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 a frequency of 40&nbsp;[[Hertz|GHz]] (7.5&nbsp;mm wavelength), 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 ==

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== 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. ]]

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 |issn=0004-637X}}</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 |doi=10.3847/1538-4357/acf293 |doi-access=free |arxiv=2305.01045 |bibcode=2023ApJ...956...77L |issn=0004-637X}}</ref> 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 |last=Shi |first=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=2024-08 |title=Sensitivity-improved Polarization Maps at 40 GHz with CLASS and WMAP Data |url=https://iopscience.iop.org/article/10.3847/1538-4357/ad5313 |journal=The Astrophysical Journal |language=en |volume=971 |issue=1 |pages=41 |doi=10.3847/1538-4357/ad5313 |issn=0004-637X}}</ref>

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>