THE scale is hard to fathom. But if you zoom out far enough, the structure of the universe reveals itself: a “cosmic web” in which thread-like filaments of gas tangle around gigantic voids, connecting disparate clusters of galaxies. These are some of the most mysterious structures in the cosmos – and recent glimpses have revealed a surprising presence among them.
Earlier this year, astronomers led by at the Commonwealth Scientific and Industrial Research Organisation in Perth, Australia, confirmed the detection of between galaxy clusters.
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It was one of the first demonstrations that magnetism exists at such gargantuan scales. But the real excitement is that the sheer size of the fields suggests they could be relics from the birth of the universe in the big bang.
Cosmologists dream about finding such “primordial” magnetic fields, because they could hold long-sought secrets about how everything came to be and might even resolve the biggest problem in modern cosmology. But conclusively identifying them is a problem. Staring deep into a universe saturated with magnetic fields, how can you be certain you are squinting at one from the dawn of time?
Discoveries such as Vernstrom’s are giving us more confidence that, even if we might not actually be there yet, we’ve now got the theoretical nous and observational tools to finally make the breakthrough. “The net is closing,” says , an astrophysicist at the University of Bologna in Italy.
Fields of influence
Magnetism is a familiar force. The magnetic fields spreading out across space have ultimately the same origin as the field produced by a magnet on your fridge: the motions and alignments of electrically charged particles such as electrons. Magnetic fields are invisible, yet their influence stretches far and wide because magnetism is the only fundamental force, apart from gravity, that can be felt across vast distances.
All of which makes it odd that magnetism is often overlooked by cosmologists. “You can sit in a week-long conference about cosmology and not hear the word ‘magnetism’ once, which is sort of ridiculous,” says Bryan Gaensler, director of the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto in Canada.
Magnetism isn’t a grand cosmic unknown like black holes or dark energy: we know plenty about what it is, what it does and what generates it. Earth’s magnetic field protects us from solar radiation. The sun’s magnetism causes solar flares and magnetic explosions called coronal mass ejections. Then there is a class of stars known as magnetars, thought to be the most magnetic objects in the universe. Viewed with the right equipment, field lines ripple out from these astrophysical sources like fingerprints or the contours of topographical maps.
In the beginning?
What we don’t know is how old magnetic fields are or the extent to which they influenced the evolution of the cosmos, particularly in its formative moments. “We have this incredibly detailed picture of what happened in the universe, starting with a tiny fraction of a second after the big bang through to the point where galaxies and stars formed,” says Gaensler. “But there are a few things we haven’t filled in yet, and I would argue that one of the biggest is where magnetic fields fit into this.”
Arguably the biggest question of all is whether magnetic fields were a feature of the early universe or came later. Depending on when and how they were formed, primordial magnetic fields might have contributed to inflation, the split-second flash of exponential expansion that is believed to have set the infant universe on the path to what we see today. In any case, they could hold some memory of that event that we could interrogate to test our best ideas about how it all went down.
They may also hold clues about how the first stars formed. “Either the first stars didn’t need magnetic fields, or they did need them – which means they had to be there before the first stars were formed,” says Gaensler. “You can’t completely solve that question unless you understand what magnetic fields were doing.”
For other researchers, there is perhaps an even bigger prize: solving the current big crisis of cosmology. Known as the Hubble tension, this is the increasingly inescapable observation that the universe is expanding faster today than it should be, according to our best understanding of how the universe evolved, known as the standard model of cosmology. A small army of cosmologists is working to resolve the Hubble tension, or at least understand what it means for cosmology. Last year, a new hypothesis surfaced: that primordial magnetic fields might do the trick.
“Currently, people do not take magnetic fields into account when they describe the evolution of the universe,” says , a physicist at Simon Fraser University in Burnaby, Canada. What Pogosian and at the University of Montpellier in France, found, however, is that if you add magnetic fields to simulations of how the universe evolves under the standard model of cosmology, the prediction it spits out for . “It’s potentially a very exciting development,” says Pogosian.
, an astrophysicist at Johns Hopkins University in Baltimore, Maryland, who won a share of the physics Nobel prize for his part in discovering that the universe is expanding at an accelerated rate, agrees. “The best feature of the primordial magnetic field idea to me is that it doesn’t require any new component or feature of the universe,” says Riess. “It utilises something that must exist at some level, but we know little about that level, and poses it as a solution to the tension.”
The catch is that we don’t know if primordial magnetic fields actually exist. If they do, they are expected to be extremely weak, almost ghostly remnants of a very different universe that stretch across unimaginable distances. They may even encompass all of space as a faint, universal signal created everywhere by some unknown phenomenon close to the beginning of time. “An intriguing possibility is that even magnetic fields in celestial objects are all produced by amplification of pre-existing seed magnetic fields,” says , an astronomer at the National Institute for Astrophysics in Cagliari, Italy.
In this scenario, fields produced by black holes or other astrophysical means supersede the older fields, obscuring them from view. The most plausible way to confirm the presence of primordial fields, then, is to find signs of magnetism in the sparsest parts of the universe: the voids between the filaments connecting the cosmic web.
If you find a field in one of these voids, you couldn’t explain its existence with astrophysical processes. There is so little matter in voids that the only explanation for any faint magnetism would be that it had been around since the beginning of time, or near enough. “The detection of magnetic fields in the voids of the cosmic web would be a smoking gun,” says Govoni.
