The Field that Ate the Milky Way
Magnetic fields would make great movie monsters. A field has already swallowed our entire galaxy, and no one knows where it came from or when it arrived. In fact, the most complete computer simulation to date–described in the 19 December PRL–bolsters the controversial theory that magnetic fields have lurked around every galaxy since its formation. The results may lead to predictions that astronomers can check, and so to a definitive theory for the origin of these mysterious magnetic fields.
Astronomers detect magnetic fields by their effects on cosmic rays and on the polarization of light from distant bodies. The fields of about 1 microgauss (one millionth of the earth’s magnetic field strength) seem to exist in all galaxies observed so far, but their origins remain one of the great mysteries of the Universe’s evolution. One school of thought argues that weak, previously-existing “seed fields” in small pockets within galaxies might have been stirred up by random turbulence and been amplified by a so-called dynamo effect. The problem is that theoreticians don’t know exactly how the dynamo would work.
Others believe the fields were created soon after the big bang. As the fireball cooled and the four fundamental forces came into being through processes called phase transitions, theories predict that random and disordered magnetic fields ought to have appeared as a side effect. These early fields may even have helped the galaxies to coalesce. But, says Karsten Jedamzik of the University of Montpellier in France, most researchers believe that the primordial fields would have dissipated over time, so even in this scenario, a dynamo process must have amplified them to generate the fields observed today.
To find out if such amplification would be required, Jedamzik collaborated with Robi Banerjee of McMaster University in Hamilton, Canada. The pair modeled the evolution of magnetic fields with a more complete computer simulation than had been run by others. The team used non-linear equations for the simulation, a more accurate but time-intensive method than the usual linear approximations.
Linear equations can’t easily capture magnetic field helicity, a property that prevents much of the field’s energy from dissipating as heat, says Banerjee. By adding helicity to the mix, the researchers were able to make realistic predictions for the evolution of magnetic fields from the big bang up to the era when galaxies began to form, 10 billion years ago. From that point on, the team used results from previous simulations by others to predict today’s magnetic fields.
Jedamzik and Banerjee found that the energy from the phase transitions alone was enough to explain today’s galactic fields, without any need for an amplification mechanism. Although the field strength was a trillion times smaller at the end of the simulation than it was at the start, this decline was not as great as other astrophysicists had expected. The results also showed that the primordial fields gradually spread out and became smoother on the size scale of a galaxy cluster, which matches astronomers’ observations.
Banerjee says the simulation could be used to make predictions for properties of the cosmic microwave background radiation–the ubiquitous glow left over from the big bang–which is currently being measured in great detail. Such calculations could potentially bring an end to the controversy over the fields’ origins if they were confirmed or contradicted by future measurements.
Hector Rubinstein of Stockholm University in Sweden agrees that more observation is needed to solve the mystery. Banerjee and Jedamzik have written “a good paper,” he says, but “it’s very difficult to tackle this question just with equations.”
–Shawna Williams
Shawna Williams is a freelance science writer.