Researchers from the University of Portsmouth, UK have solved an intriguing cosmological mystery by creating the largest ever map of how matter is distributed in the universe.
Dr. Seshadri Nadathur and Professor Robert Crittenden looked at how photons from the cosmic microwave background (CMB), the light left over from the Big Bang, are affected by the areas of the cosmos that are full of galaxies, the superclusters, and the uninhabited cosmic voids.
According to Einstein’s theory of general relativity, photons traveling through the voids should look slightly colder, and the ones traveling through the superclusters slightly hotter. This phenomenon is known as the Integrated Sachs-Wolfe (ISW) effect.
“When this effect was studied by astronomers at the University of Hawai’i in 2008 using an older catalog of voids and superclusters, the effect seemed to be five times bigger than predicted,” lead author Nadathur said in a statement.
“This has been puzzling scientists for a long time, so we looked at it again with new data.”
The effect is very subtle, so the Portsmouth team had to construct an incredibly detailed map of the structure in the universe. They used the distribution of 750,000 galaxies from the Sloane Digital Sky Survey and applied to the microwave background map taken by the Planck satellite. This map is 300 times larger than the one previously used.
The researchers showed, in a paper published in the Astrophysical Journal Letters, that the observed effect is in agreement with general relativity and with our current understanding of the universe. But, as in all good physics stories, every discovery leads to new questions.
“Our results resolve one long-standing cosmological puzzle, but doing so has deepened the mystery of a very unusual ‘Cold Spot’ in the CMB," added Nadathur.
The region is related to a cosmic supervoid but that is not enough to explain the unusual coolness. There must be something wrong with general relativity to explain that, and we haven’t found any evidence suggesting this is the case yet.
“It was thought that there was some exotic gravitational effect contradicting Einstein which would simultaneously explain both the Cold Spot and the unusual ISW results from Hawai’i," continued Nadathur. "But this possibility has been set aside by our new measurement.”
The mystery of the Cold Spot remains without an explanation. At least for now.