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| [[Image:ColdSpot.jpg|thumb|right|Circled area is the cold spot.]]
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| The '''CMB Cold Spot''' or '''WMAP Cold Spot''' is a region of the sky seen in [[microwaves]] which analysis found to be unusually large and cold relative to the expected properties of the [[cosmic microwave background radiation]] (CMB). The "cold spot" is approximately 70 µK colder than the average CMB temperature (approximately 2.7 [[Kelvin|K]]), whereas the [[root mean square]] of typical temperature variations is only 18 µK.<ref>{{cite book|last=Wright|first=E.L.|year=2004|chapter=Theoretical Overview of Cosmic Microwave Background Anisotropy|editor=W. L. Freedman|title=Measuring and Modeling the Universe|series=Carnegie Observatories Astrophysics Series|publisher=[[Cambridge University Press]]|page=291|isbn=0-521-75576-X|arxiv=astro-ph/0305591 }}</ref><ref group="nb">After the [[dipole]] anisotropy, which is due to the [[Doppler shift]] of the microwave background radiation due to our [[peculiar velocity]] relative to the [[comoving coordinates|comoving]] cosmic rest frame, has been subtracted out. This feature is consistent with the Earth moving at some 627 km/s towards the constellation [[Virgo (constellation)|Virgo]].</ref>
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| The radius of the "cold spot" [[Subtended angle|subtends]] about 5°; it is centered at the [[Galactic coordinate system|galactic coordinate]] l<sub>''II''</sub> = 207.8°, b<sub>''II''</sub> = −56.3° ([[Equatorial coordinate system|equatorial]]: [[Right ascension|''α'']] = {{RA|03|15|05}}, [[Declination|''δ'']] = {{Dec|−19|35|02}}). Thus it is in the [[southern hemisphere]] of the [[celestial sphere]], in the direction of the constellation [[Eridanus (constellation)|Eridanus]].
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| Typically, the largest fluctuations of the primordial CMB temperature occur on angular scales of about 1°. Thus a cold region as large as the "cold spot" appears very unlikely, given generally accepted theoretical models. Various alternative explanations exist, including a so-called '''Eridanus [[Supervoid]]''' or '''Great Void'''. This would be an extremely large region of the universe, roughly 150 to 300 [[megaparsec|Mpc]] or 500 million to one billion [[light-years]] across,<ref>[http://space.newscientist.com/article/mg19626311.400-the-void-imprint-of-another-universe.html The void: Imprint of another universe?], [[New Scientist]], 2007-11-24</ref> at redshift <math>z\simeq 1</math>, containing a density of matter much smaller than the average density at that redshift.{{Citation needed|date=September 2013}} Such a void would affect the observed CMB via the [[Sachs-Wolfe effect|integrated Sachs-Wolfe effect]]. If a comparable [[supervoid]] did exist, it would be one of the [[Large-scale structure of the universe|largest structures]] in the [[observable universe]].
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| ==Discovery and significance==
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| In the first year of data recorded by the [[Wilkinson Microwave Anisotropy Probe]] (WMAP) a region of sky in the constellation [[Eridanus (constellation)|Eridanus]] was found to be cooler than the surrounding area.<ref>Cruz, Martínez-González, Vielva & Cayón (2005), [http://arxiv.org/abs/astro-ph/0405341 "Detection of a non-Gaussian Spot in WMAP"], MNRAS 356 29-40</ref> Subsequently, using the data gathered by WMAP over 3 years, the statistical significance of such a large, cool region was estimated. The probability of finding a deviation at least as high in [[Gaussian random field|Gaussian]] simulations was found to be 1.85%.<ref>Cruz, Cayón, Martínez-González, Vielva & Jin (2007), [http://arxiv.org/abs/astro-ph/0603859 "The non-Gaussian Cold Spot in the 3-year WMAP data"], Astrophys.J. 655 11-20</ref> Thus it appears unlikely, but not impossible, that the cold spot was generated by the standard mechanism of [[quantum fluctuation]]s during [[cosmological inflation]], which in most inflationary models gives rise to Gaussian statistics. The cold spot may also, as suggested in the references above, be a signal of non-Gaussian primordial fluctuations.
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| Some authors called into question the statistical significance of this cold spot.<ref name="arxiv.org">Zhang & Huterer (2009), [http://arxiv.org/abs/0908.3988 "Disks in the sky: A reassessment of the WMAP "cold spot"], http://arxiv.org/abs/0908.3988 , Astroparticle Physics, 33, 69 (2010)</ref>
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| Recently the CMB Cold Spot has also been observed by the [[Planck (spacecraft)|Planck]] satellite<ref>PLANCK Collaboration (2013), [http://arxiv.org/abs/1303.5083 "Planck 2013 results. XXIII. Isotropy and Statistics of the CMB"], submitted to A&A </ref> at similar significance, discarding the possibility of being caused by a [[systematic error]] of the WMAP satellite.
