The Protogalactic Origin for Cosmic Magnetic Fields

Abstract

It is demonstrated that strong magnetic fields are produced from a zero initial magnetic field during the pregalactic era, when galaxies are first forming. Their development proceeds in three phases. In the first phase, weak magnetic fields are created by the Biermann battery mechanism, acting in shocked parts of the intergalactic medium where caustics form and intersect. In the second phase, these weak magnetic fields are amplified to strong magnetic fields by the Kolmogoroff turbulence endemic to gravitational structure formation of galaxies. During this second phase, the magnetic fields reach saturation with the turbulent power, but they are coherent only on the scale of the smallest eddy. In the third phase, the magnetic field strength increases to equipartition with the turbulent energy, and the coherence length of the magnetic fields increases to the scale of the largest turbulent eddy, comparable to the scale of the entire galaxy. The resulting magnetic field represents a galactic magnetic field of primordial origin. No further dynamo action is necessary, after the galaxy forms, to explain the origin of magnetic fields. However, the magnetic field may be altered by dynamo action once the galaxy and the galactic disk have formed. It is first shown by direct numerical simulations, that thermoelectric currentsassociated with the Biermann battery, build the field up from zero to $10^{-21}$ G in the regions about to collapse into galaxies, by $z\sim3$. For weak fields, in the absence of dissipation, the cyclotron frequency ${\bf \omega_{cyc}}=e{\bf B } /m_H c $ and $ {\bf \omega}/(1+ \chi )$, where ${\bf \omega = \nabla \times v }$ is the vorticity and $\chi$ is the degree of ionization, satisfy the same equations, and initial conditions ${\bf \omega_{cyc}=\omega}=0$, so that, ${\bf \omega_{cyc}}({\bfComment: Princeton Univ Obs, submitted to Ap

Primordial magnetic fields from dark energy

“…The non-linear turbulence effect plays an important role in generating and subsequently amplifying the created PMF which overcomes the damping mechanism due to the high plasma conductivity. The numerical simulations showed that the rms value for ∇·v ∼ ∇×v is about 10 −16 s −1 on comoving scale of Mpc at z ∼ 3 [20]. This is just the inverse of the time scale τ LSS for LSS formation.…”

“…It could be understood from the recent study in Ref. [20] where the battery mechanism has been proposed as a source for generating a small initial magnetic field during LSS formation. From the theory of small fluctuations, v will grow with density perturbations.…”

“…However, we will not pursue numerical simulations similar to Ref. [20], combining their battery source with the quintessence dynamics. As a first step, we will simply omit the σ term and solve for the photon equation self-consistently, while following closely their results and estimating the effect of the magneto-hydrodynamical dissipation.…”

String Cosmology: The pre-Big Bang Scenario

“…Thus, converting the primordial seeds into those that may have existed at the protogalaxy formation epoch is by no means a trivial exercise (see, e.g. [53]). The question of whether or not the primordial seeds generated in PBB cosmology can evolve into the observed galactic magnetic fields thus remains, to this date, an unsolved, yet very interesting, problem.…”

Magnetic Helicity and Large Scale Magnetic Fields: A Primer

“…Braithwaite & Spruit 2004;Kulsrud et al 1997;Lovelace et a. 2009;Subramanian 2010;Widrow et al 2012).…”