In 2019, Govoni and her colleagues made just such a find in a cosmic filament, the . As with the more recent detection by Vernstrom and her team, the researchers inferred the faint presence of magnetism by spotting synchrotron radiation – radio waves generated when charged particles twist in spirals as they pass through a magnetic field.
“The voids could yet give up their secrets thanks to exotic signals”
The detections are a big deal because they demonstrate that we can tease out weak fields at great distances. “They’re spread over large areas of the sky and the emission that’s expected from them is pretty faint,” says Vernstrom. “You take faint emission over a large area and that makes it even harder to detect. And then you sprinkle in all the other stuff that’s brighter than that – regular galaxies, our galaxy, plus instrumental noise – and they’re expected to be fainter than most of that.”
To see something – anything – in the voids is far harder, simply because there are barely any particles to interact with the magnetic fields and so betray their presence. But the voids could yet give up their secrets thanks to some exotic phenomena that pass through these vacuums on their epic journeys across space.
Blazars are one example. These are galaxies powered by supermassive black holes that spit out jets of ionised matter at close to the speed of light. They are among the brightest and most energetic objects in the sky. As early as 2010, researchers had noted that if a blazar lacks a distinctive halo of low-energy gamma rays, then .
Twisted waves
What has really injected fresh impetus into the hunt for primordial magnetic fields, though, is the detection of dozens of fast radio bursts (FRBs), short-lived pulses of radio waves that burst from faraway galaxies with the intensity of several suns. These signals are relatively new to astronomers, and there is still no consensus about what produces them. But in the past few years, we have begun to pick up more and more of them. In the process, it has become clear that they encode two distinct signals about the depths of space through which they pass.
The most common way to study cosmic magnetic fields is by measuring the polarisation of radiation that passes through them. This polarised radiation, whose waves vibrate only in a single plane, could be the light of a star or a galaxy, but it could also be a fast radio burst. When an FRB encounters a magnetic field, its polarised waves become twisted so that they spiral as they propagate through space. By measuring the extent of this change, known as Faraday rotation, we can infer the presence of any magnetism it must have passed through, as well as its intensity.
“The strengths of the magnetism that we can now measure are 100 times weaker than they were in the past,” says Gaensler. Even so, our best radio telescopes are just barely sensitive enough to detect the faint signals we might expect from primordial magnetic fields. And that is the beauty of FRBs, because they possess another advantage over other signals from outer space, says Gaensler: “The thing that’s game-changing about fast radio bursts is that there’s another effect we can measure.”
That effect is called dispersion. As an FRB passes through space, the radiation is shifted to lower frequencies as a result of the radiation scattering off electrons and other particles, to give it a smeared-out appearance. By measuring that dispersion, astronomers can also learn about the density of the region it has passed through. So if a particular FRB has passed through a weakly magnetised region of space that also has an extremely low density of matter, say, then it would suggest the signal has travelled through a void – and that, in turn, would suggest the magnetism there is primordial. “Fast radio bursts are perfect for this,” says Vazza.
Even if primordial magnetic fields turn out to be ubiquitous, however, their expected frailty means that we’ll need statistical methods to spot them. That means you need data. Lots of data.
As an example, when Vernstrom and her colleagues detected magnetism along a cosmic filament, they merged hundreds of thousands of images of galaxies. The technique, known as image stacking, amplifies the signal from the noise so that a faint wisp of glowing radio emissions emerges from the composite. “It’s really noisy and you really have to convince yourself,” says Vernstrom. “That’s when we started doing all kinds of tests to try to make the signal go away. And if you can’t, then that’s when you think you’ve got something.”
The coming deluge
Likewise, to conclusively discover ancient magnetism using fast radio bursts, Vazza believes we will need a deluge of data. Specifically, he and his colleagues have , all with a detected Faraday rotation. Fortunately, a new generation of radio telescopes is already rising to the challenge. “There is an instrument in Canada called CHIME which can detect industrial quantities of FRBs, the way it’s designed,” says Pogosian. “In the future, we will have thousands, perhaps tens of thousands, possibly hundreds of thousands, of fast radio bursts.”
“Cosmology as we know it doesn’t account for ancient magnetism”
The Australian Square Kilometre Array Pathfinder telescope – a precursor to the largely based in Australia and South Africa – will also bring thousands of fast radio bursts. Then there is the full-fat SKA, set to be the largest radio telescope in the world when fully operational, which astronomers are awaiting with great anticipation. “That will push us towards 10 million measurements and also be able to go further out and further back in time,” says Gaensler. The SKA will ultimately produce a three-dimensional grid of Faraday rotation in the skies all around us, he adds – something akin to a map of magnetism in the universe.
Just don’t expect a Eureka moment. “This is a field where the answers come from a steady accumulation of statistics rather than discovering a particular thing,” says Gaensler. But whatever such an arrival at consensus lacks in drama, it would more than make up for in significance.
Cosmology as we know it – everything from the creation of the elements to the expansion of space-time – doesn’t currently account for the existence of magnetic fields, says Vazza. “So if we discover that magnetic fields were actually created in very primordial epochs, this is a sign of new physics that we need to include in our cosmological models.”
We could even learn that magnetism had a pivotal role in the creation of stars and galaxies, although of course we would still have to work out what magnetised the cosmos in the first place. “It’s definitely been a big unknown for a long time and I think the tide is really turning,” says Vernstrom. “We’re starting to open the window on that part of the universe.”
