{{Cite book |last1=Shi |first1=Rui |last2=Brewer |first2=Michael |last3=Chan |first3=Carol |last4=Chuss |first4=David T. |last5=Couto |first5=Jullianna D. |last6=Eimer |first6=Joseph R. |last7=Karakla |first7=John |last8=Shukawa |first8=Koji |last9=Valle |first9=Deniz |last10=Appel |first10=John W. |last11=Bennett |first11=Charles L. |last12=Dahal |first12=Sumit |last13=Essinger-Hileman |first13=Thomas |last14=Marriage |first14=Tobias T. |last15=Petroff |first15=Matthew A. |chapter=Design and characterization of a 60-cm reflective half-wave plate for the CLASS 90has GHZnow bandput telescopevery |date=2024-08-16strong |editor-last=Zmuidzinaslimits |editor-first=Jonason |editor2-last=Gaothese |editor2-first=Jian-Rong |title=Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XII |chapter-url=https://wwwtheories.spiedigitallibrary.org/conference-proceedings-of-spie/13102/3016346/Design-and-characterization-of-a-60-cm-reflective-half-wave/10.1117/12.3016346.full |publisher=SPIE |pages=125 |doi=10.1117/12.3016346 |arxiv=2407.08912 |isbn=978-1-5106-7527-8}}</ref> to compare VPM performance to this alternative polarization modulation technology.

Because water vapor in the atmosphere emits at microwave frequencies, CLASS observes 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 [[Atacama Cosmology Telescope|ACT]], [[Atacama Pathfinder Experiment|APEX]], [[Atacama Large Millimeter Array|ALMA]], [[Fred YoungAtacama Submillimeter Telescope Experiment|ASTE]], [[POLARBEARCosmic Background Imager|CBI]][[POLARBEAR|,]] [[Fred Young Submillimeter Telescope|CCAT-prime]], Nanten[[NANTEN2_Observatory|NANTEN2]], [[POLARBEAR]], [[Simons Observatory]], and [[University of Tokyo Atacama Observatory|TAO]].

== Current status and results ==

[[File:Maps QUV vert.pdf|thumb|Maps of linearly-polarized Stokes parameters Q and U, as well as circularly-polarized Stokes parameter V from the CLASS 40 GHz survey. The plane of the Milky Way Galaxy is horizontal in this projection. |390x390px]]

CLASS is currently observing the sky in all four frequency bands. The CLASS 40&nbsp;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, a first 90&nbsp;GHz telescope was installed on the same mount as the 40&nbsp;GHz telescope, achieving first light on 30 May 2018. In 2019, the dual-frequency 150/220&nbsp;GHz telescope was deployed, along with a second telescope mount, and achieved first light on 21 September 2019.

CLASS has released results from the first five years of observations with the 40 GHz telescope, through mid-2022, and has made the most sensitive maps covering approximately 74% of the sky in this frequency band at angular scales of approximately 2° to 20°.<ref name=":1">{{Cite journal |last1=Eimer |first1=Joseph R. |last2=Li 李 |first2=Yunyang 云炀 |last3=Brewer |first3=Michael K. |last4=Shi 时 |first4=Rui 瑞 |last5=Ali |first5=Aamir |last6=Appel |first6=John W. |last7=Bennett |first7=Charles L. |last8=Bruno |first8=Sarah Marie |last9=Bustos |first9=Ricardo |last10=Chuss |first10=David T. |last11=Cleary |first11=Joseph |last12=Dahal |first12=Sumit |last13=Datta |first13=Rahul |last14=Denes Couto |first14=Jullianna |last15=Denis |first15=Kevin L. |date=2024-03-01 |title=CLASS Angular Power Spectra and Map-component Analysis for 40 GHz Observations through 2022 |journal=The Astrophysical Journal |volume=963 |issue=2 |pages=92 |doi=10.3847/1538-4357/ad1abf |doi-access=free |arxiv=2309.00675 |bibcode=2024ApJ...963...92E |doi=10.3847/1538-4357/ad1abf |issn=0004-637X |doi-access=free}}</ref><ref>{{Cite journal |last1=Li 李 |first1=Yunyang 云炀 |last2=Eimer |first2=Joseph R. |last3=Osumi |first3=Keisuke |last4=Appel |first4=John W. |last5=Brewer |first5=Michael K. |last6=Ali |first6=Aamir |last7=Bennett |first7=Charles L. |last8=Bruno |first8=Sarah Marie |last9=Bustos |first9=Ricardo |last10=Chuss |first10=David T. |last11=Cleary |first11=Joseph |last12=Couto |first12=Jullianna Denes |last13=Dahal |first13=Sumit |last14=Datta |first14=Rahul |last15=Denis |first15=Kevin L. |date=2023-10-01 |title=CLASS Data Pipeline and Maps for 40 GHz Observations through 2022 |journal=The Astrophysical Journal |volume=956 |issue=2 |pages=77 |arxiv=2305.01045 |bibcode=2023ApJ...956...77L |doi=10.3847/1538-4357/acf293 |issn=0004-637X |doi-access=free}}</ref> and from observations with the 90 GHz telescope through 2024.<ref>{{cite arXiv |last1=Li |first1=Yunyang |title=A Measurement of the Largest-Scale CMB E-mode Polarization with CLASS |date=2025-01-21 |eprint=2501.11904 |last2=Eimer |first2=Joseph |last3=Appel |first3=John |last4=Bennett |first4=Charles |last5=Brewer |first5=Michael |last6=Bruno |first6=Sarah Marie |last7=Bustos |first7=Ricardo |last8=Chan |first8=Carol |last9=Chuss |first9=David|class=astro-ph.CO }}</ref> CLASS has made the most sensitive maps covering approximately 74% of the sky at 40 GHz at angular scales of approximately 2° to 20°. These maps have been combined with those of the [[Wilkinson Microwave Anisotropy Probe|Wilkinson Microwave Anisotropy Probe (WMAP)]] satellite at a similar frequency to produce even more sensitive combined maps.<ref>{{Cite journal |last1=Shi |first1=Rui |last2=Appel |first2=John W. |last3=Bennett |first3=Charles L. |last4=Bustos |first4=Ricardo |last5=Chuss |first5=David T. |last6=Dahal |first6=Sumit |last7=Denes Couto |first7=Jullianna |last8=Eimer |first8=Joseph R. |last9=Essinger-Hileman |first9=Thomas |last10=Harrington |first10=Kathleen |last11=Iuliano |first11=Jeffrey |last12=Li |first12=Yunyang |last13=Marriage |first13=Tobias A. |last14=Petroff |first14=Matthew A. |last15=Rostem |first15=Karwan |date=August 5, 2024 |title=Sensitivity-improved Polarization Maps at 40 GHz with CLASS and WMAP Data |journal=The Astrophysical Journal |language=en |volume=971 |issue=1 |pages=41 |doi=10.3847/1538-4357/ad5313 |doi-access=free |arxiv=23052404.0104517567 |bibcode=2023ApJ2024ApJ...956971...77L41S |issn=0004-637X}}</ref> CLASS maps at 90 GHz are comparable in sensitivity to those of the [[Planck (spacecraft)|Planck Satellite]] at similar frequencies and have been used to place constraints on [[reionization]], a first for a ground-based telescope.

