Testing the creation of the universe: The primordial helium abundance

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, Yp, the ratio of He to H by mass, is measured from emission lines that form during the recombination-decay process, He⁺ + e^- → He⁰(nL) + hv followed by radiative cascade. A definitive test of the Big Bang requires an accuracy of 1% or better. Yp 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 coeficients. All previous studies of He I recombination coeficients 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 coeficients 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.