Wednesday, February 6, 2013

Evidence For Only N eff =3 Weakened

Peter Woit, at Not Even Wrong reports:
Resonaances has an excellent posting about the latest WMAP9 CMB measurements, and the value Neff for the number of implied light degrees of freedom. When the WMAP numbers were released late last year, they quoted
Neff=3.89+/-.67, 3.26+/-.35, 2.83+/-.38
for the results of fits to their data and others (see section 4.3.2). Jester described this as “like finding a lump of coal under the Christmas tree”: the value Neff=3 implies no new light degrees of freedom beyond the known 3 light neutrinos. A rumor soon appeared on his blog that this result was in error and would be corrected.
The corrected version is now out, with new results
Neff=3.89+/-.67, 3.84+/-.40, 3.55+/-.49
and a note about the correction: “slight correction to Neff for case with BAO.” which seems reasonable if you regard the difference between finding no unknown degrees of freedom and discovering a new unknown one as “slight”.
His sardonic take on the understated "slight" statement in the correction is on target.  The uncorrected results were discussed at this blog a couple of weeks ago.

The uncorrected data was more consistent with a scenario in which there are only the three light neutrinos, i.e. with no beyond the Standard Model Particles, than it was with any beyond the Standard Model physics, particularly given that we already know from other evidence that the answers must come in discrete integer multiples and that the true value of N eff can't be lower than three.

The corrected WMAP data is more consistent with a fourth light particle in the formative stage of the universe than it is with only the three light neutrinos, e.g., an as yet undiscovered single species of keV mass scale sterile neutrino that could explain dark matter.  In other words, it suggests beyond the Standard Model physics of great cosmological importance.

Just a "slight" difference indeed.

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