On the origin of cosmic magnetic fields

Abstract

We review the extensive and controversial literature concerning how the cosmic magnetic fields pervading nearly all galaxies and clusters of galaxies actually got started. Some observational evidence supports a hypothesis that the field is already moderately strong at the beginning of the life of a galaxy and its disc. One argument involves the chemical abundance of the light elements Be and B, while a second one is based on the detection of strong magnetic fields in very young high red shift galaxies.Since this problem of initial amplification of cosmic magnetic fields involves important plasma problems it is obvious that one must know the plasma in which the amplification occurs. Most of this review is devoted to this basic problem and for this it is necessary to devote ourselves to reviewing studies that take place in environments in which the plasma properties are most clearly understood. For this reason the authors have chosen to restrict themselves almost completely to studies of dynamos in our Galaxy. It is true that one can get a much better idea of the grand scope of galactic fields in extragalactic systems. However, most mature galaxies share the same dilemma as ours of overcoming important plasma problems. Since the authors are both trained in plasma physics we may be biased in pursuing this approach, but we feel it is justified by the above argument. In addition we feel we can produce a better review by staying close to that which we know best.In addition we have chosen not to consider the saturation problem of the galactic magnetic field since if the original dynamo amplification fails the saturation question does not arise.It is generally accepted that seed fields, whose strength is of order 10 −20 G, easily spring up in the era preceding galaxy formation. Several mechanisms have been proposed to amplify these seed magnetic fields to a coherent structure with the microgauss strengths of the currently observed galactic magnetic fields.The standard and most popular mechanism is the α-mean field dynamo theory developed by a number of people in the late sixties. This theory and its application to galactic magnetic fields is discussed in considerable detail in this review. We point out certain difficulties with this theory that make it seem unlikely that this is the whole story. The main difficulty with this as the only such amplification mechanism is rooted in the fact that, on galactic scales, flux is constant and is frozen in the interstellar medium. This implies that flux must be removed from the galactic discs, as is well recognized by the standard theory.For our Galaxy this turns out to be a major problem, since unless the flux and the interstellar mass are somehow separated, some interstellar mass must also be removed from the deep galactic gravitational well. This is very difficult. It is pointed out that unless the field has a substantial field strength, much larger than that of the seed fields, this separation can hardly happen. And of course, it must if the α-dynamo is to start from the ultra ...

Effects of theUY(1)Chern-Simons term and its baryonic contribution on matter asymmetries and hypermagnetic fields

“…Interestingly, similar magnetic fields have been observed in other spiral and barred galaxies [12][13][14] as well as galaxy clusters [15][16][17] and high redshift protogalactic structures [18]. It is generally believed that these magnetic fields are produced from the amplification of some seed fields [19] whose strength and origin are largely unknown [11,20]. The fact that the magnetic fields are present ubiquitously at high redshifts, strengthens the idea that their origin is cosmological, and magnetic fields may have pervaded the Universe in its hot early stages [18].…”

Magnetic fields in primordial accretion disks

“…Grasso & Rubinstein 2001;Kulsrud & Zweibel 2008;Ryu et al 2012;Widrow et al 2012). The first cosmological simulations had shown that primordial stars form in dark matter halos with 10 5 −10 6 M at z ∼ 20 (Abel et al 2002;Bromm et al 2002) or even earlier (Reed et al 2005;Gao et al 2010).…”

Extragalactic magnetic fields constraints from simultaneous GeV–TeV observations of blazars

“…However, the origin of these fields remains largely uncertain (see Kronberg 1994;Grasso & Rubinstein 2001;Widrow 2002;Beck 2009, for reviews). A commonly accepted hypothesis is that relatively strong galactic and cluster magnetic fields result from the amplification of much weaker pre-existing "seed" fields via compression and turbulence/dynamo amplification in the course of structure formation processes (Kulsrud & Zweibel 2008).…”