> Please note that the paper is “Groeneboom and Eriksen”, not “Eriksen et al.”…

Oops, my mix-up! (literally)

> But do you think the current experiments are deep/wide enough to really get a good handle on this effect?

At Fisher matrix level the SDSS photometric samples should be able to detect g* if it’s really 0.15 … at back of the envelope level since there are Fourier modes spanning a wide range of directions you need ~1/g*^2~44 linear modes to measure it. The factors of order unity are unfortunately not so kind: the Fisher matrix has a factor of 2/45 in it (because the variations in the power spectrum as a function of angle are 10^4 modes. BUT: No promises until the systematics tests are all in

Chris

A few comments to your posts:

1) The problem with a cross-correlation analysis (between, say, V and W) for this particular analysis is that it’s very difficult to handle the signal covariance matrix on a cut sky. The only reason it’s sparse in our analysis is that we’re using the Gibbs sampling algorithm, which essentially “fills in” the cut region. I don’t see many alternatives to this, really, if one wants to go to high l’s. However, the Gibbs sampler is an exact likelihood approach, and as such, intrinsically an auto-correlation method; it’s not straightforward to get rid of these auto-correlations, while still have a correct likelihood. Of course, the problem becomes smaller the more independent bands you have, but it’s always going to be there to some extent. But of course, in principle it’s of course possible that one may construct some “pseudo-Cl” approach for this particular model, but right now, I don’t see how.. For the moment, I think the best approach is simply to analyse realistic WMAP5 noise simulations, and see if something similar pops up.

2) Personally, I don’t think asymmetric beams is relevant for this result. The model signature has a substantial (as in several degrees, I’d say) correlation length along the plane normal to the preferred axis, and even though the WMAP beams are somewhat asymmetric, they’re not *that* asymmetric.. Correlated noise is definitely my biggest concern here.

and

3) Please note that the paper is “Groeneboom and Eriksen”, not “Eriksen et al.”…

Finally, I’m really looking forward to see what comes out of the LSS analyses! But do you think the current experiments are deep/wide enough to really get a good handle on this effect? Or do we need to wait for the next generation surveys?

Thanks!

That said, of the possible systematics that could produce an asymmetry in the power spectrum, the first one on my list would be beam ellipticity because WMAP does not hit each pixel at a uniform distribution of angles of attack. (Same will be true, more so, for Planck.) The cross-power analysis won’t solve this problem, ultimately one needs to simulate it using the known beam maps and see what happens.

Regarding the search in large scale structure: Anthony Pullen (here at Caltech) is working on it, so stay tuned. I’m sure there will also be a lot more poring over WMAP and soon Planck, and probably other LSS data sets shortly after that. I for one find the situation exciting. A few years ago I went to conferences where people presented “explanations” of the low-multipole anomalies that made no predictions that I could hope to see verified at many sigmas in my lifetime. Well, with this particular anomaly I hold out hope that in 5-10 years it will either have gone away or be seen at many sigma in both CMB and LSS …

If we incorporated this model into, say Cosmomc, we would most likely observe very little changes in the “standard” model parameters. This is because the only parameter affecting the angular power spectrum is the anisotropy amplitude g*, and this would only alter the overall amplitude of the angular power spectrum (sigma_8, if you want). The anisotropy direction itself would not affect the angular power spectrum as it only contains isotropic contributions, and hence not contribute to shifting any of the remaining standard parameters (as all code / theories usually are based on an isotropic theory). Even more, in order to make the code consistent, we “re-scaled” the anisotropy amplitude g* such that it is not degenerate with the amplitude of the power spectrum any more.

Nicolaas

No, unfortunately, COBE has much too low sensitivity and angular resolution to be relevant for this analysis. The effect is just about starting to become visible when considering angular scales down to ~2 degrees, and to get strong results, one needs ~0.5 degrees. COBE, on the other hand, had 7 degrees resolution, and much too high noise. So there doesn’t seem to be many alternatives around besides waiting for Planck, really, although it’s possible that galaxy catalogs like SDSS or 2dF could be relevant.

Jason Dick at 9:12 pm:

Neil,

If what you said were true, then we couldn’t ever be confident about anything. Any experimental results that we ever obtain are always statistical in nature: there’s always a probability that we’re wrong, either due to statistical or systematic errors. The best that we can do, and what is done on any good analysis, is to place statistical limits upon any such results.

Well Jason, you’ve formally contradicted yourself but confirmed exactly what I said before – you just don’t realize that I am working the “matter of principle” and you are referencing the matter of degree (talking past each other, not apparently a direct disagreement.) First, I am right and you unwittingly acknowlege it: we can’t ever be sure, it’s a matter of degree. Sure, the chance of there being such a long run is tiny, but in degree not principle, and makes it formally impossible to falsify in Popperian terms. We can be “confident” (in the loose informal sense) but not certain nor can even define a category boundary of “reasonably certain logical type”, because as I said, we make judgment calls about how unlikely something is along a continuum and pick arbitrary pigeonholes thereby.

But even then, in an infinite universe/s there will be regions or sequences of grotesquely improbable events, and the problems I raised are germane. Or, is “probability” even possible to define in an infinite universe at all, given the incommensurable nature of relative proportions in infinite sets (i.e., the Hilbert Hotel problem etc.?) But I find it odd that I thus shouldn’t worry about the risk of not wearing a seat belt etc. because of the boundary condition that the universe is infinite, and thus contains infinite copies of me/similar wearing or not wearing set belts to disallow any finite frequentist mass comparison – yet even if having a volume of say 10^30,000 light years, this would all be conventionally meaningful instead. I bring this up partly since some critics use infinite statistical measure problems to fend off some of my arguments about the chance of laws and universe behavior having such and such conveniently anthropic or even predictable form etc.

REM also those problems occur even in a very very huge yet finite universe, as long as there’s plenty of space for extremely odd things like unexpected discernible patterns of radioactive decay, etc, to likely occur.)

(I couldn’t find this http://map.gsfc.nasa.gov/media/ContentMedia/990095b.jpg map in Galactic coords.)

Is this what you meant Sean, by “correlated noise”, that the effect is correlated with the lowest noise points on the map? Or are you referring to a different noise correlation?

There are lots of ways to put priors on different models even in ignorance, though. Typically the best priors to use are those based upon Occam’s Razor: downweight theories that have more parameters. There are different ways of doing this, but the basic idea is just that we need greater statistical significance to gain confidence in a theory that has more parameters.

In this case, though, I think the only interesting question is whether or not the correlated noise has something to do with the it. Hans and collaborators have already demonstrated that correlated noise can replicate the effect, so it just remains to test the correlated noise with the WMAP analysis. It is also worrying that the apparent axis for this affect appears to coincide quite closely with the poles of the scanning strategy. The statistics are solid, the systematics need to be understood better.

BTW in radar detection theory Baysian models are used frequently but only when realistic priors are available otherwise one ends up arguing about assumptions. Great theory if you have the data.