Cosmic voids — vast underdense regions, forming an essential feature of the cosmic web and occupying most of the volume of the Universe — could contain as much as 20% of the ‘normal’ matter (i.e. the matter that makes up stars, planets, gas and dust) in the cosmos, according to an international team of astronomers led by Dr. Markus Haider of the University of Innsbruck, Austria.
This slab cut from the cube generated by the Illustris simulation shows the distribution of baryonic (normal) matter, with a width and height of 350 million light-years and a thickness of 300,000 light-years. Image credit: Markus Haider / Illustris Collaboration.
Looking at cosmic microwave radiation, modern space observatories have gradually refined our understanding of the composition of the Universe, and the most recent measurements suggest it consists of 4.9% ‘normal’ matter, or ‘baryons,’ whereas 26.8% is the unseen dark matter and 68.3% is the mysterious dark energy.
Ground-based observatories have mapped the positions of galaxies and, indirectly, their associated dark matter over large volumes, showing that they are located in filaments that make up a ‘cosmic web.’
Dr. Haider and his colleagues investigated this in more detail, using data from the Illustris project.
“Illustris is a large computer simulation of the evolution and formation of galaxies, to measure the mass and volume of these filaments and the galaxies within them. It simulates a cube of space in the Universe, measuring some 350 million light years on each side,” they said.
“It starts when the Universe was just 12 million years old, a small fraction of its current age, and tracks how gravity and the flow of matter changes the structure of the cosmos up to the present day.”
“The simulation deals with both normal and dark matter, with the most important effect being the gravitational pull of the dark matter.”
When the astronomers looked at the data, they found that about 50% of the total mass of the Universe is in the places where galaxies reside, compressed into a volume of 0.2% of the Universe we see, and a further 44% is in the enveloping filaments.
Just 6% is located in the voids, which make up 80% of the volume. But they also found that a surprising fraction of normal matter — 20% — is likely to be have been transported into the cosmic voids.
The culprit appears to be the supermassive black holes found in the centers of galaxies.
The same slice of data, this time showing the distribution of dark matter. Image credit: Markus Haider / Illustris Collaboration.
“Some of the matter falling towards the holes is converted into energy. This energy is delivered to the surrounding gas, and leads to large outflows of matter, which stretch for hundreds of thousands of light years from the black holes, reaching far beyond the extent of their host galaxies,” the astronomers explained.
Apart from filling the voids with more matter than thought, the result might help explain the missing baryon problem, where scientists do not see the amount of normal matter predicted by their models.
“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,” said Dr. Haider, who is the lead author of a paper published in the Monthly Notices of the Royal Astronomical Society.
“What we want to do now is refine our model, and confirm these initial findings.”
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M. Haider et al. 2016. Large-scale mass distribution in the Illustris simulation. MNRAS 457 (3): 3024-3035; doi: 10.1093/mnras/stw077
