CMB Hemisphere of Evil: Planck Asymmetry Examined

[Chronos]

This paper critically examines the 'hemisphere of evil' CMB asymmetry reported by Planck - CMB Aberration and Doppler Effects as a Source of Hemispherical Asymmetries, http://arxiv.org/abs/1304.3506. Its worth a look if you find this aspect of Planck vaguely disturbing [as I did].

 

[Mordred]

That is an interesting paper, its a good possibility from this papers view at least. Thanks for posting it.
Considering how sensitive and how slight CMB measurements are. Any slight error can corrupt the data

 

[Force1]

Needless to say, I'm a layman science enthusiast. I'm wondering about the peculiar velocity of our local group of galaxies, in regard to
dipole modulation, hemispherical asymmetry, and/or something called primordial dipole? A recent paper, http://arxiv.org/abs/1312.0275
says that no evidence of dipole modulation was found.

Does that mean that the dipole asymmetry can be explained away? Is dipole modulation the same thing as hemispherical asymmetry, and if the findings of no dipole modulation are correct, does that mean that there is no hemispherical asymmetry after proper corrections to the data?

Here is the abstract:
Several statistical anomalies in the CMB temperature anisotropies seem to defy the assumption of a homogeneous and isotropic universe. In particular, a dipole modulation has been detected both in WMAP and Planck data. We adapt the methodology proposed by Eriksen et al. (2007) on CMB data to galaxy surveys, tracing the large-scale structure. We analyse the NRAO VLA Sky Survey (NVSS) data at a resolution of ~2 degrees for three different flux thresholds: 2.5, 5.0 and 10.0 mJy respectively. No evidence of a dipole modulation is found. This result suggests that the origin of the dipole asymmetry found in the CMB cannot be assigned to secondary anisotropies produced at redshifts around z = 1. However, it could still have been generated at redshifts higher or lower, such as the integrated Sachs-Wolfe effect produced by the local structures. Other all-sky surveys, like the infrared WISE catalogue, could help to explore with a high sensitivity a redshift interval closer than the one probed with NVSS.

 

[Force1]

I noticed a post by Marcus here that said that the simplest explanation for the dipole is our motion through the CMB. Is that the whole story, or is there current updates that suggest that the dipole is something real, not fully explained by our motion?

 

[bapowell]

It is not about the dipole. Part of what has been seen over the years is a curious alignment of the higher-order multipoles.

 

[Force1]

It is not about the dipole. Part of what has been seen over the years is a curious alignment of the higher-order multipoles.

Thank you. As a layman, and in regard to a lot of layman level research in the last couple of days, from the papers I am reading, there are several avenues under study. Of course there is the peculiar velocity, and how to quantify it relative to the rest frame of the CMB, there is the masking used to mask out things like the plane of the Milky Way and large sturcture, dust, etc, and there is the issue of the small angle anisotropy that is disproportionately affected by the sharp edges of the masking. When all of these things are quantified, after a series of "if this, then that", all of the dipole asymmetry can be explained with, they say, 95% confidence. I'm still working on getting the latest consensus, but my guess is that there is more data that has to be analyzed, and future sky surveys that might bring new data, so I will keep looking over time.

Just a question about the mulitpoles, is the asymmetry just as easily removed at smaller angles?

 

[Force1]

Thank you. As a layman, and in regard to a lot of layman level research in the last couple of days, from the papers I am reading, there are several avenues under study. Of course there is the peculiar velocity, and how to quantify it relative to the rest frame of the CMB, there is the masking used to mask out things like the plane of the Milky Way and large sturcture, dust, etc, and there is the issue of the small angle anisotropy that is disproportionately affected by the sharp edges of the masking. When all of these things are quantified, after a series of "if this, then that", all of the dipole asymmetry can be explained with, they say, 95% confidence. I'm still working on getting the latest consensus, but my guess is that there is more data that has to be analyzed, and future sky surveys that might bring new data, so I will keep looking over time.

Just a question about the mulitpoles, is the asymmetry just as easily removed at smaller angles?

I should have included a link to this paper in my last post:
http://arxiv.org/pdf/1307.6069.pdf

 

[Force1]

Comment about a movie, mentioned in a recent post that I was notified of via email, and that has been properly deleted, along with its originators account:

My view is that some physicists, and participants in the trailer to said film, which I will not name, were not briefed on the whole project and on how the recent findings in the Planck news release (May 2013), were going to be characterized by the producers of the movie. That was just wrong, and for someone to post a reference here in response to my post is a violation of the clearly expressed forum rules and expressed standards, and for the record, offensive to me.

I'm going to abandon my efforts to get responses here, and put off my intention to better understand the data and any implications of vague anomalous residual findings in the data, until fully vetted credible material and peer review of the many papers on the Planck results are forthcoming.

 

[TEFLing]

It is not about the dipole. Part of what has been seen over the years is a curious alignment of the higher-order multipoles.

That seems very interesting and potentially important

Would you elaborate ?

 

[bapowell]

That seems very interesting and potentially important

Would you elaborate ?

There are several curious statistical anomalies in the CMB temperature spectrum which have been explored over the last decade or so in many, many papers. The best place to go for an update is probably the 2013 Planck release: http://www.aanda.org/articles/aa/abs/2014/11/aa21534-13/aa21534-13.html.

 

[TEFLing]

So the l=600 and l=900 modes, corresponding to angular sizes of 1/3 - 1/4 degree, show excess mutually correlated power?

 

[TEFLing]

There are several curious statistical anomalies in the CMB temperature spectrum which have been explored over the last decade or so in many, many papers. The best place to go for an update is probably the 2013 Planck release: http://www.aanda.org/articles/aa/abs/2014/11/aa21534-13/aa21534-13.html.

