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 He-4 and Li-7 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. (C) 1997 Elsevier Science B.V.