The Hidden Magnetic Universe Begins to Come Into View
Universe

The Hidden Magnetic Universe Begins to Come Into View

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.

Whenever astronomers develop a new method of searching for magnetic fields in ever more distant regions of the universe, they inexplicably find them.

These force fields — the same entities that emanate from fridge magnets — surround Earth, the sun and all galaxies. Twenty years ago, astronomers discovered magnetism permeating whole galaxy clusters, including the space between galaxies. Invisible field lines swoop over intergalactic space like fingerprint grooves.

Last year, astronomers were able to explore a much more sparse region of space – the space between galaxy clusters. They detected the greatest magnetic field ever discovered: 10 million light-years of magnetized space spanning the full length of this “filament” of the cosmic web. Using the same techniques, a second magnetic filament was discovered elsewhere in the universe. “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 theory is that cosmic magnetism is primordial, dating back to the creation of the cosmos. In that view, weak magnetism should exist everywhere, even in the cosmic web’s “voids” – the universe’s darkest, emptiest regions. The omnipresent magnetism would have seeded the stronger fields that blossomed in galaxies and clusters.

The cosmic web

The cosmic web, shown here in a computer simulation, is the large-scale structure of the universe. Dense regions are filled with galaxies and galaxy clusters. Thin filaments connect these clumps. Voids are nearly empty regions of space.

Primordial magnetism may also aid in the resolution of another cosmic problem known as the Hubble tension, which is now the most hotly debated topic in cosmology.

The difficulty at the heart of the Hubble tension is that the universe appears to be expanding much faster than expected based on its known components. The cosmologists Karsten Jedamzik and Levon Pogosian argue in an article published online under review by Physical Review Letters that weak magnetic fields in the early cosmos would result in the faster cosmic expansion rate observable today.

The Hubble tension is so easily relieved by primordial magnetism that Jedamzik and Pogosian’s paper attracted urgent 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.

More checks, according to Kamionkowski and others, are needed to guarantee that the early magnetism does not throw off other cosmological computations. Even if the theory holds up on paper, researchers will need to find conclusive evidence of primordial magnetism to be certain that it is the missing factor that shaped the universe.

Still, it’s strange that no one has considered magnetism in all the years of discussion regarding the Hubble tension. Most cosmologists, according to Pogosian, a professor at Simon Fraser University in Canada, don’t consider magnetism. “Everyone knows it’s one of those big puzzles,” he said. However, because there was no way to know whether magnetism was actually ubiquitous and thus a primordial component of the cosmos for decades, cosmologists mostly ignored it.

Meanwhile, astrophysicists continued to collect data. The weight of data has led the majority of them to believe that magnetism exists everywhere.

The Magnetic Soul of the Universe

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

Magnetic fields arise whenever an electric charge flows. The field of the Earth, for example, emanates from its inner “dynamo,” the current of liquid iron churning in its core. Electrons spinning around their constituent atoms generate the fields of fridge magnets and lodestones.

Two possible explanations for how magnetic fields become widespread in galaxy clusters are illustrated through cosmological simulations. The fields at left grow from uniform “seed” fields that filled the cosmos shortly after the Big Bang. At right, astrophysical processes like star formation and matter flow towards supermassive black holes generate magnetized winds that spill out from galaxies.

However, once a “seed” magnetic field is formed by charged particles in motion, it can grow larger and stronger by aligning with weaker fields. 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 charges in any region will neutralize overall. However, magnetic fields cannot be cancelled; they tend to accumulate and survive.

Yet, for all their power, these force fields keep low profiles. They are immaterial, only visible when acting on 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.

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 travelling through it in their work published last year. These charged particles emit faint “synchrotron radiation” as their paths twist in the field.

Because the synchrotron signal is strongest at low radio frequencies, it is ideal for detection by LOFAR, a network of 20,000 low-frequency radio antennas stretched 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. Because the Earth’s atmosphere refracts radio signals that flow through it, LOFAR sees the universe from the bottom of a swimming pool. The researchers solved the problem by measuring the wobble of “beacons” in the sky — radio emitters with exact locations — and correcting for this wobble in order to deblur all of the data. When they applied the deblurring algorithm to data from the filament, they saw the glow of synchrotron emissions right away.

The LOFAR telescope.
LOFAR consists of 20,000 individual radio antennas spread across Europe.

The filament appears magnetized all the way through, not only near the galaxy clusters traveling toward each other from either end. The researchers are hoping that a 50-hour data set they are now reviewing may reveal more information. Additional measurements have lately revealed magnetic fields that run the length of a second filament. The researchers intend to publish their findings soon.

The discovery of massive magnetic fields in at least these two filaments adds crucial new information. “It has spurred quite some activity,” van Weeren said, “because now we know that magnetic fields are relatively strong.”

A Light Through the Voids

If these magnetic fields emerged in the early cosmos, how did they do that? “People have been thinking about this problem for a long time,” said Tanmay Vachaspati of Arizona State University.

Vachaspati proposed in 1991 that magnetic fields could have formed during the electroweak phase transition — the split second after the Big Bang when the electromagnetic and weak nuclear forces separated. Others have proposed that magnetism develops microseconds after protons forms. Or soon after that: The late astrophysicist Ted Harrison argued in the earliest primordial magnetogenesis theory in 1973 that the turbulent plasma of protons and electrons might have spun up the first magnetic fields. Others have argued that space became magnetized before all of this, during cosmic inflation – the explosive expansion of space that is thought to have caused the Big Bang itself. It’s also plausible that it didn’t happen until structures grew billions of years later.

The pattern of magnetic fields in the most pristine patches of intergalactic space, such as the quiet parts of filaments and even more empty gaps, can be used to test hypotheses of magnetogenesis. Certain characteristics, such as whether the field lines are smooth, helical, or “curved every which way, like a ball of yarn or something” (per Vachaspati), and how the pattern changes in different places and scales, include valuable information that may be compared to theory and simulations. If, as Vachaspati proposed, magnetic fields developed during the electroweak phase transition, the resulting field lines should be helical, “like a corkscrew,” he said.

The problem is that force fields with nothing to press on are difficult to detect.

One approach, developed in 1845 by the English physicist Michael Faraday, detects a magnetic field by rotating the polarization direction of light traveling through it. The amount of “Faraday rotation” is determined by the magnetic field strength and the frequency of the light. So, by measuring polarization at various frequencies, you may infer the strength of magnetism along the line of sight. “If you do it from different places you can make a 3D map,” said Enßlin.

Graphic of synchrotron radiation and Faraday rotation.
Samuel Velasco/Quanta Magazine

Researchers have started to make rough Faraday rotation measurements using LOFAR, but the telescope has trouble picking out the extremely faint signal. Valentina Vacca, an astronomer and Govoni’s colleague at the National Institute for Astrophysics, developed an algorithm a few years ago for statistically extracting subtle Faraday rotation signals by stacking up multiple measurements of empty areas. “In principle, this can be used for voids,” Vacca said.

But the Faraday approach will truly take off when the next-generation radio telescope, the Square Kilometer Array, begins operations in 2027. “SKA should produce a fantastic Faraday grid,” Enßlin said.

For the time being, the only proof of magnetism in the voids is what observers don’t see when they look at blazars located behind voids.

Blazars are supermassive black holes that produce bright beams of gamma rays and other intense light and matter. As the gamma rays travel through space, they sometimes collide with other passing photons, morphing into an electron and a positron as a result. When these particles collide with other photons, they produce low-energy gamma rays.

The lower-energy gamma rays, however, may appear to be missing if the blazar’s light passes through a magnetic void, reasoned Andrii Neronov and Ievgen Vovk of the Geneva Observatory in 2010. The magnetic field will deflect electrons and positrons away from the observer. When they produce lower-energy gamma rays, they will not be directed towards humans.

Samuel Velasco/Quanta Magazine

Indeed, when Neronov and Vovk analyzed data from a suitable blazar, they detected high-energy gamma rays but not low-energy gamma rays. “It’s the absence of a signal that is a signal,” Vachaspati said.

A non-signal is hardly a smoking gun, and alternative explanations for the missing gamma rays have been suggested. However, follow-up observations have increasingly pointed to Neronov and Vovk’s hypothesis that voids are magnetized. “It’s the majority view,” Durrer said. In 2015, one researcher overlaid multiple measurements of blazars behind voids and succeeded in extracting a faint halo of low-energy gamma rays around the blazars. The result is exactly what would be predicted if the particles were scattered by faint magnetic fields a millionth of a trillionth as strong as a fridge magnet.

Cosmology’s Biggest Mystery

Surprisingly, this precise amount of primordial magnetism may be exactly what is required to address the Hubble tension – the problem of the universe’s unusually fast expansion.

That’s what Pogosian realized after seeing recent computer simulations by Karsten Jedamzik, a collaborator at the University of Montpellier in France. The researchers created a simulated, plasma-filled young cosmos and discovered that protons and electrons in the plasma flew along magnetic field lines and gathered in the weakest field strength regions. This clumping effect caused protons and electrons to join into hydrogen earlier than they would have otherwise, resulting in an early phase change known as recombination.

Pogosian saw that this could address the Hubble tension after reading Jedamzik’s paper. Cosmologists use ancient light emitted during recombination to calculate how rapidly space should be expanding today. The light shows a young universe studded with blobs created by sound waves swimming around in primordial plasma. If recombination occurred earlier than expected due to the clumping effect of magnetic fields, sound waves could not have propagated as far in advance, and the resulting blobs would be smaller. That means the blobs we see in the sky from recombination must be closer to us than researchers previously thought. The light coming from the blobs must have traveled a shorter distance to reach us, meaning the light must have been traversing faster-expanding space. “ It’s like trying to run on an expanding surface; you cover less distance,” Pogosian said.

As a result, smaller blobs imply a higher assumed cosmic expansion rate, putting the inferred rate considerably closer to measurements of how quickly supernovas and other astronomical objects appear to fly apart.

“I thought, wow,” Pogosian said, “this could be pointing us to [magnetic fields’] actual presence. So I wrote to Karsten immediately.” In February, just before the lockdown, the two met in Montpellier. Their calculations revealed that the amount of primordial magnetism required to address the Hubble tension agrees with blazar observations as well as the predicted size of starting fields required to build the massive magnetic fields spanning galaxy clusters and filaments. “So it all sort of comes together,” Pogosian said, “if this turns out to be right.”

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