CMB’s giant cold spot continues to challenge astronomers

The universe is full of mysteries, and one of the most intriguing lies in the cosmic microwave background (CMB), the fossil light from the Big Bang emitted when the universe was only 380,000 years old. Although this light is nearly uniformly distributed, it exhibits tiny temperature variations that offer us clues about the primordial state of our cosmos. Among these variations, one anomaly stands out: the cold spot. This region, both larger and colder than expected, has intrigued astronomers for decades.

The origin and characteristics of CMB

THE cosmic microwave background (CMB) formed about 380,000 years after the Big Bang, when the Universe transitioned from a state of hot and opaque plasma to a neutral and transparent gas. At that time it was much smaller and warmer than today, and had a temperature of about 9,700 degrees Celsius. This process, known as recombination, released a flood of photons, the intense light of the young Universe, which then traveled through space as it gradually cooled. Over the next billions of years, this primordial light stretched as the Universe expanded, reaching a temperature of nearly -270°C today, placing this radiation in the microwave band of the electromagnetic spectrum. The CMB thus became a kind of snapshot of the young Universeproviding astronomers with a unique window into the initial conditions of the cosmos. The CMB’s temperature variations, although very small, are crucial to our understanding of the structure and evolution of the cosmos. These variations, called anisotropiesare generally of the order of 1 part per million. They result from quantum fluctuations in the early Universe that were then amplified by cosmic inflation. These anisotropies then sowed the seeds for the formation of later structures such as galaxies and galaxy clusters.

A map of the sky shows the cosmic microwave background (CMB), a remnant from the period in the early Universe when this lost dark matter could have existed. Credits: ESA and Planck Collaboration

A “cold spot” that raises questions

Among these anisotropies, the cold spot stands out strikingly. Located in the direction of the constellation Eridanus, it is both bigger and colder than other observed temperature variations. While typical CMB variations show temperature deviations of a few tens of microkelvins, the cold spot shows a much more significant temperature anomaly, with deviations reaching about 70 microkelvins (0.00007 °C) on average and up to 140 microkelvins (0.00014 °C) in its deepest parts. The size of the cold spot is also remarkable. With a diameter of about five degrees, it is much larger than typical CMB spots that are typically less than one degree. This combination of anomalous size and temperature makes the cold spot a particularly intriguing object of study for astronomers, as it could reveal unknown aspects of cosmic physics or special features of the early Universe.

Explanatory hypotheses

Several hypotheses have been put forward to explain this anomaly. One of the most serious is the existence of a gigantic cosmic void between us and the CMB in this direction. Cosmic voids are large regions that contain very little matter and their influence on CMB light can be significant. When CMB light passes through a void, it can lose energy due to theSachs-Wolfe effect integrated, due to the dynamical evolution of the vacuum. A large vacuum of variable depth could therefore create a region of lower temperature observed as the cold spot. However, the confirmation of this hypothesis is problematic. Studies of galaxies in this region are indeed incomplete and current maps do not provide definitive proof of the existence of a supervoid. And even if one did exist, it is not certain that it could produce the effect necessary to fully explain the cold spot. Faced with the uncertainty surrounding the supervoid, more exotic proposals have been formulated. One of the most audacious ideas suggests that the cold spot could be a point of intersection between our Universe and a neighboring universe. While fascinating, this hypothesis remains speculative and fails to explain all of the observed properties of the cold spot. Ultimately, the CMB cold spot continues to challenge astronomers. While we don’t yet have a definitive answer about its nature, future research may offer insights. Whether it’s a supervoid, an interaction with a parallel universe, or some other as-yet-unknown phenomenon, the cold spot reminds us that our understanding of the universe is still evolving.