The Mystery of the Universe's Missing 'Normal' Matter Has Begun to Unravel


Tanimura’s team and another team led by Anna de Graaff at the University of Edinburgh in Scotland looked at data from the Planck satellite for a thermal signal called the Sunyaev-Zel’dovich effect. This effect allows for the detection of very faint  objects,  and looks for photons from the Cosmic Microwave Background as it travels through hot gas.

The interaction, which only the Planck satellite so far has been able to detect, allows astronomers to spot the presence of matter, even if it is very faint at high redshifts.

In 2015, Planck data was used to create a map of this effect throughout the observable universe. But because the filaments of gas between galaxies are so diffuse, it is very difficult to detect them directly on Planck’s map without using points of reference.

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Both of the teams of researchers used data from the Sloan Digital Sky Survey to look at galaxies that were predicted to be connected by filaments of faint gas. They stacked the Planck data to look in the areas between the galaxies.

Tanimura’s group stacked data on 260,000 pairs of galaxies, and de Graaff’s team used over a million pairs. Both teams found conclusive evidence of the baryonic gas filaments between the galaxies.

Tanimura said the results between the two groups are consistent within margins.

“The biggest surprise is that the gas we detected is very low-dense, lower than expected,” Tanimura said. “It is very surprising and very important because we prove that we can detect it now! It means that we can now start to make an entire map of the universe, including filaments as well as galaxies.”

Tanimura said that the total amount of baryons has been measured by other observations such as the CMB observations and Lyman Alpha observations, and their results are consistent within margins with cosmological simulations.

“There is already a consensus about it and we prove that it is true,” he said. “But we know more than that. We estimate the distribution and physical states of the (missing) baryons. By comparing the result (which was unknown) with current models such as cosmological simulations, we can make [a] more precise picture of the current universe and constrain the evolution of the universe.”

Tanimura noted that this finding is analogous to the first maps made of the world.

“When people went out to the ocean and started making a map of our world, it was not used by most of the people then, but we use the world map now to travel abroad,” he said. “In the same way, the map of the entire universe may not be valuable now because we do not have a technology to go far out to the space. However, it could be valuable 500 years later.”

He added, “We are in the first stage of making [a] ‘map of the entire universe.’”

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