7. ³He

The primordial abundance of ³He has not been measured. This is most unfortunate, since it is nearly as sensitive as D to the baryon density during BBN. ³He is harder to measure than D because the difference in wavelength of ³He and ⁴He lines is smaller than for D, and the Lyman series lines of He II, main absorption lines of He in the IGM, are in the far ultraviolet at 228 - 304 Å which is hard to observe because of absorption in the Lyman continuum of H I at < 912 Å.

Rood, Steigman & Tinsley [127] argued that it was unlikely that ³He could be used to supplement cosmological information from D because low mass stars should make a lot of ³He , increasing the current ISM value to well above that in the pre-solar system ISM, and in potential conflict with observations at that time. This conflict has been confirmed. Measurements do show enhanced ³He in Planetary nebulae, as expected from the production in the associated low mass stars, but this is not reflected in the ISM as a whole. The pre-solar and current ³He abundances are similar [128], in contradiction with expectation [129], [130], for unknown reasons.

It was suggested ([131], see review by Hata et al. [132]) that the uncertainty over the amount of destruction of D could be circumvented using the sum of the abundances of D + ³He , since the destroyed D should become ³He , and ³He is relatively hard to destroy. The primordial D + ³He should then be the same sum observed today, as more ³He is made in stars over time. However, there are two problems with this scenario. First, the ³He should increase over time, which it does not, implying that some stars destroy ³He , and second, the ³He abundance should be about constant in the ISM today, which it appeared not to be in early data [133]. Hence, just prior to the measurement of D in quasars, most concluded that D + ³He in the Galaxy does not provide secure cosmological information [132], and summaries by [45], [10].

Balser et al. [134] report on a 14 year program to measure ³He in the Galactic H II regions. Using models for the gas density structure, they find an average ³He / H = 1.6 ± 0.5 x 10^-5 for a sub-sample of seven simple nebulae. No variation is seen with Oxygen abundance over a factor of ten, and there is little scatter [135]. This value may represent the average in the ISM today, but it is not known how to use this to obtain primordial abundances.

These measurements are relevant to stellar nucleosynthesis and Galactic chemical evolution, and are consistent with a cosmological origin for the ³He , but we suggest that gas with much lower metal abundances will need to be observed to derive a secure primordial abundance for ³He .