New results , namely the independent determination of the deuterium abundance in several quasar absorption systems , and the complementary determination of the cosmological baryon density by observations of anisotropies in the cosmic microwave background ( CMB ) , allow for a reevaluation of the constraints on the relativistic particle content of the universe at primordial nucleosynthesis .
Expressed in terms of the neutrino energy density , we find 1.7 < N _ { \nu } < 3.5 ( 95 \% ~ { } CL ) .
In particular , we show that phenomenological four neutrino models including a sterile state ( not participating in SU ( 2 ) _ { L } \times U ( 1 ) _ { Y } interactions ) unavoidably thermalize a fourth neutrino , and are highly disfavored in the standard minimal model of primordial nucleosynthesis , if the systematic uncertainty in the primordial helium abundance is small .
We describe plausible extensions of the minimal model which evade this constraint .