New clues emerge on the origin of the universe’s magnetic fields

Role of plasma

Plasma is basically a state of matter where atoms are stripped of their electrons, forming charged particles that interact with magnetic fields. Most of the visible matter in the universe is in plasma form, but not all plasmas are the same. Some are dense and hot, like the Sun, while others are dilute and cold, like the intergalactic medium.

The researchers specifically wanted to understand how magnetic fields can emerge and survive in these low-density plasmas, where turbulence is expected to be weak. For this, they used fully kinetic particle-in-cell simulations to study the behavior of plasma particles in different conditions. They found that even in the most turbulent and dilute plasmas, magnetic fields can appear spontaneously and grow rapidly over time.

"This new research allows us to imagine the kinds of spaces where magnetic fields are born," professor Lorenzo Sironi said. Read about his new research on the origins of cosmic magnetic fields. https://t.co/tLJcxQj3dd

— Columbia University (@Columbia) August 1, 2023

The experiment demonstrated how magnetic fields can form with the smallest of fluctuations and eventually span vast distances in space. The study’s lead author, astronomy professor Lorenzo Sironi, said that their results reveal the hidden power of turbulence in shaping the universe.

“This new research allows us to imagine the kinds of spaces where magnetic fields are born: even in the most pristine, vast, and remote spaces of our universe, roiling plasma particles in turbulent motion can spontaneously give birth to new magnetic fields,” Sironi said. “The search for the ‘seed’ that can sow a new magnetic field has been long, and we’re excited to bring new evidence of that original source, as well as data on how a magnetic field, once born, can grow.”

The researchers hope their work will push future observations and experiments to test their predictions and uncover more secrets about the universe’s magnetic field.

The study was published in the journal Physical Review Letters.

Study abstract:

The mechanisms that generate “seed” magnetic fields in our Universe and that amplify them throughout cosmic time remain poorly understood. By means of fully kinetic particle-in-cell simulations of turbulent, initially unmagnetized plasmas, we study the genesis of magnetic fields via the Weibel instability and follow their dynamo growth up to near-equipartition levels. In the kinematic stage of the dynamo, we find that the rms magnetic field strength grows exponentially with rate γB≃0.4urms/L, where L/2π is the driving scale and urms is the rms turbulent velocity. In the saturated stage, the magnetic field energy reaches about half of the turbulent kinetic energy. Here, magnetic field growth is balanced by dissipation via reconnection, as revealed by the appearance of plasmoid chains. At saturation, the integral-scale wave number of the magnetic spectrum approaches kint≃12π/L. Our results show that turbulence—induced by, e.g., the gravitational buildup of galaxies and galaxy clusters—can magnetize collisionless plasmas with large-scale near-equipartition fields.