Abstract
Precision measurement of the primordial abundance of the light elements is one of the fundamental tests of the Big Bang. The primordial abundance of helium, Y[p], the ratio of He to H by mass, is measured from emission lines that form during the recombination-decay process, He^+ + e^- -> He^0(nL) + h nu followed by radiative cascade. A definitive test of the Big Bang requires an accuracy of 1% or better. Y[p] is measured from ratios of intensities of He I and H I recombination lines in H II regions. The density ratio n(He^+)/n(H+) is proportional to the observed line intensities and the inverse ratio of recombination coefficients. All previous studies of He I recombination coefficients have assumed that the triplets and singlets are distinct radiative cascade systems that communicate only through electron exchange collisions. Actually, certain large-angular-momentum levels have strongly mixed spin multiplicities, leading to conventional electric-dipole decays that act to mix the singlets and triplets. We present tentative calculations of He I recombination coefficients at a temperature 10,000 K in "Case B". Changes 1% or greater are anticipated, and changes of this magnitude may affect the deduced Y[p] by cosmologically significant amounts.
