Neutrinos, WMAP, and BBN
  New data from WMAP have appeared, related to both the fractional energy
density in relativistic species at decoupling and also the primordial helium
abundance, at the same time as other independent observational estimates
suggest a higher value of the latter than previously estimated. All the data
are consistent with the possibility that the effective number of relativistic
species in the radiation gas at the time of Big Bang Nucleosynthesis may exceed
the value of 3, as expected from a CP-symmetric population of the known
neutrino species. Here we explore the possibility that new neutrino physics
accounts for such an excess. We explore different realizations, including
neutrino asymmetry and new neutrino species, as well as their combination, and
describe how existing constraints on neutrino physics would need to be relaxed
as a result of the new data, as well as possible experimental tests of these
possibilities.
