9 Year WMAP Data Favors Three Neutrino Species

The cosmic background radiation of the universe is measured by an experiment called WMAP which has just published its ninth year of data.  The big news is that it completely consistent with the three generations of neutrinos in the Standard Model of Particle Physics (within about two-thirds of a single standard deviation), and is inconsistent at a roughly 4.97 sigma level (roughly the threshold for scientific discovery in particle physics) with more than one weakly interacting light non-Standard Model particle (e.g. with more than one species of sterile neutrino).

[C]ompared to the 7-years WMAP data, the update typically brings a 20-30% reduction of already tiny errors on the composition of the Universe. There is however one number that changed visibly. The effective number of relativistic degrees of freedom at the time of CMB decoupling, the so-called Neff parameter, is now Neff = 3.26 ± 0.35, compared to Neff = 4.34 ± 0.87 quoted in the 7-years analysis. For the fans and groupies of this observable it was like finding a lump of coal under the christmas tree...  

So, what is this mysterious Neff parameter? According to the standard cosmological model, at the temperatures above 10 000 Kelvin the energy density of the universe was dominated by a plasma made of neutrinos (40%) and photons (60%). The photons today make the CMB about which we know everything. The neutrinos should also be around, but for the moment we cannot study them directly. However we can indirectly infer their presence in the early universe via other observables. First of all, the neutrinos affect the energy density stored in radiation:

which controls the expansion of the Universe during the epoch of radiation domination. The standard model predicts Neff equal to the number of known neutrinos species, that is Neff = 3 (in reality 3.05, due to finite temperature and decoupling effects). Thus, by measuring how quickly the early Universe was expanding, we can determine Neff. If we find Neff ≈ 3 we confirm the standard model and close the store. On the other hand, if we measured that Neff is significantly larger than 3, that would mean a discovery of additional light degrees of freedom in the early plasma that are unaccounted for in the standard model. Note that these new hypothetical particles don't have to be similar to neutrinos, in particular they could be bosons, and/or have a different temperature (in which case they would correspond to non-integer increase of Neff ). All that is required from them is that they are weakly interacting and light enough to be relativistic at the time of CMB decoupling [ed. about 1 MeV in mass or less]. 

The irony is rich. 

On one hand, the results of the nine year data are a very good fit to the lambda cold dark matter model tuned to fit the previous 7 year WMAP data release. 

On the other hand, the WMAP data have ruled out the possibility that there is more than one species of cold dark matter particle and a model with no weakly interacting light particles beyond the Standard Model is disfavored over a model with only Standard Model particles by a ratio of about 4-1, although they aren't entirely excluded.

Data from other sources are consistent with a Neff (effective number of neutrino species) of 3-5.  Collider data (such as LEP) strongly support just three neutrino generations and so do neurino oscillation measurement, which just a year or two ago might have supported four or five kinds of neutrinos.

UPDATE of February 6, 2013: Corrected data largely supersede the results above as discuss here.