The cosmic web is spinning. Our universe is full of enormous filaments of matter that stretch between galaxies in a vast, interconnected web 鈥� 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鈥檛 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聽kilometres per second. They聽estimated that about 26聽per聽cent of filaments in the universe have noticeable spin.
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鈥淚鈥檇 really like to be able to measure this in the real universe,鈥� says Nick Kaiser at the 脡cole Normale Sup茅rieure in Paris. 鈥淭hat鈥檚 super hard, because the observations are very noisy, and in聽an individual object, you鈥檙e not going to see much of that effect 鈥� you have to stack them.鈥�
Although the spinning would be聽difficult to spot in the real cosmos, the prediction that it exists isn鈥檛 particularly surprising, as we expect most large objects in聽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聽astronomers call tidal torque. As a聽clump of matter 鈥� a cloud of gas, for example 鈥� falls towards a filament, the closer part聽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.
鈥淚f these filaments are spinning themselves, you would expect that that has a consequence for the galaxies that are living in them,鈥� says Rien van de Weijgaert at the University of Groningen in the Netherlands. 鈥淚f 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鈥檚 important.鈥� 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. 鈥淲e 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?鈥� says Heavens.
Magnetic fields come from the movement of charged particles, but we don鈥檛 know which particles are responsible for galactic-scale fields. If the particles that make up聽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. 鈥淭he largest structure in聽the universe is the cosmic web,鈥澛爏ays van de Weijgaert. 鈥淣ow聽we are聽starting to see its properties, and it is moving from聽something abstract into something that is real and has real聽physical characteristics.鈥�
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