For the first time ever, scientists have glimpsed the faint glow made by shocks in the cosmic web, an immense network of filaments and nodes that links the universe, reports a new study.
The mind-boggling discovery opens an unprecedented new window into the large-scale structures that stretch across hundreds of millions of light years and influence the evolution of galaxies and other mysterious phenomena, such as cosmic magnetism.
In the first moments after the Big Bang some 13.8 billion years ago, the universe was a hot primordial soup of subatomic particles without much clear structure to it. However, as this chaotic brew cooled, different forms of matter began to emerge, such as regular “baryonic” matter—the familiar stuff that makes up stars, planets, and our bodies—and dark matter, an unidentified substance that accounts for the vast majority of matter in the universe.
Over time, clumps of dark matter began to gravitationally pull in regular matter, forming recognizable structures, such as galaxies. Galaxies, in turn, coalesced together into massive galaxy clusters that are linked across huge stretches of space by filaments of dark matter, creating what is now known as the cosmic web.
For years, scientists have speculated that magnetic fields within the cosmic web would help to produce shocks that might glow dimly in radio light. Now, for the first time, astronomers have captured this “predicted emission from the formation and growth of the large-scale structure of the Universe,” according to a recent study in Science Advances.
A team led by Tessa Vernstrom, a senior research fellow at the International Centre for Radio Astronomy Research, University of Western Australia, made the discovery by “stacking” more than 600,000 images of galaxy cluster pairs, meaning two clusters that are relatively close to each other in space and therefore likely to be linked by cosmic web filaments. This approach helped to amplify the signal of this incredibly dim light from the shocks, which appear polarized due to the influence of magnetic fields.
“Honestly I actually didn’t expect to see anything, or nothing nearly as strong and convincing as what we did see,” Vernstrom said in an email to Motherboard. “The overall brightness of everything in polarized light is much much less, I thus thought any signal would just be too faint to be seen if it was there and we weren’t sure that the light coming from the cosmic web would be polarized” because some models suggest it might not be.
“So when I started running this method on the new data and seeing quite a bright looking detection in polarized light, I was skeptical,” she continued. “I think, especially when working with methods like stacking where you’re looking at the average of many things and looking for a signal that is many many times fainter than the actual noise in your image, it's best to be skeptical first.”
However, after checking the observations over and over for possible flukes or false positives, the researchers realized they had actually glimpsed this elemental glow produced by the rumblings of cosmic giants, which is thousands of times dimmer than the glow of a typical galaxy.
“We tried a number of tests to convince ourselves that what we were seeing was real and what we thought we should be seeing,” Vernstrom recalled. “When we’d done all we could do to make it go away and it didn’t and it only showed up when it should, that's when we got really excited.”
“We then compared our real data result to brand new simulations, the first large simulations to predict this polarized radio light [and] the fact that the simulated results looked very much like our real result was great confirmation,” she added.
The shocks that the team witnessed are produced as new matter—such as gas, dust, and galaxies—falls into the existing cosmic web structures. This infalling material becomes hot and energetic as it joins the structural fold, sparking bursts when it interacts with the cold universe beyond the cosmic web, which Vernstrom compared to the boom of a hypersonic aircraft.
The light from these shocks appears polarized because it is beyond the fray of the turbulent environment inside galaxy clusters. Clusters can contain thousands of gravitationally entwined galaxies that often interact in cataclysmic ways, including careening into head-on collisions or engaging in “galactic cannibalism.”
In short, these places are hot messes that send particles spinning around in all kinds of chaotic directions, which in turn scrambles the magnetic fields inside the clusters. This effect causes light inside galaxy clusters to appear disordered and unpolarized, whereas the light further out in calmer cosmic pastures, where the shocks are visible, is neatly polarized. For this reason, the discovery of these shocks could reveal exciting insights about the unexplained role of magnetism in the evolution of the universe.
“We still don’t actually know the origin of these cosmic magnetic fields,” Vernstrom explained. “There are competing or different possibilities, such as a primordial magnetic field (something from the very very early Universe left over after the inflation of the Universe)—if that's the case how strong was it ? What could that tell us about the earliest times of the Universe?”
“It’s possible these magnetic fields were produced only once the stars and galaxies were forming and they were injected into the inter-galactic space from the galaxies—that would say that the presence of the large-scale magnetic fields maybe isn’t that important for galaxy formation,” she continued. “There are a few other theories as well, but each of these theories have different predictions for the type and amount and strength of radio light we see along the cosmic web so we can compare the simulations of each model to what we observe (so far our observations do not favor only having the magnetic fields injected from galaxies).”
The observation of these shocks adds one more piece to the puzzle of cosmic magnetism, and demonstrates that the faint light of these behemoth structures is detectable by modern instruments, such as the Sloan Digital Sky Survey, which is the source of the 600,000 cluster images.
However, Vernstrom and her colleagues hope to build on their research with new observations and simulations that could potentially tease out more secrets from these enigmatic structures that link the cosmos.
“One thing is to look at how this result changes if you look at filaments and clusters from different times in our cosmic history,” Vernstrom said. “Does the signal remain and stay strong if we look at the cosmic web further back in time? Or has there been some evolution?”
“Along these lines would be comparing the results with more simulations, with different origin scenarios like mentioned above,” she continued. “In our current paper we only looked at simulations where there was this primordial magnetic field from the early Universe, but it would be good to compare against more models to start to constrain the different ideas. So, it’s about digging into where the magnetic fields came from and how they have changed across the history of the Universe.”