Reynolds numbers in the early Universe
  After electron-positron annihilation and prior to photon decoupling the
magnetic Reynolds number is approximately twenty orders of magnitude larger
than its kinetic counterpart which is, in turn, smaller than one. In this
globally neutral system the large-scale inhomogeneities are provided by the
spatial fluctuations of the scalar curvature. Owing to the analogy with the
description of Markovian conducting fluids in the presence of acoustic
fluctuations, the evolution equations of a putative magnetic field are averaged
over the large-scale flow determined by curvature perturbations. General
lessons are drawn on the typical diffusion scale of magnetic inhomogeneities.
It is speculated that Reynolds numbers prior to electron-positron annihilation
can be related to the entropy contained in the Hubble volume during the various
stages of the evolution of the conducting plasma.
