PRIMORDIAL MAGNETIC-FIELDS

The explanation of the observed galactic magnetic fields may require t he existence of a primordial magnetic field. Such a field may arise du ring the early cosmological phase transitions, or because of other par ticle physics related phenomena in the very early universe reviewed he re. The turbulent evolution of the initial, randomly fluctuating micro scopic field to a large-scale macroscopic field can be described in te rms of a shell model, which provides an approximation to the complete magnetohydrodynamics. The results indicate that there is an inverse ca scade of magnetic energy whereby the coherence of the magnetic field i s increased by many orders of magnitude. Cosmological seed fields roug hly of the order of 10(-20) G at the scale of protogalaxy, as required by the dynamo explanation of galactic magnetic fields, thus seem plau sible.