If space is closed
And if the Hubble distance were about pi x current scale factor

Then the CMB from opposite sides of the sky
Could be coming from the opposite sides of the same primordial plasma fluctuations

Is that the kind of speculation consistent with the apparent anomalies' alignments?

 

[bapowell]

Is that the kind of speculation consistent with the apparent anomalies' alignments?

No, because the alignment involves mutlipoles corresponding to sub-horizon fluctuation modes.

 

[Force1]

No, because the alignment involves mutlipoles corresponding to sub-horizon fluctuation modes.

Thanks for that terminology.
Dipole notes

A Google search on sub-horizon fluctuation modes returned this paper among others. While reading it from a layman perspective I noticed section V., and how it addresses the monopole and dipole anisotropy.

http://cosmologist.info/teaching/EU/notes_structure.pdf

Refer to Section V., page 15-16, and figure 4
The story is that, if all the photons came from a single, nearly-spherical surface ...
Please be sure to go to the linked page and look at figure 4 on page 16. I think they are starting with the premise that there was no anisotropy at the point of recombination, and so the dipole hemispherical asymmetry came about in the way the photons scattered, (or something, shrug). I would like to understand their explanation for the dipole. Is it all accounted for our solar system/galaxy motion relative to the rest frame?V. COSMIC MICROWAVE BACKGROUND ANISOTROPIES
Recombination is a relatively sudden event at redshift z ∼ 1100, so we can get a good qualitative handle on what we expect to see in the CMB anisotropies by considering all the CMB photons to come from a single nearly-spherical surface about us, where the universe suddenly transitions from being an opaque plasma to being mostly neutral hydrogen gas.
If we consider a photon propagating in a perturbed universe, integrating the geodesic equation for the conformal Newtonian gauge metric gives the energy today E(η0) in terms at the energy at a (conformal) time η (see the question sheet)

[Missing equation, wouldn't copy over from paper]

to linear order in the perturbations. Here the integral is along the photon path; since the integrand is already first order, the integral can be taken to be along the unperturbed (background, zeroth order) photon path. What we mean by “along the photon path” is that in the integrand Ψ and Φ are evaluated at the position the photon had at time η, i.e. Φ(x, η) = Φ(x0 + (η0 − η)nˆ, η) where x0 is the location of the observer and nˆ is the unit vector in the direction in which we are observing the photon. The energies are those observed by an observer with no peculiar velocity.
At zeroth order this equation agrees with the FRW result, that E ∝ E0/a. The corrections in the perturbed universe are due to the difference in potentials between the point of emission and reception, Ψ(η) − Ψ0 (the net red/blue shift as the photon climbs out/falls into the potential wells at the two points), and an integrated contribution which is call the Integrated Sachs-Wolfe (ISW) effect. This reflects the net blue/red-shift as the photon falls into and falls out of the evolving potentials along the ...

[See figure 4 images on top of page 16]
(almost) uniform 2.726K blackbody
Dipole (local motion)
O(10-5) perturbations (+galaxy)
[Image caption]
FIG. 4: The observed CMB sky: almost uniform, with the monopole subtracted showing the O(10−3) dipole due to local motion, and with both the monopole and dipole subtracted to show the anisotropies (+ foreground contamination from our galaxy).

... line of sight. In a matter dominated universe Φ′ = Ψ′ = 0, so this term vanishes: there is an exact cancellation between the blue and red shifts as the photon falls into and climbs out of the potentials. However since there is dark energy at late time, this term is not zero, with the contribution from z 1 when dark energy becomes important being called the “late-time integrated Sachs-Wolfe effect”. For the CMB, where photons decoupled at recombination, there is also some evolution of the potentials near the start of the photon paths because recombination was not completely matter-dominated (there was still some contribution to the energy density from radiation).

 

[bapowell]

Please be sure to go to the linked page and look at figure 4 on page 16. I think they are starting with the premise that there was no anisotropy at the point of recombination, and so the dipole hemispherical asymmetry came about in the way the photons scattered, (or something, shrug). I would like to understand their explanation for the dipole. Is it all accounted for our solar system/galaxy motion relative to the rest frame?

Yes. The dipole is not primordial, that is, it is not a result of the initial spectrum of density fluctuations allegedly set up by inflation. At the time of recombination, perturbations exist across a range of length scales, all the way up to the size of the horizon at recombination and larger (at least up to scales corresponding to our present-day horizon.

 

[Garth]

There are several curious statistical anomalies in the CMB temperature spectrum which have been explored over the last decade or so in many, many papers. The best place to go for an update is probably the 2013 Planck release: http://www.aanda.org/articles/aa/abs/2014/11/aa21534-13/aa21534-13.html.

Thank you for that link, most readers will have to pay to see that site but you can get the eprint of the article free here: Planck 2013 results. XXIII. Isotropy and statistics of the CMB.

I was particularly intrigued by the conclusion:

Here, it was demonstrated that while the power spectrum determined from the Planck temperature data is extremely consistent with a basic 6-parameter CDM model, the low-` multipoles (` 30) deviate from the best-fit model although at a significance that does not appear to exceed 2.7. However, this is precisely the regime where many of the anomalies presented in this paper seem to manifest themselves, and where qualitatively interesting differences are observed in the power-spectra for two hemispheres defined by the preferred direction for the dipolar power modulation.

and

It may be that the statistical anomalies currently described in this paper are a hint of more profound physical phenomena that are yet to be revealed.

Intriguing!
Garth

 

[bapowell]

And of course the spectrum at the low multipoles where the anomalies are manifested is dominated by the fundamental uncertainty called cosmic variance. This uncertainty historically tempered early claims of a "low quadrupole" as many cosmologists felt the uncertainty was simply too large on these scales to take the low quadrupole seriously. I'm not sure how much of a role cosmic variance is playing in these latest measurements.