Do intergalactic magnetic fields imply an open universe?
  The detection of magnetic fields at high redshifts, and in empty
intergalactic space, support the idea that cosmic magnetism has a primordial
origin. Assuming that Maxwellian electromagnetism and general relativity hold,
and without introducing any `new' physics, we show how the observed magnetic
fields can easily survive cosmological evolution from the inflationary era in a
marginally open Friedmann universe but fail to do so, by a very wide margin, in
a flat or a marginally closed universe. Magnetic fields evolve very differently
in open and closed Friedmann models. The existence of significant magnetic
fields in the universe today, that require primordial seeding, may therefore
provide strong evidence that the universe is marginally open rather than
marginally closed.
