Stargazing New cosmic model parameters from South Pole Telescope

[TrickyDicky]

The scientific method has the goal of finding the simplest model that supports all available data.

Usually it is stated the other way around, from the hypothetical models we look for the observational data that leads us to either reject the hypothesis or maintain it.

If we choose the parameters correctly, setting a parameter to zero makes the model simpler.

That is called tweaking (finetuning) parameters to simplify the models or to adjust them to a previous prejudice, but I'm not sure that is exactly what the scientific method is about. It is no doubt a practical way to manage data.

 

[GeorgeDishman]

==quote post#1==
http://arxiv.org/pdf/1210.7231v1.pdf
Scroll to Table 3 on page 12 and look at the rightmost column which combines the most data:

Ω[SUB]Λ[/SUB]     0.7152 ± 0.0098
H[SUB]0[/SUB]     69.62 ± 0.79
σ[SUB]8 [/SUB]    0.823 ± 0.015
z[SUB]EQ[/SUB]    3301 ± 47

Perhaps the most remarkable thing is the tilt towards positive overall curvature, corresponding to a negative value of Ω_k

For that, see equation (21) on page 14
Ω_k =−0.0059±0.0040.
Basically they are saying that with high probability you are looking at a spatial finite slight positive curvature.
==endquote==
And of course that particular study, the latest SPT, could have something wrong with it!

I think it is important also to read the associated paper by Hou et al:

http://arxiv.org/abs/1212.6267

"These extensions have similar observational consequences and are partially degenerate when considered simultaneously. These degeneracies can weaken or enhance the apparent deviation of any single extension from the CDM model. Of the 6 one-parameter model extensions considered, we find the CMB data to have the largest statistical preference for running with -0.046 < dn_s / d ln k < -0.003 at 95% confidence. This preference for dn_s / d ln k strengthens to 2.7σ for the combination of CMB+BAO+H₀. Running of this magnitude is difficult to explain in the context of single-field, slow-roll inflation models. When varying the effective number of massless neutrino species, we find N_eff = 3.62 ± 0.48 for the CMB data. Adding H₀ and BAO measurements tightens the constraint to N_eff = 3.71 ± 0.35, 1.9σ above the expected value for three neutrino species. Larger values of N_eff relieve the mild tension between the CMB, H₀, and BAO datasets in CDM."

There is significant evidence from other sources for Ʃm_\nu > 0. The value of N_eff ~ 3.7 is similar to that found by WMAP. The next most favoured seems to be a running spectral index as stated above and after that perhaps a different value of the helium fraction, Yp. Curvature seems to be some way down the list and once the degeneracies are taken into account, the evidence seems to weaken, though that's just my reading of the paper, they don't specifically look at the four-parameter combination of running with Ʃm_\nu>0 and N_eff~3.7 plus a variable curvature.

 

[GeorgeDishman]

I'm just having a first look through the Planck results but two graphs have jumped out at me.

http://arxiv.org/abs/1303.5076

For curvature, look at the top row of figure 21 on page 36.

For the degeneracy of n_s versus N_eff and Y_p see figure 24 on page 39.

There is a complete appendix starting on page 59 which discusses the discrepancy between the Planck results and those of Story and Hou from the SPT in that Planck finds no need for any 'new physics'.

 

[marcus]

Figure 21 on page 36 is nice!

There is also section 6.2.3 Curvature around page 40, where they conclude that in view of equation (68) the universe is spatially flat to within 1%. Nothing new there, the WMAP reports had a similar nearlyflat conclusions, so it is more of a confirmation of what has become accepted wisdom.

Here are equations 68a and 68b that they stress in the concluding paragraph of their section on Curvature:
==quote==
...by the addition of BAO data. We then find
100ΩK = −0.05^+0.65_-0.66 (95%; Planck+WP+highL+BAO), (68a)

100ΩK = −0.10^+0.62_-0.65 (95%; Planck+lensing+WP+highL+BAO). (68b)
==endquote==

Note that the central values are, as usual, negative. They've been coming out mostly negative for years, I guess everyone realizes. For what it's worth.

