Perhaps a supervoid doesn’t explain the mysterious CMB cold spot

For years, cosmologists had thought {that a} unusual function showing within the microwave sky, often known as the CMB chilly spot, was as a result of mild passing by a large supervoid. However new analysis casts that conclusion into doubt.

The cosmic microwave background (CMB) is the one best supply of sunshine in all of the universe. It fully permeates all of space and is made of sunshine leftover from when the cosmos was solely 380,000 years outdated. At the moment, our universe had expanded and cooled to the purpose that it may transition from a scorching, dense plasma right into a barely cooler, however impartial, fuel. That course of launched white-hot radiation with a temperature of round 10,000 Ok. Within the intervening 13.8 billion years, nevertheless, that radiation has cooled and redshifted right down to round three levels above absolute zero, which places that radiation firmly within the microwave regime.

Inside that mild there are tiny bumps, wiggles and variations in brightness. These variations unfold throughout the sky and are extremely tiny, with variations no greater than one half in 100,000. These variations symbolize tiny density variations within the early universe that might finally develop as much as grow to be galaxies and clusters. Cosmologists perceive the statistical properties of those bumps and wiggles to a fantastic diploma, however there may be one important outlier often known as the chilly spot.

The chilly spot is about 10° throughout, and whereas it’s not exceptionally chilly, the mix of its decrease temperature and its huge dimension places it outdoors of what we might anticipate from our normal mannequin of cosmology.

One believable speculation to clarify the chilly spot is that the sunshine in that course of the cosmic microwave background has traveled by a really giant patch of the universe that’s much less dense than common, one thing known as a supervoid.

As a result of the supervoid is so giant, it takes an infinite period of time for mild to cross it. And through that point the void itself grows bigger and deeper because the universe evolves. That signifies that when the sunshine entered the supervoid, the void was smaller than when the sunshine exited. This distinction saps vitality from the radiation, inflicting it to chill and redshift greater than the typical for the remainder of the CMB.

Galaxy surveys have proven the potential presence of a supervoid within the course of the chilly spot. However a brand new examine exhibits that the identified supervoids in that course can solely clarify a fraction of the dimensions and temperature of the chilly spot. Certainly, the brand new evaluation exhibits that there can’t be a big sufficient supervoid in that course, even one hidden behind the boundaries of our present observational capabilities.

The brand new analysis, printed on the preprint server arXiv, doesn’t suggest an answer to the chilly spot thriller, but it surely does inform us that now we have much more to study.