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| ==Possible causes other than primordial temperature fluctuation==
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| The large 'cold spot' forms part of what has been called an 'axis of evil' (so named because it is unanticipated to see structure<ref>http://blog.lib.umn.edu/mill1974/EGAD/041107.html</ref>) which has been explained using several contexts: "many authors
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| have commented about how the AE impacts our understanding of how structure emerged in the Universe within the framework of [[Cold dark matter|CDM]] and hydro-gravitational dynamics (HGD)"<ref>http://arxiv.org/PS_cache/arxiv/pdf/0802/0802.3229v2.pdf</ref>
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| ===Supervoid===
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| [[Image:Mean void imprint.png|thumb|168px|The mean [[ISW]] imprint 50 supervoids have on the [[Cosmic Microwave Background]]:<ref name="graneypudi">Granett, Neyrinck & Szapudi, [http://arxiv.org/abs/0805.3695] "An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect", ApJ, 683, L99</ref>{{Clarify|date=December 2013}} color scale from -20 to +20 µK.]]
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| One possible explanation of the cold spot is a huge [[void (astronomy)|void]] between us and the primordial [[Cosmic microwave background radiation|CMB]]. Voids can produce a cooler region than surrounding sightlines from the late-time [[Sachs-Wolfe effect#Late-time Integrated Sachs-Wolfe Effect|integrated Sachs-Wolfe]] effect.<ref>Inoue & Silk, 2006, [http://arxiv.org/abs/astro-ph/0602478 "Local Voids as the Origin of Large-Angle Cosmic Microwave Background Anomalies I"], ApJ 648 23-30</ref> This effect would be much smaller if [[dark energy]] weren't stretching the void as [[photon]]s went through it.
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| ''Rudnick et al''.<ref>[http://arxiv.org/abs/0704.0908 "Extragalactic Radio Sources and the WMAP Cold Spot"], ApJ, 671, pp. 40-44</ref> found a dip in [[NVSS]] galaxy number counts in the direction of the Cold Spot, suggesting the presence of a [[supervoid]]. Since then, some additional works have cast doubt on the supervoid explanation. The correlation between the NVSS dip and the Cold Spot was found to be marginal using a more conservative statistical analysis.<ref>Smith & Huterer, [http://arxiv.org/abs/0805.2751 "No evidence for the cold spot in the NVSS survey"], MNRAS, submitted</ref> Also, a direct survey for galaxies in several one-degree-square fields within the Cold Spot found no evidence for a supervoid.<ref>Granett, Szapudi & Neyrinck, [http://arxiv.org/abs/0911.2223 "Galaxy Counts on the CMB Cold Spot"], ApJ, in press</ref> However, the supervoid explanation has not been ruled out entirely; it remains intriguing, since supervoids do seem capable of affecting the CMB measurably.<ref name="graneypudi" /><ref>[http://www.ifa.hawaii.edu/cosmowave/supervoids Dark Energy and the Imprint of Super-Structures on the Microwave Background]</ref>
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| Although large voids are known in the universe, a void would have to be exceptionally vast to explain the cold spot, perhaps 1,000 times larger in volume than expected typical voids. It would be 6 billion–10 billion [[light-year]]s away and nearly one billion light-years across, and would be perhaps even more improbable to occur in the [[Large-scale structure of the universe|large scale structure]] than the WMAP cold spot would be in the primordial CMB.
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| ===Cosmic texture===
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| In late 2007, (''Cruz et al.'')<ref>{{cite journal| journal=Science| doi=10.1126/science.1148694| title=A Cosmic Microwave Background Feature Consistent with a Cosmic Texture| first=M.| last= Cruz| pmid=17962521| coauthors= N. Turok, P. Vielva, E. Martínez-González, M. Hobson| url= http://www.sciencemag.org/cgi/content/abstract/1148694| accessdate=2007-10-25| year=2007| volume=318| issue=5856| pages=1612–4|bibcode = 2007Sci...318.1612C |arxiv = 0710.5737 }}</ref> argued that the Cold Spot could be due to a [[Texture (cosmology)|cosmic texture]], a remnant of a [[phase transition]] in the early Universe.
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| ===Parallel universe===
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| A controversial claim by [[Laura Mersini|Laura Mersini-Houghton]] is that it could be the imprint of [[Multiverse|another universe]] beyond our own, caused by [[quantum entanglement]] between universes before they were separated by [[cosmic inflation]].<ref>[http://space.newscientist.com/article/mg19626311.400-the-void-imprint-of-another-universe.html The void: Imprint of another universe?], [[New Scientist]], 2007-11-24</ref> Laura Mersini-Houghton said, "Standard cosmology cannot explain such a giant cosmic hole" and made the remarkable hypothesis that the WMAP cold spot is "… the unmistakable imprint of another universe beyond the edge of our own." If true, this provides the first [[empirical evidence]] for a parallel universe (though theoretical models of parallel universes existed previously). It would also support [[string theory]]{{Citation needed|date=January 2013}}. The team claims there are [[Testability|testable]] consequences for its theory. If the parallel universe theory is true there will be a similar void in the opposite hemisphere of the [[Celestial sphere]]<ref>Phys.Rev.D77:063510,2008.
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| e-Print: hep-th/0611223, and Phys.Rev.D77:063511,2008.e-Print: hep-th/0612142</ref><ref>[http://www.theblogofscience.com/evidence-for-a-parallel-universe/ Evidence for a parallel universe?], The Blog of Science</ref> (which New Scientist reported to be the Southern hemisphere; the results of the New Mexico array study reported it as Northern hemisphere<ref>
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| Into the void. By: Chown, Marcus. New Scientist, 11/24/2007, Vol. 196 Issue 2631, p34-37, 4p</ref>).
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| A sophisticated computational analysis (using [[Kolmogorov complexity]]) has derived evidence for a north and a south cold spot in the satellite data:<ref>A&A(astronomy and astrophysics) 497,. 343-346 (2009) {{doi|10.1051/0004-6361/200911625}} Kolmogorov cosmic microwave background sky V. G. Gurzadyan1, A. E. Allahverdyan1, T. .Ghahramanyan1, A. L. Kashin1, H. G. Khachatryan1, A. A. Kocharyan1, H. Kuloghlian1, S. Mirzoyan1, E. Poghosian1, and G. Yegorian1</ref> "...among the high randomness regions is the southern non-Gaussian anomaly, the Cold Spot, with a stratification expected for the voids. Existence of its counterpart, a Northern Cold Spot with almost identical randomness properties among other low-temperature regions is revealed."
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| These predictions and others were made prior to the measurements (see [[Laura Mersini]]).{{Citation needed|date=February 2013}} However, apart from the Southern Cold Spot, the varied statistical methods in general fail to confirm each other regarding a Northern Cold Spot.<ref>Mon. Not. R. Astron. Soc. 000, 1{13 (2009) Printed 20 May 2009 Non-Gaussian Signatures in the five-year WMAP data as identified with isotropic scaling indices G. Rossmanith1?, C. Rath1, A. J. Banday2;3 and G. Morfill1</ref> The 'K-map' used to detect the Northern Cold Spot was noted to have twice the measure of randomness measured in the standard model. The difference is speculated to be caused by the randomness introduced by voids (unaccounted-for voids were speculated to be the reason for the increased randomness above the standard model).<ref>V. G. Gurzadyan and A. A. Kocharyan Kolmogorov stochasticity parameter measuring the randomness in the cosmic microwave background; Astronomy and Astrophysics, Volume 492, Issue 2, 2008, pp.L33-L34</ref>
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| ===Sensitivity to finding method===
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| The cold spot is mainly anomalous because it stands out compared to the relatively hot ring around it; it is not unusual if one only considers the size and coldness of the spot itself.<ref name="arxiv.org"/> More technically, its detection and significance depends on using a compensated [[Filter (optics)|filter]] like a [[Mexican hat wavelet]] to find it.
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| == See also ==
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| * [[Great Wall (astronomy)|CfA2 Great Wall]]
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| * [[Dark flow]]
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| * [[Great Attractor]]
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| * [[Sloan Great Wall]]
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| ==Notes==
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| {{Reflist|group=nb}}
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| == References ==
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| {{Reflist}}
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| {{Refend}}
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| == External links ==
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| * [http://www.solstation.com/x-objects/greatvoi.htm Great Void in Eridanus, (WMAP Cold Spot)]
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| * [http://www.dailytech.com/Gaping+Hole+Found+in+Universe/article8598.htm Gaping Hole Found in Universe], Daily Tech
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| * [http://www.space.com/scienceastronomy/070823_huge_hole.html Huge Hole Found in the Universe], [[Space.com]], 2007-08-23
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| * [http://news.nationalgeographic.com/news/2007/08/070824-hole-sky.html Gaping "Hole" in the Sky Found, Experts Say], [[National Geographic News]]
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| * [http://news.bbc.co.uk/1/hi/sci/tech/6962185.stm BBC News: Great 'cosmic nothingness' found]. [[BBC News]], 2007-08-24
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| {{Sky|03|15|05|-|19|35|02|1}}
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| {{DEFAULTSORT:Cmb Cold Spot}}
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| [[Category:Voids (astronomy)]]
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| [[Category:Eridanus (constellation)]]
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