The most powerful supercomputer simulation of the Universe is providing important insights into how matter is distributed across large scales. Surprisingly, a significant portion of matter resides outside of galaxies and in the cosmic voids that permeate the cosmos. Our universe is filled with all sorts of stuff, not all of it visible to the naked eye. There's your regular, everyday normal matter (also known as baryons), comprised of stars, planets, dust, gas, human beings and hoverboards that aren't actually hoverboards. And then there's the weird stuff, like invisible dark matter and mysterious dark energy, the exact nature of which still eludes cosmologists.
But the Universe is old and big, and all this stuff isn't distributed evenly. To learn more about the large-scale distribution of cosmological matter, a team of astronomers from the United States, Austria and Germany analysed data being churned out by the Illustris project — the most accurate supercomputer simulation of the universe ever developed. This model simulates a cube of space measuring 350 million light-years on each side. It begins when the universe was just 12 million years old, tracking the ebbs and flows of gravity, and how matter has changed the structure of the universe we see today.
Illustris suggests that galaxies make up only 1/500th (0.002 per cent) of the volume of the universe, and that dark voids in the outer reaches of the cosmos could contain as much as 20 per cent of the normal matter in the cosmos. The details of this work now appear in the latest edition of the Monthly Notices of the Royal Astronomical Society.
The new research suggests that half of all mass in the universe is located within galaxies, which if compressed would fit it into a single volume just 0.2 per cent the size of the visible universe. Another 44 per cent of matter is located in the concentrated filaments that connects all galaxies and stretch around the edge of enormous voids. The remaining six per cent is located within these voids themselves, which make up 80 per cent of the volume. This means these voids aren't as void-like as previously thought.
Normal matter (i.e. baryons) distribution as simulated by Illustris. Image: Markus Haider / Illustris collaboration.
The researchers were surprised to learn that a significant portion of normal matter — about 20 per cent — is located in these voids. They say supermassive black holes located in the centre of galaxies are responsible. As matter falls towards these black holes, they get converted into energy. In turn, this energy gets delivered to the surrounding gas, leading to large outflows of matter that stretch for hundreds of thousands of light-years from the black holes, and extend far out beyond the reach of the host galaxies (a process called active galactic nuclei feedback, in case you were wondering).
"This simulation, one of the most sophisticated ever run, suggests that the black holes at the centre of every galaxy are helping to send matter into the loneliest places in the universe," noted study lead author Markus Haider from the the Institute of Astro- and Particle Physics at the University of Innsbruck in Austria in a statement.
Interestingly, this could explain the so-called "missing baryon problem", a conundrum in which astronomers don't see the expected amount of normal matter predicted by models. It seems that a large fraction of the baryons that presumably exist in the cosmos resides in the thin strands between the galaxies, where they're hard to observe due to their low densities and high temperatures.
"What we want to do now is refine our model, and confirm these initial findings," added Haider.
Dark matter distribution as simulated by Illustris. (Image: Markus Haider / Illustris collaboration)