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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 fluctuations]] 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.

Recent work, however, has called into question the statistical significance of this cold spot <ref>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</ref>.

==Possible causes other than primordial temperature fluctuation==

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===Supervoid===

[[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>: color scale from -20 to +20&nbsp;µK.]]

One possible explanation of the cold spot is a huge [[void (astronomy)|void]] between us and the primordial [[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 or [[Rees-Sciama]] 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.<ref>[http://www.centauri-dreams.org/?p=1427 Centauri Dreams » Blog Archive » Dark Energy Paints the Void<!-- Bot generated title -->]</ref>

One possible explanation of the cold spot is a huge [[void (astronomy)|void]] between us and the primordial [[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 or [[Rees-Sciama]] 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.<ref>[http://www.centauri-dreams.org/?p=1427 Centauri Dreams » Blog Archive » Dark Energy Paints the Void<!-- Bot generated title -->]</ref>

In August 2007, Rudnick, Brown & Williams <ref>[http://arxiv.org/abs/0704.0908 "Extragalactic Radio Sources and the WMAP Cold Spot"], ApJ, 671, pp. 40-44</ref> claimed a dip in [[NVSS]] galaxy number counts in the direction of the Cold Spot, suggesting the presence of a [[supervoid]] at redshift ''z'' approximately equal to 1. McEwen et al. <ref>[http://arxiv.org/abs/0704.0626 "Probing dark energy with steerable wavelets through correlation of WMAP and NVSS local morphological measures"], 2008, MNRAS, 384, pp. 1289-1300</ref> independently found the correlation using a wavelet analysis of the entire area of sky covered by the survey, though they did not explicitly advance the supervoid suggestion.

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><ref>[http://www.ifa.hawaii.edu/cosmowave/supervoids Dark Energy and the Imprint of Super-Structures on the Microwave Background]</ref>

Although large voids are known in the universe, a void would have to be unusually large to explain the cold spot, perhaps 1000 times larger in volume than expected typical voids. It would be 6-10 billion [[light-year]]s away and nearly one billion [[light-year]]s across, and would be perhaps even more improbable to occur in the [[large scale structure]] than the WMAP cold spot would be in the primordial [[CMB]]. <ref>[http://news.bbc.co.uk/2/hi/science/nature/6962185.stm BBC NEWS | Science/Nature | Great 'cosmic nothingness' found<!-- Bot generated title -->]</ref> <ref>[http://www.nrao.edu/pr/2007/coldspot/ Astronomers Find Enormous Hole in the Universe (August 2007)] </ref> <ref>[http://www.nrao.edu/pr/2007/coldspot/graphics.shtml graphics] </ref>

Although large voids are known in the universe, a void would have to be unusuallyexceptionally largevast to explain the cold spot, perhaps 1000 times larger in volume than expected typical voids. It would be 6-10 billion–10 billion [[light-year]]s away and nearly one billion [[light-year]]s across, and would be perhaps even more improbable to occur in the [[large scale structure]] than the WMAP cold spot would be in the primordial [[CMB]]. <ref>[http://news.bbc.co.uk/2/hi/science/nature/6962185.stm BBC NEWS | Science/Nature | Great 'cosmic nothingness' found<!-- Bot generated title -->]</ref> <ref>[http://www.nrao.edu/pr/2007/coldspot/ Astronomers Find Enormous Hole in the Universe (August 2007)] </ref> <ref>[http://www.nrao.edu/pr/2007/coldspot/graphics.shtml graphics] </ref>

[[Image:Mean_void_imprint.png|thumb|168px|The mean [[ISW]] imprint 50 supervoids have on the [[Cosmic Microwave Background]]: color scale from -20 to +20&nbsp;µK (adapted from Granett, Neyrinck & Szapudi).]]

In May 2008, two papers appeared on [[arXiv|astro-ph]] arguing against, and indirectly supporting, the supervoid explanation. Smith & Huterer <ref>[http://arxiv.org/abs/0805.2751 "No evidence for the cold spot in the NVSS survey"], MNRAS, submitted</ref> found that although there is a significant dip in [[NVSS]] galaxy density in the Cold Spot as Rudnick et al. claimed, it is not at the center of it, and there are many other possible circles to draw in the Cold Spot in which there is no dip. This does not prove that the Cold Spot cannot be due to a supervoid as suggested in the [[NVSS]] data; it merely uses a [[Bayesian]] statistical argument to assess the existing evidence that the Cold Spot is entirely due to a supervoid as quite weak.

In the second paper, Granett, Neyrinck & Szapudi

<ref>[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><ref>[http://www.ifa.hawaii.edu/cosmowave/supervoids Dark Energy and the Imprint of Super-Structures on the Microwave Background]</ref> found that supervoids and [[superclusters]] in the [[Sloan Digital Sky Survey|SDSS]] Luminous Red Galaxy catalog produce cold and hot spots (respectively) on the CMB, highly significant when the signals from 50 superclusters and 50 supervoids are added together. This could be considered a detection of the [[Integrated Sachs-Wolfe]] (ISW) effect, caused by [[dark energy]] accelerating the expansion of the universe, but in fact it's surprisingly strong to be ISW. This finding is relevant to the supervoid explanation for the Cold Spot because it supports the idea of supervoids having a measurable effect on the CMB.

===Cosmic Texture===

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===Parallel universe===

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]]. 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 northernsouthern hemisphere of the [[Celestial sphere]].<ref>[http://www.theblogofscience.com/evidence-for-a-parallel-universe/ Evidence for a parallel universe?], The Blog of Science</ref>

===Sensitivity to finding method===

Researchers at the University of Michigan pointed out that 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>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</ref>. More technically, its detection and significance depends on using a compensated filter like a [[Mexican hat wavelet]] to find it.

== See also ==

*[[Sloan Great Wall]]

*[[Great Wall (astronomy)|CfA2 Great Wall]]

*[[Dark flow]]

*[[Great Attractor]]

== NoteReferences ==

{{Reflist}}

<references/>

{{Refend}}

== External links ==

*[http://www.solstation.com/x-objects/greatvoi.htm Great Void in Eridanus, (WMAP Cold Spot)]

*[http://www.dailytech.com/Gaping+Hole+Found+in+Universe/article8598.htm Gaping Hole Found in Universe], Daily Tech

*[http://www.space.com/scienceastronomy/070823_huge_hole.html Huge Hole Found in the Universe], [[Space.com]], [[2007-08-23]]

*[http://news.nationalgeographic.com/news/2007/08/070824-hole-sky.html Gaping "Hole" in the Sky Found, Experts Say], [[National Geographic News]]

*[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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<nowiki>[[Category:Voids]]