CosmoCoffee

Having recently worked on the vector and tensor modes (interesting for B-mode polarization), I was interested to see this paper focussing on the scalar modes.

* There is a lot of analysis, but I'm puzzled why the author doesn't actually compute the CMB anisotropy and plot the [tex]C_l[/tex] for some example models. I would have thought modifying CAMB/CMBFAST would be quite straightforward. The observational implications of the paper are at the moment obscure.

* People familiar with astrophysical MHD tell me that magnetic fields of order 1 nG required to make interesting signatures on the CMB are far too large to be compatible with galaxy/cluster evolution models. I'd be interested to hear more opinions on this. There are quite a few papers on magnetic fields and the CMB (guilty) - are they all a waste of time?

Hi Antony,
I have heard that mentioned at Cosmo04, about the constraints from galaxy evolution models. I am curious to learn more about that, because that's not what I remember seeing in review papers when I looked at them last time (about 2 years ago).
But regardless of the outcome, I don't think looking for magnetic fields in the CMB is a waste of time. If one looks at actual observational constraints on magnetic fields then CMB is probably the most competitive test. The constraint on a homogeneous B is something like nG and comes from CMB. On scales you consider in your papers (l \sim 1000) the existing constraints are much weaker, at least by an order of magnitude. So if you were sensitive to a nG on those scales, you would improve the constraint by a lot!
Levon

The paper astro-ph/0410032 has quite a nice criticism of CMB analyses, in particular citing astro-ph/0202272 for the numerical claim that comoving fields of order [tex]10^{-12}G[/tex] are sufficient to reproduce observed cluster magnetic fields, and hence that the nG fields required for interesting CMB signatures (at l < 10000) are inconsistent with this.

However I do agree that it is probably worth being aware of possible CMB signals just in case the numerical simulations are unreliable and as a general cross-check.

The paper astro-ph/0410032 has quite a nice criticism of CMB analyses, in particular citing astro-ph/0202272 for the numerical claim that comoving fields of order 10 - 12G are sufficient to reproduce observed cluster magnetic fields, and hence that the nG fields required for interesting CMB signatures (at l < 10000) are inconsistent with this.

I saw that paper yesterday, it looks like a very nice and thorough study of the evolution of magnetic fields. They cite two reasons for being pessimistic about nG fields. I find their first reason more convincing -- it is indeed very hard to generate nG fields by causal physics. As they say, there is a simple qualitative reason, which is the smallness of the Hubble parameter at the time when magnetic fields could have been produced in the early universe. But if we believe that something like inflation took place then there could be ways around these causal constraints (see e.g. astro-ph/0108093).

Their second reason -- the inconsistency with the simulations, as Antony noted as well, is not an original contribution of this paper, they cite numerical claims of astro-ph/0202272. That paper went largely unnoticed -- only 2 citations in over 2 years
and only 2 citations for the previous paper, from 1999, by the same authors on the same subject with similar claims. I wonder if this is a case of people simply not knowing this result, disagreeing with it or deliberately ignoring it.