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en:CMB cold spot

L'area cerchiata è la macchia fredda

La macchia fredda nella radiazione cosmica di fondo o macchia fredda del WMAP, è una porzione della volta celeste che, se vista nelle microonde, appare più larga e fredda rispetto alle proprietà medie della radiazione cosmica di fondo (CMB). Mediamente, la fluttuazione tipica delle temperature della CMB è nell'ordine di 10-5, mentre la macchia fredda è di 70 µK più fredda della temperatura media della CMB (approssimativamente di 2,7 K).

Il raggio della macchia è di 5° circa; il centro si trova alle coordinate galattiche: lII = 207.8°, bII = −56.3° (equatoriale: α: = 03h15m05s, δ = −19d35m02s). Questa si trova nell'emisfero sud della volta celeste, nella costellazione dell'Eridano.

Secondo i modelli teorici generalmente accettati dalla comunità scientifica, le fluttuazioni più larghe della radiazione di fondo avvengono entro una scala angolare di circa 1°; per questo motivo, macchie fredde grandi come queste appaiono alquanto improbabili. Esistono alcune spiegazioni alterniative per questa macchia, quali il Supervuoto di Eridano o il Grande Vuoto. Questa regione sembra essere grande all'incirca 150 Mpc, ovvero 500 milioni di anni luce, ad uno spostamento verso il rosso di , con una densità di materia di molto minore rispetto alla densità media allo spostamento verso il rosso. Un vuoto così grande influisce sulla radiazione di fondo osservata tramite l'effetto Sachs-Wolfe. Se esistesse un'altra regione di supervuoto comparabile a questa, sarebbe una delle più larghe strutture dell'universo osservabile.


Scoperta e significato

Al primo anno di risultati del WMAP, è stata scoperta una regione nella costellazione dell'Eridano più fredda dell'area circostante.[1]. In seguito, usando i dati al terzo anno del WMAP, è stato calcolato il significato statistico di quest'area: la probabilità di trovare


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.[2] 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%.[3] 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 [4].

Possible causes other than primordial temperature fluctuation

Supervoid

 
The mean ISW imprint 50 supervoids have on the Cosmic Microwave Background[5]: color scale from -20 to +20 µK.

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.[6] This effect would be much smaller if dark energy weren't stretching the void as photons went through it.

Rudnick et al. [7] 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. [8] Also, a direct survey for galaxies in several one-degree-square fields within the Cold Spot found no evidence for a supervoid. [9] However, the supervoid explanation has not been ruled out entirely; it remains intriguing, since supervoids do seem capable of affecting the CMB measurably. [5][10]

Although large voids are known in the universe, a void would have to be exceptionally vast to explain the cold spot, perhaps 1000 times larger in volume than expected typical voids. It would be 6 billion–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.

Cosmic Texture

In late 2007, Cruz et al. [11] 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.[12] 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 southern hemisphere of the Celestial sphere.[13]

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[14]. More technically, its detection and significance depends on using a compensated filter like a Mexican hat wavelet to find it.

See also

References

  1. ^ (EN) Cruz, Martínez-González, Vielva e Cayón, Detection of a non-Gaussian Spot in WMAP, MNRAS 356 29-40, su arxiv.org, 18-05-2004. URL consultato il 27-06-2009.
  2. ^ Cruz, Martínez-González, Vielva & Cayón (2005), "Detection of a non-Gaussian Spot in WMAP", MNRAS 356 29-40
  3. ^ Cruz, Cayón, Martínez-González, Vielva & Jin (2007), "The non-Gaussian Cold Spot in the 3-year WMAP data", Astrophys.J. 655 11-20
  4. ^ Zhang & Huterer (2009), "Disks in the sky: A reassessment of the WMAP "cold spot", http://arxiv.org/abs/0908.3988
  5. ^ a b Granett, Neyrinck & Szapudi, [1] "An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect", ApJ, 683, L99
  6. ^ Inoue & Silk, 2006, "Local Voids as the Origin of Large-Angle Cosmic Microwave Background Anomalies I", ApJ 648 23-30
  7. ^ "Extragalactic Radio Sources and the WMAP Cold Spot", ApJ, 671, pp. 40-44
  8. ^ Smith & Huterer, "No evidence for the cold spot in the NVSS survey", MNRAS, submitted
  9. ^ Granett, Szapudi & Neyrinck, "Galaxy Counts on the CMB Cold Spot", ApJ, in press
  10. ^ Dark Energy and the Imprint of Super-Structures on the Microwave Background
  11. ^ 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.
  12. ^ The void: Imprint of another universe?, New Scientist, 2007-11-24
  13. ^ Evidence for a parallel universe?, The Blog of Science
  14. ^ Zhang & Huterer (2009), "Disks in the sky: A reassessment of the WMAP "cold spot", http://arxiv.org/abs/0908.3988


[[Category:Voids]] [[Category:Eridanus constellation]] [[da:Hul i universet]] [[fr:Point froid]] [[ja:WMAPコールドスポット]] [[zh:WMAP冷斑點]]

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