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[astro-ph/0509039] Local Pancake Defeats Axis of Evil
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Authors:Chris Vale
Abstract:Among the biggest surprises revealed by COBE and confirmed by WMAP measurements of the temperature anisotropy of the CMB are the anomalous features in the 2-point angular correlation function on very large angular scales. In particular, the $\ell = 2$ quadrupole and $\ell = 3$ octopole terms are surprisingly planar and aligned with one another, which is highly unlikely for a statistically isotropic Gaussian random field, and the axis of the combined low-$\ell$ signal is perpendicular to ecliptic plane and the plane defined by the dipole direction. Although this $< 0.1 %$ 3-axis alignment might be explained as a statistical fluke, it is certainly an uncomfortable one, which has prompted numerous exotic explanations as well as the now well known ``Axis of Evil'' (AOE) nickname. Here, we present a novel explanation for the AOE as the result of weak lensing of the CMB dipole by local large scale structures in the local universe, and demonstrate that the effect is qualitatively correct and of a magnitude sufficient to fully explain the anomaly.
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Gil Holder



Joined: 25 Sep 2004
Posts: 27
Affiliation: McGill University

PostPosted: September 08 2005  Reply with quote

Thanks for the clarifications, Martin. It makes it clear that the important thing here is vlens relative to the CMB. For the alignments to be from lensing (or ISW, from another perspective) we need the great attractor to be moving in a particular direction at a particular speed (although this can be traded off with the mass, presumably). What is remarkable is that this direction happens to be the same direction we are moving, suggesting there is a Much Greater Attractor which is pulling both us and the Great Attractor.

What is the upper limit on how much of the measured dipole is primordial? Aside from the peculiar velocity studies, there must be limits on the primordial dipole from people doing kSZ studies, but it is possible that no one bothered to report their numbers.
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Dragan Huterer



Joined: 18 Jul 2005
Posts: 24
Affiliation: University of Michigan

PostPosted: September 08 2005  Reply with quote

On the other hand Tonry et al (ApJ, 530, 625, 2000) claim that the region with R<3000km/s (that is, <30 Mpc/h - exactly the distance to Great Attractor) is "essentially at rest with respect to the CMB". (By this they mean velocity less than 150 km/s I think - small compared to the Local Group velocity of 627 km/s). This doesn't sound good for the model - the Much Greater Attractor that Gil mentioned seems disfavored. But is it still possible that the GA is moving with respect to the CMB at a much greater speed than the 30Mpc region?

This paper, by the way, was pointed out to me because they determine the mass of the Great Attractor to be 9x1015 Msun. Also not good as it's a bit small for Chris - but perhaps there are newer and more accurate studies.
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Garth Antony Barber



Joined: 19 Jul 2005
Posts: 71
Affiliation: Published independent

PostPosted: September 08 2005  Reply with quote

Dragan Huterer wrote:
This paper, by the way, was pointed out to me because they determine the mass of the Great Attractor to be 9x1015 Msun. Also not good as it's a bit small for Chris - but perhaps there are newer and more accurate studies.

Or a closer and smaller lensing mass?

Garth
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Antony Lewis



Joined: 23 Sep 2004
Posts: 1351
Affiliation: University of Sussex

PostPosted: September 08 2005  Reply with quote

Incidentally, people have looked at using the moving lens effect to constrain velocities of (other) clusters: the motion of the cluster gives a dipole-like anisotropy pattern over the cluster, though observing it seems to be difficult.

Some relevant references are

M. Birkinshaw and S. F. Gull
A test for transverse motions of clusters of galaxies
Nature 302 (1983), 315

astro-ph/9501059 and astro-ph/9803040.

In the rest frame of the CMB the moving lens constitutes a time-evolving potential, so as discussed the mechanism is like the ISW/Rees-Sciama effects. In the rest frame of the lens it is dipole lensing.
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Håkon Dahle



Joined: 04 Oct 2004
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Affiliation: Inst. of Theoretical Astrophysics, University of Oslo

PostPosted: September 08 2005  Reply with quote

Quote:
This paper, by the way, was pointed out to me because they determine the mass of the Great Attractor to be 9x1015 Msun. Also not good as it's a bit small for Chris - but perhaps there are newer and more accurate studies.


