# CMB cold spot

(Redirected from Eridanus Supervoid)
Circled area is the cold spot.

The CMB Cold Spot or WMAP Cold Spot is a region of the sky seen in microwaves that has been 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 K), whereas the root mean square of typical temperature variations is only 18 µK.[1][nb 1]

The radius of the "cold spot" subtends about 5°; it is centered at the galactic coordinate lII = 207.8°, bII = −56.3° (equatorial: α = 03h 15m 05s, δ = −19° 35′ 02″). It is therefore in the Southern Celestial Hemisphere, 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 to 300 Mpc or 500 million to one billion light-years across,[2] at redshift $z\simeq 1$, containing a density of matter much smaller than the average density at that redshift.[citation needed] 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.

A study published in 2015 shows the most likely explanation for the CMB cold spot is the supervoid one (see below)[3]

## 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.[4] 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%.[5] 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.

Some authors called into question the statistical significance of this cold spot.[6]

In 2013 the CMB Cold Spot was also observed by the Planck satellite[7] at similar significance, discarding the possibility of being caused by a systematic error of the WMAP satellite.

## Possible causes other than primordial temperature fluctuation

The large 'cold spot' forms part of what has been called an 'axis of evil' (so named because it is unanticipated to see structure[8]) which has been explained using several contexts: "many authors have commented about how the AE impacts our understanding of how structure emerged in the Universe within the framework of CDM and hydro-gravitational dynamics (HGD)"[9]

### Supervoid

The mean ISW imprint 50 supervoids have on the Cosmic Microwave Background:[10][clarification needed] 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.[11] This effect would be much smaller if dark energy was not stretching the void as photons went through it.[3]

Rudnick et al.[12] 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.[13] Also, a direct survey for galaxies in several one-degree-square fields within the Cold Spot found no evidence for a supervoid.[14] However, the supervoid explanation has not been ruled out entirely; it remains intriguing, since supervoids do seem capable of affecting the CMB measurably.[10][15][16]

A 2015 study shows the presence of a supervoid that has a radius of 1.8 billion light years and is centered at 3 billion light-years of our galaxy in the direction of the Cold Spot, likely being associated with it.[3] This would make it the largest void detected, and one of the largest structures known.[17][note 1]

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-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.)[18] argued that the Cold Spot could be due to a cosmic texture, a remnant of a phase transition in the early Universe.

### 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.[2] 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]. 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 opposite hemisphere of the Celestial sphere[19][20] (which New Scientist reported to be the Southern hemisphere; the results of the New Mexico array study reported it as Northern hemisphere[2]).

A sophisticated computational analysis (using Kolmogorov complexity) has derived evidence for a north and a south cold spot in the satellite data:[21] "...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."

These predictions and others were made prior to the measurements (see Laura Mersini).[citation needed] However, apart from the Southern Cold Spot, the varied statistical methods in general fail to confirm each other regarding a Northern Cold Spot.[22] 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).[23]

### Sensitivity to finding method

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

## Notes

1. ^ A claim by Szapudi et al states that the newly found void is the "largest structure ever identified by humanity". However, another source reports that the largest structure is the supercluster corresponding to the NQ2-NQ4 GRB overdensity at 10 billion light years.

## Notes

1. ^ 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 cosmic rest frame, has been subtracted out. This feature is consistent with the Earth moving at some 627 km/s towards the constellation Virgo.

