BBN In Non-Standard Models

As just discussed in 5.6, there is some tension between the SBBN-predicted abundances of D and 4He and their primordial abundances inferred from current observational data (see Fig. 13). Another way to see the challenge is to superpose the data on the BBN predictions from Fig. 2, where the Yp versus D/H relations are shown for several values of Nj, (SSG). This is done in Fig. 15 where it is clear that the data prefer nonstandard BBN, with N closer to 2 than to the standard model value of 3. 

It is easy to understand this result on the basis of the earlier discussion (see 3.3). The adopted abundance of D serves, mainly, to fix the baryon density which, in turn, determines the SBBN-predicted 4 He abundance. The corresponding predicted value of YP is too large when compared to the data. A universe which expands more slowly S 1 Nj, 3) will permit more neutrons to transmute into protons before BBN commences, resulting in a smaller 4He mass fraction. However, there are two problems (at least ) with this solution . The main issue is that there are three flavors of 

FIGURE 14. The band is the SBBN-predicted lithium abundance (by number relative to hydrogen) versus the SBBN-predicted deuterium abundance. The width of the band accounts for the theoretical uncertainties in the SBBN predictions. