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en:CMB cold spot

Circled area is the cold spot.

The WMAP Cold Spot or CMB Cold Spot is a region of the sky seen in microwaves which is unusually large and cold relative to the expected properties of the cosmic microwave background radiation (CMB). It should though be remembered that the typical fractional temperature fluctuations of the CMB are of order 10-5, and the "cold spot" is approximately 70 µK colder than the average CMB temperature (approximately 2.7 K).

The radius of the "cold spot" is about 5°; it is centered at the galactic coordinate lII = 207.8°, bII = −56.3° (equatorial: α = 03h15m05s, δ = −19d35m02s). Thus it is in the Southern hemisphere of the Celestial sphere, in the direction of the constellation Eridanus.

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 Mpc or 500 million light years across, at redshift , containing a density of matter much smaller than the average density at that redshift. Such a void would affect the observed CMB via the integrated Sachs-Wolfe effect. If a comparable supervoid did exist, it would be one of the largest structures in the observable Universe.

Discovery and significance

In the first year of data recorded by the Wilkinson Microwave Anisotropy Probe (WMAP) a region of sky in the constellation Eridanus was found to be cooler than the surrounding area.[1] 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 simulations was found to be 1.85%.[2] 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.

Possible causes other than primordial temperature fluctuation

Supervoid

One possible explanation of the cold spot 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.[3] This effect would be much smaller if dark energy weren't stretching the void as photons went through it.[4]

In August 2007, Rudnick, Brown & Williams [5] 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. [6] 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 large scale structure than the WMAP cold spot would be in the primordial CMB. [7] [8] [9]

 
The mean ISW imprint 50 supervoids have on the Cosmic Microwave Background: color scale from -20 to +20 µK (adapted from Granett, Neyrinck & Szapudi).

In May 2008, two papers appeared on astro-ph arguing against, and indirectly supporting, the supervoid explanation. Smith & Huterer [10] 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 [11][12] 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. [13] 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.[14] 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.[15]

See also

References

  1. ^ Cruz, Martínez-González, Vielva & Cayón (2005), "Detection of a non-Gaussian Spot in WMAP", MNRAS 356 29-40
  2. ^ Cruz, Cayón, Martínez-González, Vielva & Jin (2007), "The non-Gaussian Cold Spot in the 3-year WMAP data", Astrophys.J. 655 11-20
  3. ^ Inoue & Silk, 2006, "Local Voids as the Origin of Large-Angle Cosmic Microwave Background Anomalies I", ApJ 648 23-30
  4. ^ Centauri Dreams » Blog Archive » Dark Energy Paints the Void
  5. ^ "Extragalactic Radio Sources and the WMAP Cold Spot", ApJ, 671, pp. 40-44
  6. ^ "Probing dark energy with steerable wavelets through correlation of WMAP and NVSS local morphological measures", 2008, MNRAS, 384, pp. 1289-1300
  7. ^ BBC NEWS | Science/Nature | Great 'cosmic nothingness' found
  8. ^ Astronomers Find Enormous Hole in the Universe (August 2007)
  9. ^ graphics
  10. ^ "No evidence for the cold spot in the NVSS survey", MNRAS, submitted
  11. ^ "An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect", ApJL, submitted
  12. ^ Dark Energy and the Imprint of Super-Structures on the Microwave Background
  13. ^ M. Cruz, N. Turok, P. Vielva, E. Martínez-González, M. Hobson, A Cosmic Microwave Background Feature Consistent with a Cosmic Texture, in Science, vol. 318, 2007, p. 1612, DOI:10.1126/science.1148694. URL consultato il 25 ottobre 2007.
  14. ^ The void: Imprint of another universe?, New Scientist, 2007-11-24
  15. ^ Evidence for a parallel universe?, The Blog of Science

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