## References

1. ^ Wright, E.L. (2004). "Theoretical Overview of Cosmic Microwave Background Anisotropy". In W. L. Freedman. Measuring and Modeling the Universe. Carnegie Observatories Astrophysics Series. Cambridge University Press. p. 291. arXiv:astro-ph/0305591. ISBN 0-521-75576-X.
2. ^ a b c Chown, Marcus (2007). "The void: Imprint of another universe?". New Scientist 196 (2631): 34–37. doi:10.1016/s0262-4079(07)62977-7.
3. ^ a b c Szapudi, I.; Kovacs, A.; Granett, B. R.; Frei, Z.; Silk, J.; Burgett, W.; Cole, S.; Draper, P. W.; Farrow, D. J.; Kaiser, N.; Magnier, E. A.; Metcalfe, N.; Morgan, J. S.; Price, P.; Tonry, J.; Wainscoat, R. (2015). "Detection of a supervoid aligned with the cold spot of the cosmic microwave background". Monthly Notices of the Royal Astronomical Society 450 (1): 288–294. arXiv:1405.1566. Bibcode:2015MNRAS.450..288S. doi:10.1093/mnras/stv488. Lay summary.
4. ^ Cruz, M.; Martinez-Gonzalez, E.; Vielva, P.; Cayon, L. (2004). "Detection of a non-Gaussian Spot in WMAP". Monthly Notices of the Royal Astronomical Society 356: 29–40. arXiv:astro-ph/0405341. Bibcode:2005MNRAS.356...29C. doi:10.1111/j.1365-2966.2004.08419.x.
5. ^ Cruz, M.; Cayon, L.; Martinez-Gonzalez, E.; Vielva, P.; Jin, J. (2006). "The non-Gaussian Cold Spot in the 3-year WMAP data". The Astrophysical Journal 655: 11–20. arXiv:astro-ph/0603859. Bibcode:2007ApJ...655...11C. doi:10.1086/509703.
6. ^ a b Zhang, Ray; Huterer, Dragan (2009). "Disks in the sky: A reassessment of the WMAP "cold spot"". Astroparticle Physics 33 (2): 69. arXiv:0908.3988. Bibcode:2010APh....33...69Z. doi:10.1016/j.astropartphys.2009.11.005.
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8. ^ Milligan on March 22, 2006 10:31 PM. "WMAP: The Cosmic Axis of Evil - EGAD". Blog.lib.umn.edu. Retrieved 2014-05-11.
9. ^ Schild, Rudolph E.; Gibson, Carl H. (2008). "Goodness in the Axis of Evil". arXiv:0802.3229v2 [astro-ph].
10. ^ a b Granett, Benjamin R.; Neyrinck, Mark C.; Szapudi, István (2008). "An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect". The Astrophysical Journal 683 (2): L99–L102. arXiv:0805.3695. Bibcode:2008ApJ...683L..99G. doi:10.1086/591670.
11. ^ Kaiki Taro Inoue; Silk, Joseph (2006). "Local Voids as the Origin of Large-angle Cosmic Microwave Background Anomalies I". The Astrophysical Journal 648: 23–30. arXiv:astro-ph/0602478. Bibcode:2006ApJ...648...23I. doi:10.1086/505636.
12. ^ Rudnick, Lawrence; Brown, Shea; Williams, Liliya R. (2007). "Extragalactic Radio Sources and the WMAP Cold Spot". The Astrophysical Journal 671: 40–44. arXiv:0704.0908. Bibcode:2007ApJ...671...40R. doi:10.1086/522222.
13. ^ Smith, Kendrick M.; Huterer, Dragan (2008). "No evidence for the cold spot in the NVSS radio survey". Monthly Notices of the Royal Astronomical Society 403 (2): 2. arXiv:0805.2751. Bibcode:2010MNRAS.403....2S. doi:10.1111/j.1365-2966.2009.15732.x.
14. ^ Granett, Benjamin R.; Szapudi, István; Neyrinck, Mark C. (2009). "Galaxy Counts on the CMB Cold Spot". The Astrophysical Journal 714 (825): 825. arXiv:0911.2223. Bibcode:2010ApJ...714..825G. doi:10.1088/0004-637X/714/1/825.
15. ^ Dark Energy and the Imprint of Super-Structures on the Microwave Background
16. ^ Finelli, Fabio; Garcia-Bellido, Juan; Kovacs, Andras; Paci, Francesco; Szapudi, Istvan (2014). "A Supervoid Imprinting the Cold Spot in the Cosmic Microwave Background". arXiv:1405.1555 [astro-ph.CO].
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18. ^ Cruz, M.; N. Turok; P. Vielva; E. Martínez-González; M. Hobson (2007). "A Cosmic Microwave Background Feature Consistent with a Cosmic Texture". Science 318 (5856): 1612–4. arXiv:0710.5737. Bibcode:2007Sci...318.1612C. doi:10.1126/science.1148694. PMID 17962521. Retrieved 2007-10-25.
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