Even in the vastness of space, galaxies don’t like to be alone.
Here in the Milky Way, we live in an impossibly small slice of a vast cosmic web—the intricate, lattice-like superstructure of the known universe. On these grand scales, the universe is mostly emptiness, and what matter exists is manifests in a three-dimensional, web-like architecture, coalescing into clumps connected by tenuous threads called filaments. Vacant caverns are left in the space in between, like the holes in a loosely woven tapestry.
Within the web’s nodes, galaxies crowd together in droves of hundreds and thousands, yielding thickly settled cosmic cities teeming with fast-moving particles, including electrons moving nearly at the speed of light, and hosting enormous magnetic fields. The energetic signatures of these galaxy clusters burn so bright and hot that they can even be detected from billions of light-years away.
That’s good news for Earthbound astronomers attempting to, metaphorically speaking, unravel the very fabric of the universe. But compared to galaxy clusters, the thin, sparsely populated threads between them are energetically dim, and far harder to study. While a handful of observations have hinted that these intergalactic highways are paved with matter, much of the true nature of filaments—and thus, the architecture of the universe—has remained elusive.
That might soon change. In a study published today in the journal Science, researchers report the first evidence of radio waves emanating from a cosmic web thread, in the form of a filament that spans the gap between two galaxy clusters in the midst of merging. These emissions suggest that, like the clusters they connect, filaments can support magnetic fields, and might even carry their own cache of high-energy particles, traveling close to the speed of light.
“This is exciting work,” says Priyamvada Natarajan, an astrophysicist at Yale University who was not involved in the study. “It suggests filaments are more than just mass, and that they may have [high-energy] particles that were hitherto unknown. Filaments might have a bigger role to play in the cosmic web.”
Homing in on the filament’s emissions was a feat in and of itself. Previous observations had pinpointed the presence of a cosmic thread spanning the 10-million-light-year-wide gap between Abell 0399 and Abell 0401—two galaxy clusters in the midst of a painfully slow collision about 1 billion light-years from Earth. Both galaxy clusters had been shown to host their own magnetic fields, but far less had been found in the gap between. Curious to see if this filament contained the same types of energy emanating from the clusters it bridged, a team of researchers led by Federica Govoni, an astronomer at Cagliari Observatory at Italy’s National Institute for Astrophysics, collected data on the region using the Low Frequency Array (LOFAR), a radio telescope network based mainly in the Netherlands. What they found was faint—a series of emissions in the radio band of the electromagnetic spectrum that were just barely detectable, Govoni says.
“But they were exactly what we were looking for,” she adds. “For us, this is a beautiful result...like an aurora on cosmic scales.”
The observation itself was just the beginning. Govoni and her team aren’t yet sure what exactly underlies the filament’s celestial magic—and a few of their results still defy explanation.
Similar radio emissions have been found around individual galaxies and galaxy clusters (including Abell 0399 and Abell 0401 themselves), and are thought to indicate the presence of swarms of high-energy particles hurtling around a gargantuan magnetic field—the galactic equivalent of cars circling a bustling metropolis. Obtaining these readings from a filament, then, requires the presence of both crucial ingredients: a cavalry of freshly fueled vehicles (energetic particles), and a network of streets for them to traverse (the magnetic field).
The fact that this magnetic field exists isn’t really surprising, says Elizabeth Blanton, an astronomer at Boston University who was not involved in the study. Where there’s matter, magnetic fields are common, and they’re common in galaxy clusters. There’s no reason the same shouldn’t hold true for a filament, she says. “Clusters are denser, but fundamentally, it’s all kind of the same stuff.”
That said, however, “this is an important and exciting result…[that confirms] models of the content and the structure of the cosmic web,” she adds. “It’s a rare look into a large-scale filament in the universe, which are notoriously hard to observe, and where magnetic fields have traditionally been harder to detect.” (Finding this particular magnetic field was especially gratifying, Govoni says, because it seems about a million times weaker than Earth’s.)
What’s more, this magnetic field is a first for another important reason: its sheer size, Natarajan says. Up until this point, a magnetic field of this scale, spanning the gap between two galaxy clusters separated by millions of light-years, has never been detected. “You need an intergalactic magnetic field to explain this [radio wave] phenomenon,” she says. “And we don’t yet have an explanation for what can generate that.”
Then there’s the enigma of the filament’s glut of high-energy particles—the motor vehicles zooming up and down its length. While a swarm of cars might be a given in the packed urban environment of a galaxy cluster, it’s a bit less typical to find on a rural stretch of highway.
The find is especially perplexing given just how long the filament is, says Felipe Andrade-Santos, an astrophysicist at the Clay Center Observatory at Dexter Southfield and the Harvard-Smithsonian Center for Astrophysics who was not involved in the study. According to the study, the energetic lifespan of the filament’s particles would be, at most, about 230 million years (not much on cosmic timescales), after which they would effectively run out of mojo and no longer be detectable from Earth. That’s just barely enough time for these particles to cross a distance of 300,000 light-years—a paltry fraction of the 10 million light-years between Abell 0399 and Abell 0401.
In other words, the surprise of these results doesn’t just amount to finding congestion on a remote, 10-million-light-year-long highway. It’s more like finding congestion on a remote, 10-million-light-year-long highway completely devoid of gas stations.
One possible explanation for this conundrum would be a source—or, probably, many sources—of high-energy particles along the filament itself. A smattering of remote car dealerships could inject a pre-existing population of supercharged particles onto the cosmic interstate, where the magnetic field would be ready and waiting. The already-speedy particles would also probably get a tiny energetic boost from the ongoing Abell galactic collision, helping them reach top speeds.
There’s no guarantee that this exact scenario is actually playing out in space. But when Govoni and her colleagues modeled this idea in a computer simulation, it produced nearly identical radio emissions to what they’d actually observed with LOFAR.
If that’s the case, there could already be good cosmic candidates for the source of this pre-existing population of high-energy particles: black holes, shrouded within the hearts of galaxies studding the length of the filament, says Andrade-Santos, whose previous work has hinted at a similar phenomenon. Now-dormant galaxies might have belched matter onto the concourse at various points in the past.
These theoretical black holes haven’t yet been detected, but the prospect is exciting, Andrade-Santos says. Compared to the cosmic web’s populous clusters, filaments are lacking in just about everything: matter, gas, galaxies. And yet, it would seem the universe’s connective tissue still packs some serious particulate punch.
“The possibility of this pre-existing population in the filament itself is very, very exciting,” Natarajan says. “This tells you the filament isn’t just a passive place with dark matter and a bit of a gas. It could actually be energetically and magnetically active, and have a large-scale magnetic field.”
Much of this awaits further confirmation, and many questions remain. It’s also not yet clear if this particular filament is a fluke—or if this observation will be the first of many.
Either way, Govoni says, “this will hopefully open up new possibilities to study the cosmic web.”