The studies in astro-ph/0412329 and astro-ph/0403275 (using type Ia supernovae and X-ray clusters, respectively) seem to support the basic picture needed – the Great Attractor having a significant flow into the more distant Shapley concentration.
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Anais Rassat



Joined: 20 Oct 2004
Posts: 30
Affiliation: LASTRO (EPFL)

PostPosted: September 10 2005  Reply with quote

Martin White wrote:
Hi,
one finds dE/E = 2 vlens.Φ dt
Integrating the ith component rather than the 0th gives the bend-angle formula – but this doesn't matter if you're lensing a monopole.


Martin.


Gil Holder wrote:
Thanks for the clarifications, Martin. It makes it clear that the important thing here is vlens relative to the CMB.


Hi, I thought that Chris Vale was assuming no instrinsic fluctuations, that the lensed dipole was only due to the observer's and the lens' motions. Then in the CMB frame there is only a monopole , so doesn't that mean that the effect is inexistant (in the CMB frame)?

Anais.
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Gil Holder



Joined: 25 Sep 2004
Posts: 27
Affiliation: McGill University

PostPosted: September 10 2005  Reply with quote

Quote:
Hi, I thought that Chris Vale was assuming no instrinsic fluctuations, that the lensed dipole was only due to the observer's and the lens' motions. Then in the CMB frame there is only a monopole , so doesn't that mean that the effect is inexistant (in the CMB frame)?


In the CMB rest frame you get differential redshifts from the moving potential. It is easiest to see why if the potential is moving transverse to the line of sight. One way to think about this is that photons on the upstream side of the motion fall into a shallower potential than they climb out of, giving a net redshift, while the ones that are "behind" the motion get a net blueshift. Certainly the bending angle is irrelevant if you only have an intrinsic monopole, but net redshifts and blueshifts are relevant.

This is purely from the monopole, so an observer at rest in the CMB frame will see this effect. The motion of the observer (as Martin shows explicitly) relative to the CMB frame just gives a slight distortion to the whole pattern at the level of v/c, which is negligible for these purposes. The observer's motion is largely irrelevant here, other than to generate our observed dipole (and a small bit of the quadrupole).
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Mike Hudson



Joined: 09 Sep 2005
Posts: 6
Affiliation: U Waterloo

PostPosted: September 12 2005  Reply with quote

Hakon Dahle wrote:
Quote:

The studies in astro-ph/0412329 and astro-ph/0403275 (using type Ia supernovae and X-ray clusters, respectively) seem to support the basic picture needed – the Great Attractor having a significant flow into the more distant Shapley concentration.


As coauthor of one of the above-mentioned studies, I can confirm this. It is likely that the Great Attractor is moving at 200−300 km/s towards l=300, b=10 approximately. Therefore it should see a significant dipole in the CMB. However its mass is close to the Tonry et al. estimate, which is much lower than what Chris assumed.

Of course, there are other more distant attractors. As noted by others, one such is the Shapley Concentration. Its mass is likely to be in the range of a few times 1016, but it is ~3x farther away and ~3x the size, hence subtends a similar angle as the GA. Is Shapley moving? Probably. Generically in a LambdaCDM Universe bulk flows are a very slowly decreasing function of scale: spheres of 100 Mpc have an rms bulk flow of ~a few hundred km/s.

For a fairly recent summary of large-scale flows see astro-ph/0404386
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Chris Vale



Joined: 06 Sep 2005
Posts: 5
Affiliation: Fermilab

PostPosted: October 07 2005  Reply with quote

I've posted version 2 of the Pancake paper, it should be out on astro-ph on Monday (but I'm happy to email it early to anyone who's interested). The basic conclusions of the original are unchanged, as I *think* I'm able to show pretty concvincingly that gravitational interactions (whether you prefer to think of them as lensing or ISW, either way is fine with me... after all, they are related by the equivalence principal) cannot be safely ingnored, and indeed might be the whole enchilada.

Asantha and I had a nice chat after the original, and we each set about calculating the effect from structures related to the local flow - he comes to a rather different conclusion astro-ph/0510137 - but I think he may have neglected the most important bit. The key idea is that the velocity of local structures must add coherently, and the local flow does precisely this. Most of the local universe is not particularly overdense, but it *is* flowing toward some mass dipole, so that most of the signal actually comes from ubiquitous, ordinary structures. The total mass out to a radius of 2 or 3 hundred Mpc really adds up...
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Anais Rassat



Joined: 20 Oct 2004
Posts: 30
Affiliation: LASTRO (EPFL)

PostPosted: February 01 2006  Reply with quote

Discussed in today's Independent:

http://news.independent.co.uk/world/science_technology/article342351.ece
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