CMB Cold Spot

The CMB Cold Spot or WMAP Cold Spot is a region of the sky seen in microwaves which analysis 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.

The radius of the "cold spot" subtends about 5°; it is centered at the galactic coordinate l_II = 207.8°, b_II = −56.3° (equatorial: α = 03h 15m 05s, δ = −19° 35′ 02″). 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.