Black holes stash matter in cosmic voids

Vast vacant slabs of space, stretching millions of light-years wide, comprise around 80% of the Universe's volume. But a new study indicates they're not as empty as we thought, thanks to the explosive gluttony of supermassive black holes. Belinda Smith explains.

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Simulations suggest 50% of the Universe's total mass is where galaxies are found, or just 0.2% of the visible Universe.
Markus Haider / Illustris collaboration

Supermassive black holes are responsible for banishing almost a fifth of "normal" matter into intergalactic voids, computer modelling suggests.

A group of astronomers in Europe and the US simulated the vast voids of space, and calculated they might contain as much as 20% of the normal matter in the Universe.

The culprits? Supermassive black holes that live in the centre of galaxies.

The team, led by Markus Haider of the University of Innsbruck in Austria, published their cosmic simulations in the Monthly Notices of the Royal Astronomical Society.

We live in a Universe dominated by unseen matter: only around 5% of it is "normal" – that is, made of atoms – with the rest being dark matter and dark energy.

Matter isn't spread evenly through the Universe. Galaxies and everything they contain are concentrated into a "cosmic web": filaments millions of light-years long that stretch around the edge of enormous voids.

These voids comprise around 80% of the Universe's volume.

Haider and his colleagues probed the web using data from the Illustris project, a supercomputer simulation of galactic formation and evolution, to measure how much mass is retained in the filaments and how much has spilt into the voids.

(The largest Illustris simulation was run on 8,192 computer cores, and took 19 million processing hours – the equivalent of one computer running for around 2,000 years.)

A slab cut from the cube generated by the Illustris simulation. It shows 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
The same slice of data, this time showing the distribution of 'normal' or baryonic matter. – Markus Haider / Illustris collaboration

Dark matter, shown in the top image above, is neatly concentrated in the filaments. But normal matter is a different story – it's "fluffier", with around a fifth found in the enormous voids.

Haider thinks supermassive black holes are the exilers. As matter falls into a supermassive black hole, some of it is converted to energy.

This energy shoots out into the surrounding gas and thrusts it hundreds of thousands of light-years away from the black hole, farther than the border of its host galaxy.

Dark matter, on the other hand, is impervious to the blasts, and stays neatly in its filaments.

“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," Haider says.

The team plans to run new simulations with Illustris to confirm and refine their model, and say these should be available in a few months.

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Belinda Smith is a science and technology journalist in Melbourne, Australia.