A Big Bang Nucleosynthesis Likelihood Analysis of the Baryon-to-Photon   Ratio and the Number of Light Particle Degrees of Freedom
  We extend the model-independent likelihood analysis of big bang nucleosynthesis (BBN) based on He4 and Li7 to allow for numbers of degrees of freedom which differ from the standard model value characterized by $N_\nu = 3$. We use the two-dimensional likelihood functions to simultaneously constrain the baryon-to-photon ratio and the number of light neutrinos. The upper limit thus obtained is $N_\nu < 4.0$ (at 95% C.L.). We also consider the consequences if recent observations of deuterium in high-redshift QSO absorption-line systems (QSOALS) are confirmed. 
Keith A. Olive and David Thomas