Which Fields Survive to Recombination: Alfven Damping Scales for a Bounced Primordial Field

A primordial field must survive the plasma era to be observed, and the plasma erases fields below a damping scale set by Alfven-wave propagation before recombination. For a present field of 1 nanogauss this paper computes that scale: the Alfven speed at recombination is 1.91e-04 of light speed, and the damping length is 2.45e+04 parsecs comoving. Fields organised on megaparsec scales, as the void evidence requires, are therefore entirely safe from plasma damping, with five orders of magnitude to spare. The danger zone is the opposite end: a field born at the turnaround sits on the turnaround horizon scale of 5.01e-11 parsecs comoving, more than eleven orders of magnitude below the damping scale, and would be erased unless an inverse cascade moves its power upward before recombination. Damping thus acts as a filter that any bounce magnetogenesis history must pass, and the filter's passband is computed here explicitly as a function of field strength. The damping estimate is computed from a stated model, not measured.