# 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.

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I added the hyperlink to the paper. It looks like an intriguing argument. This is not an endorsement yet. ;-) – Luboš Motl Oct 6 '12 at 16:00
Thanks! I was in the process of adding the link and abstract as you were editing also.... – FrankH Oct 6 '12 at 16:03
@FrankH I'd be curious to here your answer... – user11547 Oct 26 '12 at 21:11
It's actually not a recent argument. Check the references in the paper, esp [8], it dates back several years. I went and read some of that but I'm not much wiser. The thing is the following: what they call "ricci identities" are just that, identities. They are always valid, you learn nothing from this. Since they don't write down a Lagrangian, I can't tell whether or not the coupling of electromagnetism to GR is the standard coupling or not. If it's the standard coupling, it would be convincing. If not, not. – WIMP Dec 25 '12 at 15:48
Shtanov and Sahni claim Barrow is wrong: arxiv.org/abs/1211.2168 – Ben Crowell Jun 7 '13 at 21:38

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.

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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.

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I must admit that I don't really understand some of the points here. The last paragraph doesn't make sense, especially because of the lack of an absolute/privileged reference frame. I'm also confused as to why things beyond the observable universe should impact the implications of magnetic fields on a Friedmann model to a strong extent, given that we generally assume homogeneity. – HDE 226868 Jan 13 at 23:08
Lack of of an absolute/privileged reference frame? In an obtainable sense yes, but where do you think assumptions like homogeneity are taken from? Same place, we can only assume homogeneity(which does not need to be preserved as seen in our visible sphere if over a larger sphere is different but truer) from the absolute/privileged reference frame by viewing all infinity at once. As for Magnetic Fields, and all things within the universe, they are limited by c but NOT the universe (space-time) itself, so what we cannot see can still influence the open/closed factors of the universe. – Void Serpent Jan 14 at 4:37
Small fact: if you have plasma in your Universe, you break Lorentz invariance. Because there is a special frame — the rest frame of the plasma – Andrii Magalich Jun 25 at 0:12

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.)

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Did you read the paper? They report other people who have measured significant intergalactic fields that are not part of any galaxy. Please read the paper before you criticize it. – FrankH Feb 21 at 23:55
They contribute to preventing the interstellar medium ISM against gravitational collapse; not the mass structure of the galaxy as a whole. – Nick Feb 23 at 10:55