Overview of cosmic magnetic fields

In order to understand the formation, the evolution and the current properties of astrophysical objects, one needs to estimate the forces acting on matter. With its infinitely extending range, gravity is the main candidate for shaping the large-scale structure of the Universe. However, with an estimated fraction of more than 99 %, baryonic matter is predominately in a plasma state, i.e. it includes electrically charged particles. The dynamics of these are influenced by the second long-range fundamental force, the electromagnetic force. Indeed, observations show that the local Universe is highly magnetized. Magnetic fields are observed on all scales, from planets and stars via galaxies up to the intergalactic medium, where fields might be correlated on Mpc-scales. In many cases, for instance in the interstellar medium, the magnetic energy density is comparable to the thermal energy density and the one of turbulence and cosmic rays. This phenomenon is known as energy equipartition and indicates a strong coupling between the individual energy components.

Observations of magnetic fields are difficult and thus we do not know much about their state in objects at large distance which corresponds to the Universe at earlier times. If back then magnetic fields have been as strong as they are at present day, the process of structure formation could be affected significantly. Especially the formation of the very first stars, the so-called Population III stars, will be different in that case. Strong magnetic fields can change the mass distribution of the first stars with implications for their lifetime and the enrichment of their environment with metals. Also the formation and evolution of galaxies is sensitive to the presence of magnetic energy. Future radio telescopes will be able to look deeper into the Universe and study the time evolution of cosmic magnetic fields which will help us to better understand structure formation.

Various possibilities to create magnetic seed fields have been suggested. The first fields might already have been generated in the very early Universe, during the epoch of inflation (from ≈ 10−36 until ≈ 10−32 seconds after the Big Bang) or during cosmological phase transitions (e.g. QED phase transition). In addition, plasma batteries or other plasma instabilities can result in magnetic seed fields. A common property of all generation mechanisms is that the resulting seed fields are extremely weak with a strength of 10−20 Gauss resulting from a typical plasma battery. This is tremendously (in fact 15 orders of magnitude) smaller than the field strength of 10−5 Gauss measured in galaxies today! When and how became magnetic fields so strong? I suggest that the answer to this question could be the small-scale turbulent dynamo which is introduced in the next section.