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What's the significance of large-scale anomalies in CMB that are confirmed by Planck? I've read somewhere that the cold spots can provide support for string theory or it may be due to a parallel universe . To what extent is that true ? Are there models that can explain these anomalies?

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Yep, it picks me somehow too that this cold spot is still there and I'd like to know what's up with this: Too bad that the polarization results are not yet out ... – Dilaton Mar 28 '13 at 15:35
I think we could be more careful with the term "large-scale anomalies", i.e. the "deficit" at low multipoles. There's a small range of values which show a small tension with the LambdaCDM-prediction, but there's no "honest" tension. If you produce such a plot, it's completely reasonable to have 7-8 consecutive points on the same side of the predicted curve. (It's not OP's fault, Planck themselves stressed that feature of the plot too much.) – Vibert Mar 28 '13 at 22:44
What is CMB? I don't know the term – Bernhard Jul 28 '13 at 6:11

Taking your meaning of "significance" in the statistical sense, the answer is ${\sim}3\sigma$, as said in $\S9.2$ of the Planck overview paper. Of course, they are very wishy-washy as to what this means. If you do a thousand different tests on a given dataset, you expect to find a $3\sigma$ result. On the other hand, maybe they mean there is a $99.7\%$ chance that such anomalies do not arise from the baseline, null model, but in that case the numbers depend very much on what your underlying and alternative models are, neither of which are made terribly precise.

The anomalies, especially the Cold Spot, are discussed further in $\S5.8$ and $\S5.9$ of the 23rd paper in the series. Despite phrases like "these results could have profound implications for cosmology," the numbers are not so fantastic that they can't be explained by a host of ordinary phenomena. The Planck paper cites a number of others that explain these phenomena as being:

  • Diffuse emission from our Solar System, as discussed by Schwarz, Copi, Maris, and Hansen;
  • An artifact of the integrated Sachs-Wolfe effect, as discussed by Francis and Efstathiou; or
  • A true cosmological effect. (The Planck team spends a good deal of time discussing Bianchi $\mathrm{VII}_h$ cosmologies.)

The Cold Spot in particular is reviewed by Vielva. The Planck paper mentions possible explanations include:

  • Details of the evolution of large-scale structure, as discussed by Inoue;
  • The Sunyaev-Zel'dovich effect, as discussed (and discarded) by Cruz;
  • An artifact of foreground subtraction, as discussed (and more or less dismissed) by Cruz;
  • Gravitational lensing, as discussed by Das; and
  • A "cosmic texture," if you believe in such things, as discussed by Cruz once again.

In all honesty, there's no compelling reason at the moment to believe any of these ideas over the simple explanation of "statistical fluctuation of a boring universe." No doubt, though, many more papers will be written over the next couple years on the subject.

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The dismissal of these anomalies seems a bit contrary to what Feynman has suggested is the appropriate attitude of a scientist.

The mutual alignment of quad and octupole is only one aspect.

The alignment of both with the ecliptic and equinoxes is another.

The observed preferred galaxy spin handedness along the same general direction is another.

The galaxy cluster problem along the same general direction is another.

All of these things taken together indicate that the universe is not isotropic and homogeneous, and that we occupy a special location, which is completely heretical of course :-)

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