Having reviewed the basic physics and cosmological evolution underlying BBN and summarized the observational data leading to a set of adopted primordial abundances, the predictions may now be confronted with the data. There are several possible approaches that might be adopted. The following option is chosen here. First, concentrating on the predictions of SBBN, deuterium will be used as the baryometer of choice to fix the baryon-to-photon ratio . This value and its uncertainty are then used to "predict" the 3He, 4He, and 7Li abundances, which are compared to those adopted above. This comparison can provide a test of the consistency of SBBN as well as identify those points of "tension" between theory and observation. This confrontation is carried further to consider the two extensions beyond the standard model [S 1 ( N 0); e 0].
4.1. Testing the Standard Model
For SBBN, the baryon density corresponding to the D abundance adopted here (yD = 2.6 ± 0.4) is 10 = 6.1+0.7-0.5, corresponding to b = 0.022+0.003-0.002. This is in outstanding agreement with the estimate of Spergel et al. (2003), based largely on the new CBR (WMAP) data (Bennett et al. 2003): b = 0.0224 ± 0.0009. For the baryon density determined by D, the SBBN-predicted abundance of 3He is y3 = 1.0 ± 0.1, which is to be compared to the outer-Galaxy abundance of y3 = 1.1 ± 0.1, which is suggested by Bania et al. (2002) to be nearly primordial. Again, the agreement is excellent.
The tension between the data and SBBN arises with 4He. Given the very slow variation of YP with , along with the very high accuracy of the SBBN-predicted abundance, the primordial abundance is tightly constrained: YSBBN = 0.248 ± 0.001. This should be compared with our adopted estimate of Y = 0.238 ± 0.005 (Olive et al. 2000). Agreement is only at the ~ 5% level. This tension is shown in Figure 10. This apparent challenge to SBBN is also an opportunity. As already noted, while the 4He abundance is insensitive to the baryon density, it is very sensitive to new physics (i.e. nonstandard universal expansion rate and/or neutrino degeneracy).
Figure 10. The SBBN-predicted relation between the primordial abundances of D and 4He (mass fraction) is shown by the band, whose thickness represents the uncertainties in the predicted abundances. Also shown by the point and error bars are the adopted primordial abundances of D and 4He (see the text).
There is tension, too, when comparing the SBBN-predicted abundance of 7Li with the (very uncertain) primordial abundance inferred from the data. For SBBN the expected abundance is [Li]P = 2.65+0.09-0.11. This is to be compared with the various estimates above that suggested [Li]P 2.2 ± 0.1. In Figure 11 is shown the analog of Figure 10 for lithium and deuterium. Depending on the assessment of the uncertainty in the primordial abundance inferred from the observational data, the conflict with SBBN may or may not be serious. In contrast to 4He, 7Li is more similar to D (and to 3He) in that its BBN-predicted abundance is relatively insensitive to new physics. As a result, this tension, if it persists, could be a signal of interesting new astrophysics (e.g., have the halo stars depleted or diluted their surface lithium?).
Figure 11. The SBBN-predicted relation between the primordial abundances of D and 7Li is shown by the band, whose thickness reflects the uncertainties in the predicted abundances. The data points are for the primordial abundance of D adopted here along with the Ryan et al. (2000) Li abundance (filled circle) and the Pinsonneault et al. (2002) Li abundance (filled triangle).