WMAP cold spot

In 2004 the Wilkinson Microwave Anisotropy Probe (WMAP) mapped out a region in the Eridanus Constellation which had cosmic microwave background radiation (CMB) that was cooler than the surrounding area. [Vielva, Martínez-González, Barreiro, Sanz & Cayon, 2004, [http://arxiv.org/abs/astro-ph/0310273 "Detection of Non-Gaussianity in the Wilkinson Microwave Anisotropy Probe First-Year Data Using Spherical Wavelets"] , ApJ 609 22-34] Such a large, cool region is estimated to be only ~0.2% likely to occur in the primordial CMB. [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]

Position

It is centered at the galactic coordinate l_"II" = 207.8°, b_"II" = −56.3° (equatorial: "α" = 03^h15^m05^s, "δ" = −19^d35^m02^s). It is in the Southern hemisphere of the Celestial sphere.

Possible causes

upervoid

A leading explanation is a huge void between us and the primordial CMB. Voids can produce a cooler region than surrounding sightlines from the late-time integrated Sachs-Wolfe effect or Rees-Sciama effect. [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 ] This effect would be much smaller if dark energy weren't stretching the void as photons went through it. [ [http://www.centauri-dreams.org/?p=1427 Centauri Dreams » Blog Archive » Dark Energy Paints the Void ] ]

In August 2007, Rudnick, Brown & Williams [ [http://arxiv.org/abs/0704.0908 "Extragalactic Radio Sources and the WMAP Cold Spot"] , ApJ, 671, pp. 40-44] found a dip in NVSS galaxy number counts in the Cold Spot, suggesting there might be a supervoid there. McEwen et al. [ [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] 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.

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-years away and nearly one billion light-years across, and would be perhaps even more improbable to occur in the universe than the WMAP cold spot would be in the primordial CMB. [ [http://news.bbc.co.uk/2/hi/science/nature/6962185.stm BBC NEWS | Science/Nature | Great 'cosmic nothingness' found ] ] [ [http://www.nrao.edu/pr/2007/coldspot/ Astronomers Find Enormous Hole in the Universe (August 2007)] ] [ [http://www.nrao.edu/pr/2007/coldspot/graphics.shtml graphics] ]

In late May 2008, two papers appeared on astro-ph arguing against, and indirectly supporting, the supervoid explanation. Smith & Huterer [ [http://arxiv.org/abs/0805.2751 "No evidence for the cold spot in the NVSS survey"] , MNRAS, submitted] 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 [ [http://arxiv.org/abs/0805.3695 "An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect"] , ApJL, submitted] [ [http://www.ifa.hawaii.edu/cosmowave/supervoids Dark Energy and the Imprint of Super-Structures on the Microwave Background] ] found that supervoids and superclusters in the 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 is arguably the clearest detection so far of the Integrated Sachs-Wolfe effect, and provides evidence of dark energy accelerating the expansion of the universe. This finding supports the supervoid explanation for the Cold Spot because it shows that supervoids can have a measurable effect on the CMB, but the support is indirect because the Cold Spot is not in the SDSS survey.

Cosmic Texture

In late 2007, Cruz et al. [cite journal| journal=Science| doi=10.1126/science.1148694| title=A Cosmic Microwave Background Feature Consistent with a Cosmic Texture| first=M.| last= Cruz| 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| pages=1612] argued that the Cold Spot could be due to a cosmic texture, a remnant of a phase transition in the early Universe. This is an exotic explanation, but worth considering since a supervoid would have to be so big to produce the Cold Spot.

Parallel universe

A controversial claim by Laura Mersini-Houghton is that it could be the imprint of another universe beyond our own, caused by quantum entanglement between universes before they were separated by cosmic inflation. [ [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] 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 testable consequences for its theory. If the parallel universe theory is true there will be a similar void in the northern hemisphere of the Celestial sphere. [http://www.paternitytestinglabs.com/evidence-for-a-parallel-universe/]

See also

*Sloan Great Wall
*CfA2 Great Wall

References

External links

*http://www.dailytech.com/Gaping+Hole+Found+in+Universe/article8598.htm
*http://www.space.com/scienceastronomy/070823_huge_hole.html
*http://news.nationalgeographic.com/news/2007/08/070824-hole-sky.html
* [http://news.bbc.co.uk/1/hi/sci/tech/6962185.stm BBC News: Great 'cosmic nothingness' found]