CMB 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 (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' deviates 140 µK colder than the average CMB temperature. 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' deviates 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″). 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 and 6 to 10 billion light years away, at redshift z ≃ 1 {displaystyle zsimeq 1} , 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. 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. 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%. 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. The large 'cold spot' forms part of what has been called an 'axis of evil' (so named because it is unanticipated to see structure). One possible explanation of the cold spot is a huge void between us and the primordial CMB. A region cooler than surrounding sightlines can be observed if a large void is present, as such a void would cause an increased cancellation between the 'late-time' integrated Sachs–Wolfe effect and the “ordinary” Sachs-Wolfe effect. This effect would be much smaller if dark energy were not stretching the void as photons went through it. Rudnick et al. 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. Also, a direct survey for galaxies in several one-degree-square fields within the Cold Spot found no evidence for a supervoid. However, the supervoid explanation has not been ruled out entirely; it remains intriguing, since supervoids do seem capable of affecting the CMB measurably.