Astronomers are discovering that magnetic fields permeate much of the cosmos. If these fields date back to the Big Bang, they could solve a major cosmological mystery.

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By Natalie Wolchover
Senior Writer/Editor

Anytime astronomers figure out a new way of looking for magnetic fields in ever more remote regions of the cosmos, inexplicably, they find them.

These force fields — the same entities that emanate from fridge magnets — surround Earth, the sun and all galaxies. Twenty years ago, astronomers started to detect magnetism permeating entire galaxy clusters, including the space between one galaxy and the next. Invisible field lines swoop through intergalactic space like the grooves of a fingerprint.

Last year, astronomers finally managed to examine a far sparser region of space — the expanse between galaxy clusters. There, they discovered the largest magnetic field yet: 10 million light-years of magnetized space spanning the entire length of this “filament” of the cosmic web. A second magnetized filament has already been spotted elsewhere in the cosmos by means of the same techniques. “We are just looking at the tip of the iceberg, probably,” said Federica Govoni of the National Institute for Astrophysics in Cagliari, Italy, who led the first detection.

The question is: Where did these enormous magnetic fields come from?

“It clearly cannot be related to the activity of single galaxies or single explosions or, I don’t know, winds from supernovae,” said Franco Vazza, an astrophysicist at the University of Bologna who makes state-of-the-art computer simulations of cosmic magnetic fields. “This goes much beyond that.”

One possibility is that cosmic magnetism is primordial, tracing all the way back to the birth of the universe. In that case, weak magnetism should exist everywhere, even in the “voids” of the cosmic web — the very darkest, emptiest regions of the universe. The omnipresent magnetism would have seeded the stronger fields that blossomed in galaxies and clusters.

Primordial magnetism might also help resolve another cosmological conundrum known as the Hubble tension — probably the hottest topic in cosmology.

The problem at the heart of the Hubble tension is that the universe seems to be expanding significantly faster than expected based on its known ingredients. In a paper posted online in April and under review with Physical Review Letters, the cosmologists Karsten Jedamzik and Levon Pogosian argue that weak magnetic fields in the early universe would lead to the faster cosmic expansion rate seen today.

Primordial magnetism relieves the Hubble tension so simply that Jedamzik and Pogosian’s paper has drawn swift attention. “This is an excellent paper and idea,” said Marc Kamionkowski, a theoretical cosmologist at Johns Hopkins University who has proposed other solutions to the Hubble tension.

Kamionkowski and others say more checks are needed to ensure that the early magnetism doesn’t throw off other cosmological calculations. And even if the idea works on paper, researchers will need to find conclusive evidence of primordial magnetism to be sure it’s the missing agent that shaped the universe.

Still, in all the years of talk about the Hubble tension, it’s perhaps strange that no one considered magnetism before. According to Pogosian, who is a professor at Simon Fraser University in Canada, most cosmologists hardly think about magnetism. “Everyone knows it’s one of those big puzzles,” he said. But for decades, there was no way to tell whether magnetism is truly ubiquitous and thus a primordial component of the cosmos, so cosmologists largely stopped paying attention.

Meanwhile, astrophysicists kept collecting data. The weight of evidence has led most of them to suspect that magnetism is indeed everywhere.
The Magnetic Soul of the Universe

In the year 1600, the English scientist William Gilbert’s studies of lodestones — naturally magnetized rocks that people had been fashioning into compasses for thousands of years — led him to opine that their magnetic force “imitates a soul.” He correctly surmised that Earth itself is a “great magnet,” and that lodestones “look toward the poles of the Earth.”

Magnetic fields arise anytime electric charge flows. Earth’s field, for instance, emanates from its inner “dynamo,” the current of liquid iron churning in its core. The fields of fridge magnets and lodestones come from electrons spinning around their constituent atoms.

Cosmological simulations illustrate two possible explanations for how magnetic fields came to permeate galaxy clusters. At left, the fields grow from uniform “seed” fields that filled the cosmos in the moments after the Big Bang. At right, astrophysical processes such as star formation and the flow of matter into supermassive black holes create magnetized winds that spill out from galaxies.

However, once a “seed” magnetic field arises from charged particles in motion, it can become bigger and stronger by aligning weaker fields with it. Magnetism “is a little bit like a living organism,” said Torsten Enßlin, a theoretical astrophysicist at the Max Planck Institute for Astrophysics in Garching, Germany, “because magnetic fields tap into every free energy source they can hold onto and grow. They can spread and affect other areas with their presence, where they grow as well.”

Ruth Durrer, a theoretical cosmologist at the University of Geneva, explained that magnetism is the only force apart from gravity that can shape the large-scale structure of the cosmos, because only magnetism and gravity can “reach out to you” across vast distances. Electricity, by contrast, is local and short-lived, since the positive and negative charge in any region will neutralize overall. But you can’t cancel out magnetic fields; they tend to add up and survive.

Yet for all their power, these force fields keep low profiles. They are immaterial, perceptible only when acting upon other things. “You can’t just take a picture of a magnetic field; it doesn’t work like that,” said Reinout van Weeren, an astronomer at Leiden University who was involved in the recent detections of magnetized filaments.

In their paper last year, van Weeren and 28 co-authors inferred the presence of a magnetic field in the filament between galaxy clusters Abell 399 and Abell 401 from the way the field redirects high-speed electrons and other charged particles passing through it. As their paths twist in the field, these charged particles release faint “synchrotron radiation.”

The synchrotron signal is strongest at low radio frequencies, making it ripe for detection by LOFAR, an array of 20,000 low-frequency radio antennas spread across Europe.

The team actually gathered data from the filament back in 2014 during a single eight-hour stretch, but the data sat waiting as the radio astronomy community spent years figuring out how to improve the calibration of LOFAR’s measurements. Earth’s atmosphere refracts radio waves that pass through it, so LOFAR views the cosmos as if from the bottom of a swimming pool. The researchers solved the problem by tracking the wobble of “beacons” in the sky — radio emitters with precisely known locations — and correcting for this wobble to deblur all the data. When they applied the deblurring algorithm to data from the filament, they saw the glow of synchrotron emissions right away.