The Universe is magnetized. This is true on “small” length scales, such as in planets and stars, and over much larger scales, such as across the tenuous gas in galaxies and galaxy clusters and, possibly, the even more rarefied intergalactic medium. Physicists are fairly certain that these magnetic fields weren’t created in the big bang (the reason has to do with the symmetry of Maxwell’s equations). Rather, for the most part, they assume that small “
seed fields,” which formed some time after the big bang, were amplified into what we observe today. But how these seed fields materialized remains one of the great, unsolved problems in cosmology.
In
Physical Review Letters, Smadar Naoz and Ramesh Narayan at the Harvard-Smithsonian Center for Astrophysics, Massachusetts, propose a possible solution. They have revisited a model for the generation of small magnetic fields in a plasma, called the Biermann battery, and shown that this process could have generated seed fields in the Universe much earlier than was previously thought possible. Although the fields they calculate are weak, the fact that they could have existed early in the age of the Universe means there was more time for other processes to amplify them into the fields we observe today.
Theories of the origin of cosmological fields, or, magnetogenesis, are either top down or bottom up. Top down theories invoke a process that operates everywhere, producing a pervasive field. In bottom up theories, magnetogenesis occurs in small objects, and the magnetic fields are then dispersed to large scales. Both types of theory require two stages. The first is to create a seed field. In the second phase, the existing field grows by a process called a dynamo, in which the kinetic energy in the flowing magnetic plasma is converted, by induction, into magnetic energy. (The 22-year solar magnetic activity cycle is a famous example of an astrophysical dynamo.) Although it’s still unclear exactly how plasma flows amplify magnetic fields, large-scale shear and small-scale turbulence are both thought to play a role.
