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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 (CMBR).

The "Cold Spot" is approximately 70 μK (0.00007 K) colder than the average CMB temperature (approximately 2.7 K), whereas the root mean square of typical temperature variations is only 18 μK. At some points, the "cold spot" is 140 μK colder than the average CMB temperature. 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″). 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 that may exist between us and the primordial CMB (foreground voids can cause cold spots against the CMB). Such a void would affect the observed CMB via the integrated Sachs–Wolfe effect, and would be one of the largest structures in the observable universe.

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

One possible explanation of the cold spot is a huge void between us and the primordial CMB. Rudnick et al. found a dip in NVSS galaxy number counts in the direction of the Cold Spot, suggesting the presence of a large void. The correlation between the NVSS dip and the Cold Spot was found to be marginal using a more conservative statistical analysis. A 2015 study shows the presence of a supervoid that has a diameter of 1.8 billion light years and is centered at 3 billion light-years from our galaxy in the direction of the Cold Spot, likely being associated with it. 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. A 2017 study reported surveys showing no evidence that associated voids in the line of sight could have caused the CMB Cold Spot and concluded that it may instead have a primordial origin. In December 2021, the Dark Energy Survey (DES), analyzing their data, put forward more evidence for the correlation between the Eridanus supervoid and the CMB cold spot.

A sophisticated computational analysis (using Kolmogorov complexity) has derived evidence for a north and a south cold spot in the satellite data: "...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." However, apart from the Southern Cold Spot, the varied statistical methods in general fail to confirm each other regarding a Northern Cold Spot.

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.

Quick Facts

• Existence of its counterpart, a Northern Cold Spot with almost identical randomness properties among other low-temperature regions is revealed." However, apart from the Southern Cold Spot, the varied statistical methods in general fail to confirm each other regarding a Northern Cold Spot.
• At some points, the "cold spot" is 140 μK colder than the average CMB temperature.
• The "Cold Spot" is approximately 70 μK (0.00007 K) colder than the average CMB temperature (approximately 2.7 K), whereas the root mean square of typical temperature variations is only 18 μK.
• Various alternative explanations exist, including a so-called Eridanus Supervoid or Great Void that may exist between us and the primordial CMB (foreground voids can cause cold spots against the CMB).
• Thus a cold region as large as the "cold spot" appears very unlikely, given generally accepted theoretical models.

Source material: Wikipedia - "CMB cold spot". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.