Primordial He-4 as a test of big bang nucleosynthesis

It is discussed how Yp, the observationally inferred primordial He-4 abundance, becomes a stringent test of both the standard homogeneous and inhomogeneous big bang models. It is argued that the standard procedure of extrapolating the straight line fit to the He-4 data to obtain the zero metallicity intercept is inappropriate and places inordinate weight on high-metallicity points. Extrapolating lower metallicity data versus N, Yp = 0.220 + or - 0.010 is obtained. At face value, this result just precludes the standard model with three light neutrinos and places severe constraints on inhomogeneous models. However, systematic errors are potentially large, and therefore no definitive conclusion can be reached at this time. 43 refs.