[astro-ph/0503213] Evidence of vorticity and shear at large angular scales in the WMAP data: a violation of cosmological isotropy?

Authors:  T. R. Jaffe, A. J. Banday, H. K. Eriksen, K. M. Gorski, F. K. Hansen
Abstract:  Motivated by the large-scale asymmetry observed in the cosmic microwave background sky, we consider a specific class of anisotropic cosmological models -- Bianchi type VII_h -- and compare them to the WMAP first-year data on large angular scales. Remarkably, we find evidence of a correlation which is ruled out as a chance alignment at the 3 sigma level. The best fit Bianchi model corresponds to x=0.55, Omega_0=0.5, a rotation axis in the direction (l,b)=(222degr,-62degr), shear (sigma/H)_0=2.4x10^-10 and a right--handed vorticity (omega/H)_0=4.3x10^-10. Correcting for this component greatly reduces the significance of the large-scale power asymmetry, resolves several anomalies detected on large angular scales (ie. the low quadrupole amplitude and quadrupole/octopole planarity and alignment), and can account for a non--Gaussian "cold spot" on the sky. It appears that the WMAP data do indeed violate cosmological isotropy.
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Anze Slosar
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Anze Slosar » March 10 2005

Ah, joys of cosmology! Another very interesting paper this week!

I have a few questions for discussion:
  • Figure 1. b: It would be interesting to see how does this Bianchi template looks in more neutral coordinates (say with axis to the pole). As far as I can gather these models produce a spirally looking pattern and my question is whether such pattern could result as an artifact of the scanning strategy. If this is the case, the the "telescope" would add this kind of template and then, of course, you will get all those cold-spot detections, assymmetries, etc. without them really be there;
  • The WMAP best fit theoretical squadropole of 869 [tex]\mu {\rm K}^2[/tex]: surely this is the running spectral index value? And the n_run was there to fit the low quadrupole (among other things), so it would make more sense to compare it to the pure LCDM of 1150 or so [tex]\mu {\rm K}^2[/tex]. BUt yes, anything higher than 300 is OK with normal cosmology.
  • This model brings 6 or 7 new params, which is quite a lot. Looking in pixel space, after subtracting template, by how much does the probability that what remains is a realisation of the pure LCDM increases? I am slightly confused whether this kind of analysis is neccessary, if we can just get rid of all NG detections by introducing 5 extra params? How does one go and quantify the improvement of fit in such cases...
  • Finally, and most importantly: it seems that we cannot really get considerably higher statistical significance from CMB alone. If the universe is indeed described by such models, what other observational signatures could one get? Could you discard such models on the basis of LSS considerations?

Niayesh Afshordi
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Niayesh Afshordi » March 11 2005

It seems to me that Jaffe et al. are introducing 7 new parameters, using which \chi^2 is improved by 9 (since S/N =3).
This doesn't look very significant! Am I right?

Tess Jaffe
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Tess Jaffe » March 11 2005

The Bianchi component requires six additional parameters: x (or h), sigma (and thereby omega), three Euler angles, and the handedness. However, it's difficult to assess the improvement in the likelihood, because the anomalies are all non-Gaussian/anisotropic. Therefore, a simple traditional power spectrum vs. parameters approach based on the usual likelihood does not work, and a more complicated framework (with new associated parameters) is indeed required. The beauty of this particular model is that it appears to explain virtually all claimed anomalies and is both simply and physically motivated (which is more than can be said about ad-hoc inflationary potentials :-)). However, this letter does not attempt to address the complicated issue of quantifying the statistical improvement of the model by the addition of these parameters. It is meant simply to pique interest in models which have for some time been assumed irrelevant.

Pablo Fosalba
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Pablo Fosalba » March 12 2005

I have a couple of concerns about this model:
assuming there is a contribution from such model at high multipoles,
it remains to be seen what is the prediction for the power spectrum
for l>200 when one subtracts the Bianchi template from the WMAP map.
The acoustic peak structure should be preserved.
Related to the previous concern,
isn't this model in conflict with inflation ?

Apart from this, I find this paper is one of the nicest I have seen
dealing with WMAP data, so congratulations to the authors!

Levon Pogosian
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Levon Pogosian » March 14 2005

I also enjoyed reading this paper.
Are there experts in cosmocoffee who could comment on Anze's first question --could the Bianchi template be due to the scanning sequence or some other measurement artifact?
Do we have to wait for Planck to find out?