==quote==
These limits are consistent with (and slightly tighter than) the results reported by Hinshaw et al. (2012) from combining the nine-year WMAP data with high resolution CMB measurements and BAO data.
==endquote==
http://arxiv.org/abs/1303.5076

The idea of near but not necessarily exact flatness is appealing for several reasons, and by now I would imagine it is widely accepted. For one thing it makes calculation easier---you get to use perfect flatness because (without being assured that it is really the case) it is such a good approximation!

People who, for philosophical/aesthetic reasons, prefer to imagine the universe as exactly flat, infinite and containing an infinite amount of matter and energy, can picture it according to their taste. Others, who like an infinite panorama of bubbles, can imagine things according to their taste. And those who for different philosophical/aesthetic reasons, are more comfortable with the large hypersphere picture, can think along those lines with equal justification. The "near flat" conclusion accommodates everybody without prejudice.

The main thing though, to repeat, is that you get to *calculate* using hypothetical exact flatness.

And FWIW in study after study the central values of the Omega_k confidence intervals keep on coming out negative in most cases, particularly before the addition of late-universe observations like BAO (i.e. galaxy counting, census-taking in the more contemporaneous universe), but also as in equations 68a and 68b, FWIW, with the inclusion of BAO data. I don't think we have any idea what that means, if anything. Maybe someone has a guess.

 

[marcus]

Maybe I should mention that early universe observation (like Planck mission) is viewed as the main testing arena in other words a proving ground for Quantum Gravity.
This paper which came out a couple of days ago examples that.

http://arxiv.org/abs/1303.4989
Loop Quantum Gravity and the The Planck Regime of Cosmology
Abhay Ashtekar
(Submitted on 20 Mar 2013)
The very early universe provides the best arena we currently have to test quantum gravity theories. The success of the inflationary paradigm in accounting for the observed inhomogeneities in the cosmic microwave background already illustrates this point to a certain extent because the paradigm is based on quantum field theory on the curved cosmological space-times. However, this analysis excludes the Planck era because the background space-time satisfies Einstein's equations all the way back to the big bang singularity. Using techniques from loop quantum gravity, the paradigm has now been extended to a self-consistent theory from the Planck regime to the onset of inflation, covering some 11 orders of magnitude in curvature. In addition, for a narrow window of initial conditions, there are departures from the standard paradigm, with novel effects, such as a modification of the consistency relation involving the scalar and tensor power spectra and a new source for non-Gaussianities. Thus, the genesis of the large scale structure of the universe can be traced back to quantum gravity fluctuations in the Planck regime. This report provides a bird's eye view of these developments for the general relativity community.
23 pages, 4 figures. Plenary talk at the Conference: Relativity and Gravitation: 100 Years after Einstein in Prague. To appear in the Proceedings to be published by Edition Open Access. Summarizes results that appeared in journal articles [2-13]

According to Ashtekar, LQG can be used to model an era of expansion before inflation in which conditions might arise that affect how it plays out in novel and measurable ways. In papers leading up to this one synonyms like "pre-inflationary era" have been used in place of "Planck regime".

 

[GeorgeDishman]

Maybe I should mention that early universe observation (like Planck mission) is viewed as the main testing arena in other words a proving ground for Quantum Gravity.
This paper which came out a couple of days ago examples that.

http://arxiv.org/abs/1303.4989
Loop Quantum Gravity and the The Planck Regime of Cosmology
Abhay Ashtekar
(Submitted on 20 Mar 2013)

Interesting that he was one of the originators of LQG too.

http://en.wikipedia.org/wiki/Loop_quantum_gravity#History

... In papers leading up to this one synonyms like "pre-inflationary era" have been used in place of "Planck regime".

I always assumed there had to be a gap since the horizon problem is supposedly solved by allowing inflation to expand a region which had reached thermodynamic equilibrium to larger than our horizon. That seems to imply a period before inflation started during which equilibrium could be achieved. "Planck regime" would then be a subset of "pre-inflationary era" with the former ending around 10^-43s and the latter starting around 10^-36s.

Thanks for bringing this up, LQG is something I hadn't look at before, it seems it's going to become more relevant as Planck is reaching the sensitivity where it might become testable.

From your previous message:

Note that the central values are, as usual, negative.

Yeah, by 0.08 sigma