Weak magnetic fields are "roaming" across the universe, according to a new study that may have solved the mystery of where the huge magnetic fields around galaxies come from.
Galaxies such as our Milky Way have their own large-scale magnetic fields. Although these fields are weak compared to planetary fields, scientists think the galactic versions help establish rates of star formation, guide cosmic rays, and regulate the dynamics of interstellar gas.
Most scientists believe the stronger magnetic fields of today's adult galaxies grew from weaker "seed" fields. But it's unclear where these older fields originated.
The two leading theories: The seed fields were created by the movement of charged gas in protogalaxies, or they were produced outside of galaxies by some unseen processes in the early universe.
New observations made with NASA's Fermi Gamma-ray Space Telescope support the idea that the seeds were there all along, even before galaxies themselves.
Based on Fermi's data, "we've found that these weak magnetic fields should be everywhere. They should be outside the galaxies, filling the whole universe, even where there are no galaxies, no clusters, no anything," said study co-author Andrii Neronov of the University of Geneva's ISDC Centre for Astrophysics in Switzerland.
Since the new findings suggest magnetic fields can form outside galaxies, "perhaps those magnetic fields were created before the galaxies were formed," Neronov said.
Sowing the Seeds for Galactic Fields
According to the theory, primordial seed fields could have been created from charged particles spit out during violent events such as supernovae.
Over time, the theory goes, a seed field could bulk up inside a galaxy, because the galaxy's slow spin causes charged particles and gases to align along the seed's magnetic field lines. (Related: "Earth's Core, Magnetic Field Changing Fast, Study Says.")
But other seed fields would remain roaming through intergalactic space—and that's what Neronov and colleagues think they've found.
More precisely, the team saw a lack of very high-energy gamma rays in Fermi data on blazars, galaxies with supermassive black holes at their centers that spew jets of particles at near the speed of light.
The gamma rays that reach Earth from blazars should be at a certain energy level. But the gamma rays Neronov's team saw appear to have been sapped of some of their strength, which is exactly what would have happened if the gamma rays had interacted with weak magnetic fields along the way.
"What we've detected could be this initial weak field, and that could resolve the problem of the origin of [modern] magnetic fields in the Milky Way and other galaxies, because we may now know the initial conditions," Neronov said.
Magnetic Mysteries Remain
The scientists aren't sure which high-energy processes might have created the very first magnetic fields in a young, galaxy-less universe, although there's no shortage of candidates.
It's also unclear whether the roaming seed fields played a role in the subsequent formation of galaxies and galaxy clusters, since the fields' exact intensities have yet to be measured.
"In general, I tend to think that they do not play a significant role in the formation of the galaxies, because they are too weak" at the low levels the Fermi team observed, Neronov said.
The new Fermi data on blazars are published in this week's issue of the journal Science.