CLASS has made a first detection of [[circular polarization]] from the atmosphere at a frequency of 40&nbsp;GHz, which is in agreement with models of atmospheric circular polarization due to [[Zeeman effect|Zeeman splitting]] of [[Allotropes of oxygen|molecular oxygen]] in the presence of the Earth's magnetic field.<ref>{{Cite journal|last1=Petroff|first1=Matthew A.|last2=Eimer|first2=Joseph R.|last3=Harrington|first3=Kathleen|last4=Ali|first4=Aamir|last5=Appel|first5=John W.|last6=Bennett|first6=Charles L.|last7=Brewer|first7=Michael K.|last8=Bustos|first8=Ricardo|last9=Chan|first9=Manwei|last10=Chuss|first10=David T.|last11=Cleary|first11=Joseph|date=2020-01-30|title=Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A First Detection of Atmospheric Circular Polarization at Q band|journal=The Astrophysical Journal|volume=889|issue=2|pages=120|doi=10.3847/1538-4357/ab64e2|issn=1538-4357|arxiv=1911.01016|bibcode=2020ApJ...889..120P |s2cid=207870198 |doi-access=free }}</ref> The atmospheric circular polarization is smoothly-varying over the sky, allowing it to be separated from celestial circular polarization. This has allowed CLASS to constrain celestial circular polarization at 40&nbsp;GHz to be less than 0.1 μK at angular scales of 5 degrees and less than 1 μK at angular scales around 1 degree.<ref name=":1" /><ref>{{Cite journal|last1=Padilla|first1=Ivan L.|last2=Eimer|first2=Joseph R.|last3=Li|first3=Yunyang|last4=Addison|first4=Graeme E.|last5=Ali|first5=Aamir|last6=Appel|first6=John W.|last7=Bennett|first7=Charles L.|last8=Bustos|first8=Ricardo|last9=Brewer|first9=Michael K.|last10=Chan|first10=Manwei|last11=Chuss|first11=David T.|date=2020-01-29|title=Two-year Cosmology Large Angular Scale Surveyor (CLASS) Observations: A Measurement of Circular Polarization at 40 GHz|journal=The Astrophysical Journal|volume=889|issue=2|pages=105|doi=10.3847/1538-4357/ab61f8|issn=1538-4357|arxiv=1911.00391|bibcode=2020ApJ...889..105P |s2cid=207870170 |doi-access=free }}</ref> This is an improvement upon previous limits on circular polarization in the CMB by more than a factor of 100.<ref>{{Cite journal|last1=Mainini|first1=R.|last2=Minelli|first2=D.|last3=Gervasi|first3=M.|last4=Boella|first4=G.|last5=Sironi|first5=G.|last6=Baú|first6=A.|last7=Banfi|first7=S.|last8=Passerini|first8=A.|last9=Lucia|first9=A. De|last10=Cavaliere|first10=F.|date=August 2013|title=An improved upper limit to the CMB circular polarization at large angular scales|journal=Journal of Cosmology and Astroparticle Physics|language=en|volume=2013|issue=8|pages=033|doi=10.1088/1475-7516/2013/08/033|issn=1475-7516|arxiv=1307.6090|bibcode=2013JCAP...08..033M |s2cid=119236025}}</ref><ref>{{Cite journal|last1=Nagy|first1=J. M.|last2=Ade|first2=P. A. R.|last3=Amiri|first3=M.|last4=Benton|first4=S. J.|last5=Bergman|first5=A. S.|last6=Bihary|first6=R.|last7=Bock|first7=J. J.|last8=Bond|first8=J. R.|last9=Bryan|first9=S. A.|last10=Chiang|first10=H. C.|last11=Contaldi|first11=C. R.|date=August 2017|title=A New Limit on CMB Circular Polarization from SPIDER|journal=The Astrophysical Journal|language=en|volume=844|issue=2|pages=151|doi=10.3847/1538-4357/aa7cfd|arxiv=1704.00215 |bibcode=2017ApJ...844..151N |issn=0004-637X|hdl=10852/60193|s2cid=13694135|hdl-access=free |doi-access=free }}</ref>