Our universe is highly magnetized, but no one knows exactly why. The current theory is that cosmic turbulence amplified tiny “seed” magnetic fields to create the powerful ones that govern galaxies today. Astrophysicists are still working to fully understand this process but a recent lab experiment mimicking galactic collisions might bring scientists one step closer to figuring out the mysterious origins of cosmic magnetism.
The matter in our universe forms a web of densely populated galaxy clusters and connecting filaments separated by vast voids, interrupted only by the occasional stray galaxy. When astronomers first started to observe magnetic fields in space, they noticed something peculiar: The universe is magnetized. Scientists had expected to find magnetism in active regions, where plasma currents such as those inside stars might spawn magnetic fields. But apparently even the most vacant cosmic stretches, where scientists expected very little to be happening, are threaded with magnetism. Cosmic magnetic fields are key players in governing the motion and evolution of stars and galaxies, so scientists are keen on understanding how they were born and how they became so strong.
Astrophysicists suspect that intergalactic magnetism originated as “primordial magnetic fields,” explains Jena Meinecke, an astrophysicist at the University of Oxford who led the work. “They’re basically the grandparent magnetic fields of all the magnetic fields that are around.” Scientists have created these so-called “seed” fields from scratch by a process called the “Biermann battery mechanism.” Essentially, by generating a very strong shock wave, researchers can make their own magnetic fields in the lab. But their seed fields are about a quadrillion times weaker than those seen in galaxy clusters.
To account for the incredible strength of magnetic fields in space, scientists have proposed that seed magnetic fields can be amplified by cosmic turbulence. Galaxy clusters, which are rife with galaxies spewing jets of material and crashing into one another, are pretty turbulent environments. As the plasma in these clusters shifts around, the magnetic fields embedded within them should twist and stretch, too. Some of the moving particles’ kinetic energy is transformed into magnetic energy, augmenting the magnetic fields.
To test this theory, astrophysicists wanted to see if turbulence could amplify seed magnetic fields in the lab. In a study published on June 22 in Proceedings of the National Academy of Sciences, Meinecke and her colleagues attempted to replicate galaxy mergers with laser-produced plasma clouds and observed how the resulting turbulence affected seed magnetic fields. For their experiment, Meinecke’s team used the Vulcan laser at the Rutherford Appleton Laboratory in England, one of the most powerful lasers in the world. They set up two parallel carbon foil sheets about six centimeters apart in a gas-filled chamber and focused multiple laser beams on the outside face of each foil. These lasers drove shocks through each foil, which created a jet of plasma that blasted from the inside faces of both. “So we had these two jets that came toward each other and then eventually collided in the middle,” Meinecke says. “Upon collision, they created…kind of this turbulent ball that expanded out, which is really beautiful to see.”
This turbulence tangled and amplified seed magnetic fields Meinecke’s team had generated between the foil sheets, providing experimental evidence for the current theory of how cosmic magnetic fields became so powerful. The researchers also simulated their experiment using a computer model, which produced similar results. “When people sometimes see this stuff, the first thing they say is, ‘Oh, how ridiculous, space is huge [compared with the lab], how can it possibly be the same?’” says Joseph Cross, a laboratory astrophysicist in Meinecke’s research group at Oxford, who was not involved with this study. “But there are lots of nice papers that show the equations that determine how everything is happening can be written in a way that doesn’t depend on the scale.” According to Cross, Meinecke’s work demonstrates that despite the enormous difference in size between real-life galaxy mergers and lab plasma collisions, the same principles apply for amplifying magnetic fields.
Andrii Neronov, an astrophysicist at the University of Geneva who was not involved with the study, however, cautions that a model of turbulence-amplified magnetic fields does not necessarily capture all the nuances of a real galaxy cluster environment. According to Neronov, the creation of cosmological magnetic fields is a multistage process controlled by many unknown physical parameters.
Still, Neronov says it is reassuring that Meinecke’s team demonstrated turbulent amplification in the lab experiment and in their simulations. The result “demonstrates that one particular stage of the process works as expected. In this respect it is an important step toward understanding the generation of intracluster magnetic fields,” he says.
Although researchers can only replicate such gargantuan events on a tiny scale, there is much they can learn on Earth about the magnetism that rules the universe.