CMB cold spot

The CMB Cold Spot is a mysterious region of the sky that challenges our understanding of the universe. It is located within the cosmic microwave background radiation, or CMBR. The CMBR is a faint glow of microwave radiation that fills all of space. This radiation is the leftover heat from the early universe. Most of this background radiation has a steady temperature of about 2.7 K. However, the Cold Spot is unusually cold compared to this average.

It is approximately 70 μK colder than the rest of the sky. In some areas, the temperature drops by as much as 140 μK.

To find this spot, scientists first had to account for the dipole anisotropy. This is a temperature change caused by the Doppler shift. It happens because the Earth moves at about 627 km/s toward the constellation Virgo. Once this movement is subtracted, the Cold Spot becomes clear. It is located in the Southern Celestial Hemisphere. Specifically, it sits within the constellation Eridanus. The spot has a radius that covers about 5° of the sky. This is much larger than the typical 1° fluctuations seen in the CMBR.

Researchers first identified this anomaly using the Wilkinson Microwave Anisotropy Probe, known as WMAP. During its first year of data collection, WMAP found this cold region in Eridanus. Scientists later used three years of WMAP data to study its significance. They found only a 1.85% probability of such a deviation occurring in standard Gaussian simulations. This low percentage suggests the spot is quite unusual. In 2013, the Planck satellite observed the same area. The Planck data showed similar significance to WMAP. This helped prove the spot was a real feature and not a mistake by the WMAP satellite.

One major theory is that a supervoid exists between us and the CMBR. A supervoid is an enormous region of space containing very little matter. Such a void would affect the light through the integrated Sachs–Wolfe effect. This effect occurs when photons travel through a changing gravitational field. As light passes through a large void, dark energy stretches the space. This causes a cancellation between the ordinary Sachs–Wolfe effect and the late-time integrated Sachs–Wolfe effect. This process results in the observed cold temperature.

Different studies have provided different measurements for this potential supervoid. A 2015 study suggested a void with a diameter of 1.8 billion light-years. This void would be located roughly 3 billion light-years from our galaxy. Some researchers claimed this might be the largest structure ever identified by humanity. However, other sources suggest even larger structures exist elsewhere in the universe. A void large enough to explain the Cold Spot would need to be massive. It might be 6 to 10 billion light-years away and nearly one billion light-years across.

Other scientists have proposed even more exotic explanations for the anomaly. Some suggest the spot is a cosmic texture. This would be a remnant from a phase transition in the early universe. Others have proposed the existence of cosmic strings or loops. A more controversial theory involves parallel universes. Some researchers suggest the spot could be an imprint from another universe. This might happen through quantum entanglement before cosmic inflation occurred. Another idea is that the spot resulted from a collision between cosmological bubbles.

Studying the Cold Spot helps scientists test their models of the early universe. Most models assume that temperature fluctuations follow Gaussian statistics. This means the fluctuations should be predictable and follow a specific pattern. The Cold Spot might be a signal of non-Gaussian primordial fluctuations. This would mean the early universe was more complex than current models suggest. Some computational analyses have even looked for a matching Northern Cold Spot. While some data suggests a counterpart exists, other statistical methods fail to confirm it. The mystery of the Cold Spot continues to drive new astronomical research.