Do intergalactic magnetic fields imply an Open Universe? According to a paper on the arXiv (now published in Phys Rev D), they do. How credible is this result?  The abstract says:

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 and not marginally closed.

 A: A 2013 paper by Shtanov and Sahni (already mentioned by Ben Crowell in the comments) says that the modes grow exponentially in conformal coordinates, and Barrow et al overlooked the fact that the conformal time changes very little during and after inflation.
A 2014 preprint by Tsagas, one of the authors of the original paper, cites Shtanov and Sahni and seems to agree with them and disagree with the original paper. So I think the original paper is wrong.
A: Magnetic fields are short range and tend to be planetary - I can't see how magnetic fields could have any influence whatsoever on cosmology.  I suspect the paper has been misinterpreted.  
(And magnetic fields interact with charged particles / other magnetic fields - there is no gravitational effect.  I suspect a mis-question and some barking up the wrong tree here.)
A: I'd say no, though the paper seems credible in its methods I believe it is neglecting to account for the fact that we can only measure the Visible Universe.
This may require a little explanation as I don't mean there are some extra layers or dimensions, but that we can only see $\dfrac{4}{3}\pi(14.5\cdot 10^{9}\ \mathit{ ly})^{3}$ of the Universe due to the limit of the speed of light, c. 
The Universe itself however is much larger because Space-Time can (and has) expanded faster than c, and, if there are "edges" (which would make it open, but irrelevant b/c it's unreachable) likely still is to contain light. 
Or if it's closed then the faster than c expansion in the early universe was what closed it off, but again made it too large with what time has passed to observe enough of it to make such a claim.
Just my thought from what I know of Astrophysics though.
There is also the notion the Universe must be Static to an Outside Omni-observer regardless of the non-static appearance internally, which an open universe doesn't scream static/conservation of everything to me.
