10. ARE THE DIFFERENT NUCLEI CONCORDANT OR IS THERE A CRISIS?

Nearly everyone believes that the primordial abundances are consistent with BBN (e.g. [153], [1], [11], [7], [72], [154]), but there are many lingering questions about the measurements. The reader will readily detect the two attitudes described by Audouze [54]: ``optimistic'', and ``agnostic and perhaps heretical'' in many papers. Each of us tends to adopt differing attitudes for each nucleus and astrophysical processes. This review favors D/H because it is simple and familiar.

Steigman [109] noted that there was ``a hint of an emerging crisis'' because the <sup>4</sup>He abundances appeared to be lower than expected using the from the other nuclei, but he recommended much more careful study of the uncertainty in BBN predictions, chemical evolution, and observational uncertainties including systematic effects. Hata et al. [155] and Steigman [156] stated that ``there is a conflict'', referring to the differences in implied by low D and low Y_p values.

Whether or not there is a crisis depends on the confidence assigned to the answers to three questions:

• Is primordial D/H low everywhere, or are there also a some places with high values?

• Is Y_p low, high, or uncertain?

• Has ⁷Li in halo stars been depleted by a factor of two?

Some combinations of answers are not consistent with SBBN. Recent data make low D/H seem secure in three quasars plus the ISM, hence the issue is whether there are also other places with high primordial D. Low D/H is compatible with high Y_p and depleted ⁷Li , but not with low Y_p or undepleted ⁷Li . High D is compatible with low Y_p and undepleted ⁷Li , but it is incompatible with the three sites which show low D/H and with Galactic chemical evolution. A factor of ten D depletion would be required in all four places. Low Y_p is compatible with undepleted ⁷Li and high D, but is incompatible with the low D.

A good case has been made for high Y_p , explanations have been given why earlier results gave lower values, and the uncertainty appears to be larger than quoted. Hence D and Y_p are in agreement.

The ⁷Li observed in stars on the Spite plateau is lower than values consistent with low D. Depletion might provide an explanation, but the amount of depletion and the dominant mechanism are not known. The lack of scatter implies little depletion, less than expected, which [113], [49] conclude is not sufficient to match low D. Bonifacio & Molaro [140] find a higher ⁷Li , but still below the level required to match low D without depletion.