Tuesday, March 18, 2014

Scalar-Tensor Ratio From Primordial B Waves Is 0.2 + 0.07/-0.05

A core concept of cosmology is "inflation" which has been a core feature of mainstream cosmology theory for three or four decades.  The notion of inflation is that the universe expanded at a rate faster than the speed of light for a fraction of a second at a time when the entire post-Big Bang universe was smaller than downtown Denver, and that this explains why the universe is unnaturally homogeneous.

But, there are several hundred different varieties of inflation theories out there, only one of which is actually true (if any of them are true).  There are several key parameters that describe the way that inflation unfolded in the first moments after the Big Bang unfolded that allow one to distinguish between many of these several hundred theories.

Planck data and previous cosmic background radiation studies established that one key parameter, which is the exponent describing scale dependence in cosmological evolution, is on the low side of the range between 0.96 and 0.97, where 1.0 is scale invariance.

A second key parameter is called "e-fold" and roughly speaking, measures how long an early phase of the expansion of the universe from the Big Bang called "inflation" lasted - previous data has favored a value for this parameter of somewhere from 50 to 60.

A third key parameter, a measurement of which was announced yesterday, is the scalar-tensor ratio, "r". which can be measured by observing the polarization of the cosmic background radiation. Basically, r is important in distinguishing different proposed version of cosmological inflation or substitutes for it that explain features of the universe that cosmological inflation theories try to explain.

Earlier combined results from data including the preliminary Planck satellite data (without polarization data which will be released later this year), found that the value of r was not inconsistent with zero (with a best fit of about 0.01) and determined that r was less than 0.11 at the two sigma level.

A very low value of r favored the simplest inflation scenarios, such as Higgs inflation, where inflation arises from the evolution of the Higgs field of the Standard Model.

A report released yesterday by the BICEP-2 observatory at the South Pole found that r=0.20 + 0.07/-0.05.

I had predicted the weekend before, based on an analysis of the rumors and the previous science in the field, that the result would be r=0.020+/-0.07, plus or minus 0.01 in either of the numbers.  I was within my margin of error for the average margin of error, and was exactly right except for a difference of two hundredth in the downside error bar.

In particular, this report was based on an observation of "Primordial B waves" at a roughly 4 sigma significance compared to the null hypothesis that this was zero.  The news reports claim this as the first experimental evidence of gravitational waves, which are predicted in general relativity, but this isn't really true.  There was already evidence of gravitational waves from observations of binary star systems.  But, this does confirm the existence of gravitational waves with a second independent data point.

The BICEP-2 figure, taken at face value, naively implies that the energy density of the universe at the time of inflation was on the order of 2*10^16 GeV, which is about the Grand Unified Theory scale at which the coupling constants of SUSY-like theories converge, and about 1% of the Planck scale (this inference is highly model dependent).  The BICEP-2 data is the first experimental evidence for the existence of some key scale at which new physics emerges between the electroweak scale (ca. 200 GeV), and the Planck scale (ca. 10^18 GeV).

Thus, for a particle physicist, the BICEP-2 result favors a conclusion that the Standard Model does not hold without modification all of the way up to the Planck scale, although the first observable deviations from the Standard Model if it is merely a low energy effective field theory of the laws of nature could still be greatly above the scale at which any deviations can be measured in man made laboratories or experiments.

Given the previous data, the best combined fit is now r=0.10 to 0.11, assuming that the BICEP-2 result isn't flawed in methodology in some way, which is an entirely plausible possibility that will look more plausible if it is not confirmed by the Planck polarization data later this year, and several other experiments that will be reporting their results within the next year or two.  Skepticism of the result, in the absence of independent confirmation by another experiment (Jester puts the odds that this result is right at only 50-50) flows from the fact that the value reported is so different from the consensus value from all previous experiments, with the results in a roughly three standard deviation tension with each other.

Any set of the three key parameters (r, scale exponent, and e-folds) that is consistent with both the BICEP-2 data and the previous observational evidence has to fall in a quite narrow range of the parameter space, leaving only a small number of inflation theories in the viable category.

If BICEP-2 is correct, then some of the simplest theories of inflation in the early universe, such as "Higgs inflation" are ruled out, in favor of theories where the topology of the inflating universe is exactly or very nearly flat, rather than concave or convex, and in particular, inflation theories such as "natural inflation."  In a "natural inflation" scenario, the BICEP-2 data also favor a high end result for another result, called the number of "e-folds" during the inflation process of 60 (the low end of the range from other data is about 50).

UPDATE:  A new paper in the wake of BICEP-2 data argues that r=.2 implies that Neff is 4+/-0.41, thus strongly favoring the existence of a light singlet sterile neutrino.


Mitchell said...

I wouldn't give up on Higgs inflation yet.

andrew said...

Is that because you doubt the methodology or analysis in the BICEP data?

Mitchell said...

No, I just think Higgs inflation is flexible.