Anthony Banday
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Anthony Banday » March 14 2005

Scanning strategy problems are highly unlikely to explain these issues.
In fact, the WMAP scanning strategy was *designed* to avoid such
artefacts. The satellite is differential and has a short
precession period which tends to wash out systematic effects
in general, and azimuthal structure in particular. Furthermore, noise related
effects tend to be be symmetric over the two (ecliptic) hemispheres.

Beyond these generalisations, there are several specific points that argue
against a scanning strategy based explanation:

1) Such effects are not seen in the simulations of Hinshaw et al.
2003, ApJS, 148, 63, which are based on the time-ordered-data
and include any 1/f noise structure and its subsequent removal
by the map-making/filtering iteration. In particular, see Figs. 12 and 13.
2) The observed structures are *much* wider than the beam size.
3) The best-fit axis does not coincide perfectly with the ecliptic
poles.

It should also be remembered that the scanning can provides a mechanism that
allows systematic artefacts to be introduced coherently, but a physical
signal is still required. There do not appear to be any known candidates.

Tony, Tess, Hans Kristian, Kris, Frode

Levon Pogosian
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Levon Pogosian » March 14 2005

Thanks, Anthony.

Bruce Bassett
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Bruce Bassett » March 14 2005

Hi,

Fascinating paper indeed! One important point is that since Bianchi models have non-zero shear or Weyl tensor (in the background) unlike FLRW, the dynamics of linear perturbations is significantly different than in FLRW.

Hence matching to the CMB is actually a complicated project even if one is only interested in large angular scales (e.g. the ISW effect will be different).

Cheers,
Bruce

Niayesh Afshordi
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Niayesh Afshordi » March 15 2005

Something I don't understand here is how you can measure \omega/\H from CMB anisotropies so accurately. The CMB anisotropies are at the level of 10^{-5}, and the maximum level of anisotropies that I expect from rotation is O(\omega/\H), so I don't see where the number 10^{-10} comes about.

Combination of many multipoles can only improve the accuracy by a factor of 10-100, which is still a long way from 10^{-10}.

Anze Slosar
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Re: [astro-ph/0503213] Evidence of vorticity and shear at la

Post by Anze Slosar » March 15 2005

Bruce Bassett wrote:Fascinating paper indeed! One important point is that since Bianchi models have non-zero shear or Weyl tensor (in the background) unlike FLRW, the dynamics of linear perturbations is significantly different than in FLRW.
Yes, this is what I want to understand. Is it possible to trivially refute these models on the basis that LSS or CMB PS after template subtraction would look completelly different?

I just got a copy of the original Barrow et al paper and they also focus exclusivelly on the CMB, while I would naivelly expect that an extra scale associated with rotation should appear in the power spectrum... [Well, to be quite honest, haven't read the paper so I might be talking total rubbish]

Kate Land
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Kate Land » March 15 2005

Very nice paper, and an intriguing idea ;-) However, on the subject of likelihood, isn't it crucial to take account of other effects. e.g Aren't these models ruled out purely by the low omega value, and the effect this would have on the position of the 1st Doppler Peak. Am I missing something?

Bruce Bassett
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Bruce Bassett » March 15 2005

I should clarify my previous comment. The values of the shear/vorticity are so small 10^{-10} that today their impact on perturbation dynamics would be irrelevant I imagine. In that case the LSS predictions would apparently be close to FLRW.

However, the dynamics of Bianchi models is very complex and many exhibit asymptotic isotropisation - i.e. they start out very anisotropic with large shear and then end up coming very close to FLRW. Depending on the precise dynamics one could imagine models where the shear was dynamically important early on...(but still not so big as to violate BBN limits), so perturbation dynamics during radition domination could be significantly non-FLRW.

An interesting conceptual point is that the existence of vorticity means that one cannot define a single hypersurface-orthogonal four velocity for all observers (something we always take for granted in FLRW).

Tess Jaffe
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[astro-ph/0503213] Evidence of vorticity and shear at large

Post by Tess Jaffe » April 08 2005

At http://www.mpa-garching.mpg.de/~tjaffe/note.ps we've posted a quick note on the Bianchi models. Basically, it's just a summary of the relevant parameters and equations in Barrow et al. (1985) along with plots of some examples. From these, one can see that for some models, a shear of order 10^-10 can give anisotropies of the order 10uK.

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