As the discussion in the previous section should have made clear, the attempts to use a variety of observational data to infer the BBN abundances of the light nuclides is fraught with evolutionary uncertainties and dominated by systematic errors. It may be folly to represent such data by a "best" value along with normally distributed errors. Nonetheless, in the absence of a better alternative, this is what will be done in the following.
From their data along the lines-of-sight to three QSOALS, O'Meara et al. (2001) recommend (D/H)P = 3.0 ± 0.4 × 10-5. While I agree this is likely a good estimate for the central value, the spread among the extant data (see Figures 3 - 5) favors a larger uncertainty. Since D is only destroyed in the post-BBN universe, the solar system and ISM abundances set a floor to the primordial value. Keeping this in mind, I will adopt asymmetric errors (~ 1): (D/H)P = 3.0+1.0-0.5 × 10-5.
In our discussion of 4He as derived from hydrogen and helium recombination lines in low-metallicity, extragalactic HII regions it was noted that the inferred primordial mass fraction varied from YP = 0.234 ± 0.003 (OS/OSS), to YP = 0.238 ± 0.003 (PPL and GSV), to YP = 0.244 ± 0.002 (IT/ITL). Following the recommendation of OSW, here I will choose as a compromise YP = 0.238 ± 0.005.
Here, too, the spread in the level of the "Spite Plateau" dominates the formal errors in the means among the different data sets. To this must be added the uncertainties due to temperature scale and model atmospheres, as well as some allowance for dilution or depletion over the long lifetimes of the metal-poor halo stars. Attempting to accomodate all these sources of systematic uncertainty, I adopt the Pinsonneault et al. (2002) choice of [Li]P = 2.4 ± 0.2.
As discussed earlier, the stellar and Galactic chemical evolution uncertainties afflicting 3He are so large as to render the use of 3He to probe or test BBN problematic; therefore, I will ignore 3He in the subsequent discussion. There are a variety of equally valid approaches to using D, 4He, and 7Li to test and constrain the standard models of cosmology and particle physics (SBBN). In the approach adopted here deuterium will be used to constrain the baryon density ( or, equivalently, B h2). Within SBBN, this leads to predictions of YP and [Li]P. Indeed, once the primordial deuterium abundance is chosen, may be eliminated and both YP and [Li]P predicted directly, thereby testing the consistency of SBBN.