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 4He and 7Li to allow for numbers of degrees of freedom which differ from the standard model value characterized by Nv = 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 Nv < 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.