From our tiny human perspective on Earth, the universe can seem like a deceptively stable place. But in reality, our planet is a rotating orb located in a solar system that sweeps around the center of the Milky Way, which is itself just one of billions of spinning galaxies across the observable universe.
It seems that no matter where we look, objects in the universe are dizzily spinning away. Now, scientists led by Peng Wang, a postdoctoral researcher at Leibniz Institute for Astrophysics Potsdam (AIP), have discovered that objects can rotate on previously unheard-of scales that span hundreds of millions of light years.
The mind-boggling finding reveals that filaments within the cosmic web, an enormous network of dark matter and gas structures that connects the universe, are rotating, making them “the largest objects known to have angular momentum,” according to a study published on Monday in Nature Astronomy.
“It's a major finding,” said Noam Libeskind, a cosmologist at AIP who initiated the project and co-authored the study, in a joint call with Wang. “It's a pretty big deal that we've discovered angular momentum, or vorticity, on such a huge scale.”
“I think it will help people understand cosmic flows and how galaxies are moving throughout the cosmic web and through the universe,” he added. “It will help us better understand the important scales for galaxy formation and ultimately, why everything in the universe is spinning and how spin is generated. That is a really, really hard question to solve. It's an unsolved question in cosmology.”
It’s difficult to even contemplate the scales of the spinning filaments, which are threads primarily made of dark matter, a mysterious unidentified substance that is common in the universe. These structures connect distant galaxy clusters and contain gas streams and populations of galaxies within their gargantuan tubular extents. It’s clear that filaments influence the motions of galaxies inside of them to some degree, but the movements of the intergalactic structures themselves, and their exact effect on galactic dynamics, have remained elusive for years.
“There's some order in this chaotic universe.”
Wang and his colleagues set out to investigate this problem by meticulously examining hundreds of thousands of galaxies within a few billion light years of Earth captured by the Sloan Digital Sky survey based in New Mexico.
To probe angular motion in filaments, the team adopted a technique that has long been used to measure the rotations of galaxies. Scientists aiming to clock galactic spins make use of the Doppler effect, which manifests as a change in the frequency of waves that depends on where an observer is located in relation to those waves. The most famous example of this effect is the change in the pitch of a siren you hear as an ambulance drives by you; when the ambulance is approaching you, the sound waves are compressed into higher frequencies compared to the lower frequencies you hear when it passes you.
Because the Doppler effect also occurs in light, radiant objects in space produce light waves that become compressed, or “blueshifted,” into a blue color when they are moving toward Earth and “redshifted” into a lower-frequency red color when they are moving away from our perspective. Generally speaking, the more blueshifted or redshifted an object appears to us, the faster it is moving toward or away from Earth.
To apply this effect to the cosmic web, Wang’s team identified giant filament structures in the Sloane survey. The researchers then divided the dozens of galaxies within the threads into two regions placed on either side of the structure, and measured the redshifts of objects in those two regions.
“We borrowed the concept from galaxies and we calculated the redshift difference between the two parts,” Wang explained in the call, adding that his team also estimated the average velocities of all the galaxies in a filament.
As it turned out, the redshift difference between the two groups was much larger than the average velocities of the galaxies, which was a clear sign that one side of the filament was moving away from Earth. The effect could not be observed in all of the filaments due to observational constraints and the structural properties of the filaments, but the team found ”a very strong rotation signal” when the alignments were right, according to the study.
Wang and his colleagues also discovered that the strength of the rotational signal was linked to the masses of galaxy clusters located at either end of the filaments. Filaments that were hooked into more massive clumps of matter had a more obvious rotation, suggesting that the gravitational forces surrounding galaxy clusters may be part of the mechanism that makes these giant threads spin.
“It gives us a hint at what might actually be doing this; that it might actually be due to these massive gravitational fields that are created by these clusters,” Libeskind said. “But it's also quite a nice confirmation that there's no significant biases in our study, because if there were biases, they should be independent of these cluster masses.”
The exciting results suggest that galaxies follow helical (corkscrew-shaped) trajectories as they flow through these dark matter superhighways across the universe, “opening the door to a new understanding of cosmic spin,” according to the study.
That door leads to many new pathways, and Wang is already planning to follow up on the new research with advanced models that will simulate the evolution of these rotating filaments.
“This work is based on observation,” Wang said. “Next time, we'll go to a simulation so we can measure filament rotation at different redshifts to see how and when the filament spin forms.”
Libeskind added he was also interested in investigating whether the spin of the filaments aligns with the rotation of galaxies within them, which would require sophisticated follow-up observations of large galaxy populations.
“If the internal rotation of the galaxies themselves is consistent with the filament rotation—if the filament rotation is actually endowing galaxies with their spin—that would be an absolutely fascinating connection,” Libeskind said. “To actually see it in real observations, to see that galaxies are spinning with the filament, would help us understand how gas collapses to form galaxies.”
Scientists, including the authors of the new paper, have widely speculated about this possible connection between the motions of galaxies and the wider cosmic web. In some cases, galaxies that are located tens of millions of light years away seem to be rotationally synced up with each other, suggesting that they are influenced by the same large-scale structures.
However, Wang’s team is the first to actually measure the rotation of one of these structures, bringing the topic from hypothetical discussion into an observational reality. The new study not only smashes the record for measuring spin on such unprecedented scales, it also hints that objects in the universe may be far more dynamically connected—even across vast distances—than previously imagined.
“For me, at least, it’s incredibly inspiring to know that there's some sort of cosmos to the chaos,” Libeskind said. “There's some order in this chaotic universe. It's really mesmerizing to find these patterns, to look at the heavens and to look at these galaxies that are, on these scales, just specks of dust, and to say: ‘Wow, they're not just swarming randomly. There's actually an ordered motion to them. They're actually moving. They all know about each other. They're all feeling the same gravitational field. They're all moving in the same way.’”
“I'm awed by it every day of my life,” he concluded.