Astronomers have found that the largest structures in the universe spin, making twister-like strings of galaxies and dark matter in the cosmic web.
Like giant cosmic twisters, filaments of the cosmic web slowly spin around their own axis while funneling matter into galaxy clusters. These humongous structures are made of galaxies, gas, and dark matter and can measure up to hundreds of millions of light-years. And they are now the largest structures in the universe that are known to rotate.
A team led by Peng Wang (Leibniz Institute for Astrophysics Potsdam (AIP), Germany) found evidence for these filaments’ spin by studying more than 200,000 galaxies in the Sloan Digital Sky Survey. In a June 14th paper published in Nature Astronomy, the team concludes that the results “are consistent with the detection of a signal one would expect if filaments rotated.”
Much of the universe spins — stars, galaxies, and clusters. But astronomers still don’t understand if and how spins on smaller scales connect to those on (much) larger scales. “The spin of intergalactic filaments is a key piece of the puzzle,” comments theorist Miguel Aragon-Calvo (National Autonomous University of Mexico). Team member Noam Libeskind (AIP) said in a press statement, “There must be an as yet unknown physical mechanism responsible for torquing these objects.”
Wang, Libeskind, and their Estonian and Chinese colleagues used an algorithm to identify linear structures in the distribution of galaxies in clusters. By stacking data on more than 17,000 of these filaments, they found evidence that galaxies on one side of a filament “spine” generally have a higher redshift (a measure of the galaxies' radial velocity toward or away from Earth) than galaxies on the other side. These motion asymmetries suggest that galaxies are rotating around filament axes with velocities up to 100 kilometers per second (200,000 mph). (That may sound fast, but it’s generally much too slow to complete one “orbit” around the filament within the present age of the universe).
According to popular cosmological models, gravity has concentrated matter (both visible and dark) into thin sheets and elongated filaments. There’s some observational evidence of tenuous gas and dark matter in this cosmic web, though it’s still pretty scarce. However, the intricate three-dimensional structure shows up clearly in the distribution of galaxies that formed out of this raw material. Computer simulations and velocity measurements indicate that dark matter, gas and galaxies flow toward the still-growing clusters at the nodes where filaments meet.
Aragon-Calvo was part of a team that recently found indications of filament spin in the Millennium Simulation — a dark-matter-only simulation of cosmic structure growth. “Wang and his colleagues did a great job uncovering a signal that has been hiding in plain sight,” he says. “This is a direct confirmation of our prediction.”
It is not yet known how the corkscrew-like motions of galaxies in the cosmic web — the combination of filament spin and funneling — will influence the rotation of smaller structures, like galaxy clusters. Interestingly, the observed motion asymmetry is stronger in filaments that connect more massive clusters, which suggests the clusters affect the filaments’ behavior. The researchers of the new study hope that future work may also reveal at which stage in structure formation filaments spin up, and how this affects the galaxies they contain.
According to Aragon-Calvo, theory suggests that matter in the newborn universe should not exhibit any non-random, systematic rotation. “If we were to find signatures of primordial vorticity, this could point to new processes or even new physics,” he says.