Relaxing the bounds on primordial magnetic seed fields

We point out that the lower bound on the primordial magnetic field required to seed the galactic dynamo is significantly relaxed in an open universe or in a universe with a positive cosmological constant. In such universes, the increased age of galaxies gives a dynamo mechanism more time to amplify a small initial field. It is shown that, for reasonable cosmological parameters, primordial seed fields of strength ${10}^{-30}\mathrm{G}$ or less at the time of galaxy formation could explain observed galactic magnetic fields. As a consequence, mechanisms of primordial magnetic seed-field generation that have previously been ruled out could well be viable. We also comment on the implications of the observation of micro-Gauss magnetic fields in galaxies at high redshift.