"Cosmic Voids" May Hold A Fifth Of All Matter In The Universe

Shown is the distribution of dark matter, with a width and height of 350 million light-years and a thickness of 300,000 light-years. Galaxies are found in the small, white, high-density dots. Markus Haider / Illustris collaboration

If we could somehow sit on the edge of the visible universe and look at how galaxies are distributed in the cosmos, we would notice something peculiar. Galaxies might seem scattered here and there, but in reality they are distributed in long, interconnected filaments.

Between the filaments, there are the cosmic voids – large regions of the universe that were thought to be empty. But now a group of astronomers discovered that 20 percent of the visible matter in the universe is spread out in these voids, and it has been shot there by the powerful jets of supermassive black holes.

The team, which included astronomers based in Austria, Germany, and the U.S., looked at the Illustris project, a large computer simulation of the universe as we know it. Their findings are published in the Monthly Notices of the Royal Astronomical Society.

The simulation looks at how the universe evolved from 12 million years after the Big Bang to today. To do so, it uses the latest data on the composition of the universe. The universe is made of three components: 4.9 percent visible matter (which makes stars, planets, us, and so on), 26.8 percent dark matter (a mysterious form of matter that only interacts gravitationally) and 68.3 percent dark energy (the even more mysterious negative pressure that is forcing the universe to expand).

The simulation tracked how both visible and dark matter changed the structure of the cosmos, and how the filaments form. The distribution is called the cosmic web and it is a direct consequence of the composition of the universe. In the cosmic web, galaxies occupy about 1/500th of the universe and the voids make up 80 percent of the volume.

The same slice of data as above, this time showing the distribution of visible matter; compared to the top image, it can be seen how much more spread out visible matter is. Credit: Markus Haider / Illustris collaboration

When the team looked at the matter distribution from Illustris, they found that 50 percent of matter (both visible and dark) is in galaxies, 44 percent is found in the filaments and 6 percent is in the void. While dark matter usually dominates over matter one to six, in voids there’s a lot more matter than expected.

“This simulation, one of the most sophisticated ever run, suggests that the black holes at the center of every galaxy are helping to send matter into the loneliest places in the universe. What we want to do now is refine our model, and confirm these initial findings,” Dr. Markus Haider, lead author of the paper, said in a statement.

Illustris is now running new simulations to corroborate this result. For the time being, simulations are the only way to understand the material in the voids. As they are so far from galaxies, the gas is too cool and too disperse to be directly observed with current instruments. 

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