SPT-SZ Survey Results: New Neutrino Species Suggested

[Chronos]

Results from the spt-sz survey are in - http://arxiv.org/abs/1212.6267. No particularly shocking revelations, however, the suggestion of new neutrino species is interesting.

 

[marcus]

There was an earlier report in October (Story et al) that discussed the curvature estimate in more detail. This new paper basically just refers the reader to the October one.

==quote http://arxiv.org/pdf/1212.6267v1.pdf page 9==
5. CURVATURE
The SPT+WMAP7 constraint on the mean curvature of the universe has been presented by Story et al. (2012). ...
...
...
...Better constraints on curvature are possible by including low-redshift probes. For instance, CMB+BAO+H₀ leads to a constraint on curvature of Ω_k = −0.0061 ± 0.0040. Even with low-redshift datasets included, SPT data remains important. Without the SPT bandpowers (and lensing information therein), the uncertainty would be roughly 15% larger: Ω_k = −0.0019 ± 0.0047 for WMAP7+BAO+H₀.
==endquote==

So without the new SPT data you have the WMAP7+BAO+H₀ result. Including the SPT data you have what they denote CMB+BAO+H₀ which I highlighted in blue. This bounds the average curvature away from zero (i.e. flat) at 2σ or 95% confidence.

Since they begin by referring to the Story et al paper, which gave a longer more detailed treatment with essentially the same bottom line, I'll quote equation (21) in that Story et al.

==quote http://arxiv.org/pdf/1210.7231.pdf page 14==
The tightest constraint on the mean curvature that we consider comes from combining the CMB, H₀ , and BAO datasets:
Ω_k =−0.0059±0.0040. (21)
==endquote==

So the central value in the new paper, -0.0061, is just slightly more negative than the one in the October paper, -0.0059.

The way Ω_k is defined, a negative value corresponds to positive mean curvature and
Ω_total = 1 - Ω_k
So a central value of -0.006 corresponds to Ω_total = 1.006.

We still can't say anything with much assurance because if you do not include SPT data and make different choice of data set---e.g. just use WMAP, or some other combination--you get a wider errorbar and the central values are all over the place.

It just happens that for some reason the SPT in conjunction with the other main data gives this comparatively tight constraint of ±0.0040 that both papers report, and a positive curvature which translates to a finite spatial volume universe with a finite circumference.

The 95% confidence smallest positive curvature with Ω_k = -0.006 + 0.004 = -0.002
would lead to the largest estimated circumference. Basically 2π time Hubble distance (say 14 Gly) divided by sqrt(.002)
That is, 88 billion lightyears divided by sqrt(.002) = 1968 billion light years.

The 95% confidence largest positive curvature with Ω_k = -0.006 - 0.004 = -0.01
would lead to the smallest estimated circumference. Again 2π time Hubble distance (say 14 Gly) or 88 Gly, but this time divided by sqrt(.01)
That is, 88 billion lightyears divided by sqrt(.01) = 880 billion light years.

So although it's still very much undecided, one of the things about the SPT reports that continues to fascinate me is that it appears to be excluding a spatially flat, spatially infinite universe at 2 sigma or 95% confidence.

Assume that the piece of it we can observe is representative, curvature-wise.

 

[Drakkith]

New neutrino species? Where would it fit in at in the standard model? (Am I understanding that right?)

 

[Chronos]

This is another potential indicator favoring the existence of sterile neutrinos, which could be a significant fraction of dark matter. It would constitute new physics beyond the standard model.

 

[Drakkith]

Nice.