Abstract
The origin and maintenance of coherent magnetic fields in the Universe is reviewed with an emphasis on the possible challenges that arise in their theoretical understanding. We begin with the interesting possibility that magnetic fields originated at some level from the early universe. This could be during inflation, the electroweak, or the quark-hadron phase transitions. These mechanisms can give rise to fields which could be strong, but often with much smaller coherence scales than galactic scales. Their subsequent turbulent decay decreases their strength but increases their coherence. We then turn to astrophysical batteries which can generate seed magnetic fields. Here the coherence scale can be large, but the field strength is generally very small. These seed fields need to be further amplified and maintained by a dynamo to explain observed magnetic fields in galaxies. Basic ideas behind both small and large-scale turbulent dynamos are outlined. The small-scale dynamo may help to understand the first magnetization of young galaxies, while the large-scale dynamo is important for the generation of fields with scales larger than the stirring scale, as observed in nearby disk galaxies. The current theoretical challenges that turbulent dynamos encounter and their possible resolution are discussed.
Highlights
The universe is magnetized, right from the Earth, the Sun, and other stars to disk galaxies, galaxy clusters, and perhaps the intergalactic medium (IGM) in voids
We focus here on galactic magnetic fields and trace their origin and maintenance from the early to the present day universe
Seed magnetic fields could be a relic from the early Universe, arising during the inflationary epoch or in a later phase transition, when the electroweak symmetry is broken or when quarks gather into hadrons
Summary
The universe is magnetized, right from the Earth, the Sun, and other stars to disk galaxies, galaxy clusters, and perhaps the intergalactic medium (IGM) in voids. Magnetic fields are observed to have both a coherent component of order a few micro Gauss, ordered on scales of a few to ten kilo parsecs (kpc) and a random component with scales of parsecs to tens of parsecs [1,2,3] In these galaxies, both stars and the gas in the interstellar medium (ISM) are in a thin disk supported against gravity by their rotation. There is indirect evidence for a lower limit of order 10−16 G to the magnetic field contained in the intergalactic medium of large scale void regions between galaxies [8,9] (see, [10]) This strength refers to a coherence scale of a Mpc, and the field needs to be stronger if the coherence scale is smaller.
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