The cosmic web is spinning. Our universe is full of enormous filaments of matter that stretch between galaxies in a vast, interconnected web – and simulations suggest that those filaments are rotating.
We know that all galaxies spin and that their rotation can be affected by their environments, including any nearby filaments of matter, but we didn’t know whether these huge, thread-like structures also rotate. Now, Qianli Xia at the University of Edinburgh, UK, and his colleagues have used simulations of how dark matter behaves in filaments to figure out how they might be moving.
The researchers combined simulations of nearly 34,000 filaments to measure their spin, and found that their average rotational velocity was about 80 kilometres per second. They estimated that about 26 per cent of filaments in the universe have noticeable spin.
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“I’d really like to be able to measure this in the real universe,” says Nick Kaiser at the École Normale Supérieure in Paris. “That’s super hard, because the observations are very noisy, and in an individual object, you’re not going to see much of that effect – you have to stack them.”
Although the spinning would be difficult to spot in the real cosmos, the prediction that it exists isn’t particularly surprising, as we expect most large objects in space to rotate based on how they form, says Alan Heavens at Imperial College London.
In the simulation, the researchers found that this spinning occurs because of what astronomers call tidal torque. As a clump of matter – a cloud of gas, for example – falls towards a filament, the closer part of it experiences stronger gravitational forces than the most distant part, causing it to spin. When it falls into the filament, that spin gets passed on as well.
“If these filaments are spinning themselves, you would expect that that has a consequence for the galaxies that are living in them,” says Rien van de Weijgaert at the University of Groningen in the Netherlands. “If you are interested in the origin and evolution of galaxies and, in the end, where our Milky Way came from, this is the kind of stuff that’s important.” The fact that filaments spin could help refine our understanding of why galaxies spin.
It could also help explain another deeper mystery: where the magnetic fields that suffuse the cosmos come from. “We know the mechanisms which would amplify pre-existing magnetic fields, so we know of ways of taking a very small field and making it bigger, but that begs the question of where did the very small field come from?” says Heavens.
Magnetic fields come from the movement of charged particles, but we don’t know which particles are responsible for galactic-scale fields. If the particles that make up the filaments have any electric charge, they could create the small magnetic fields that are magnified by galaxies and clusters of galaxies.
At its most basic, this finding allows us to understand the backbone of matter in our universe. “The largest structure in the universe is the cosmic web,” says van de Weijgaert. “Now we are starting to see its properties, and it is moving from something abstract into something that is real and has real physical characteristics.”
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