The Universe is filled with energetic particles, electron and fully ionized atoms, that roam space with a velocity very close to the speed of light. Collectively, these particles are known as cosmic rays, and their origin is one of the fundamental unsolved problems in modern astrophysics.

The charged energetic particles, that make up cosmic rays, are a very dilute medium; each particle carries a very high energies, but the particles themselves are very few. Per volume, cosmic rays contain as much energy as the gas and the magnetic field between the stars and as the total light in a Galaxy. The processes that determine the energy and spatial distribution of cosmic rays are different from those that shape ordinary gases on earth, because they rely almost entirely on electric and magnetic fields. Most gases on Earth are thermal: the energy of the gas is distributed approximately equally among the atoms or molecules of the gas. In contrast, matter in the Universe is often far from any equilibrium. In the dilute plasmas that fill most interstellar and intergalactic space, nature chooses to endow a small number of particles with an extreme amount of energy. We witness a fundamental self-organization that, through interactions between particles and electromagnetic fields, arranges the atoms and available energy in three components: a cool or warm gas that carries the bulk of the mass, cosmic rays with a wide range of energies, and the turbulent electromagnetic fields that link the two. Why does nature produce cosmic rays? Also, what is the fate of the turbulent magnetic field? Do interactions of cosmic rays generate the large-scale magnetic field that permeates the Universe?

It appears that efficient acceleration of cosmic rays proceeds in systems with outflow phenomena, in which a fraction of the flow energy can be transferred to cosmic rays. One of those system are shell-type supernova remnants (SNR), in which material from the exploded star slams into the ambient gas, thus forming a shock front. The figure shows a radio map of the SNR Cassiopeia A, which is about 300 years old. In fact, SNRs have long been suspected as production sites of Galactic cosmic rays on account of the flow energy and the supernova rate. The question of cosmic-ray acceleration in SNRs includes aspects of the generation, interaction, and damping of magnetic turbulence in non-equilibrium plasmas. The physics of the coupled system of turbulence, energetic particles, and colliding plasma flows can best be studied in young SNRs, for which X-ray and gamma-ray observations indicate very efficient particle acceleration and the existence of a strong turbulent magnetic field.

We conduct intensive simulations in which we follow individual particles as they move in electric and magnetic fields. Directly ahead of SNR shocks one expects that cosmic rays drift relative to the ionized interstellar gas, which is an unstable situation. The movie shows simulation results for that scenario: The top panel indicates the turbulent magnetic field and the bottom panel depict the density of interstellar gas. Initially the interstellar gas is at rest and the cosmic rays drift to the left. Note that after a while the structures in both the magnetic field and the gas density appear to drift as well. In the end there is no relative drift between cosmic rays and interstellar gas, and the growth of magnetic field terminates. This saturation can only be captured in kinetic studies.