Craig J. Hogan
Published 2000-05-18, updated 2000-05-23Version 2
It is shown that the standard cosmological model predicts {\it ab initio} generation of large-scale but very small-amplitude cosmic magnetic fields at the epoch of recombination of the primeval plasma. Matter velocities dominated by coherent flows on a scale $L\approx 50h^{-1}(1+z)^{-1}$ Mpc lead to a dipole of radiation flux in the frame of the moving matter. Thomson scattering of the radiation differentially accelerates the electrons and ions, creating large-scale coherent electric currents and magnetic fields. This process is analyzed using magnetohydrodynamic equations which include a modification of Ohm's law describing the effect of Thomson drag on the electrons. The field strength is estimated to be $B\simeq 10^{-20}$G.
Comments: 6 pages, AAS Latex. The amplitude of the generated field is much smaller than estimated in the original version, due to slow growth rate of B by induction
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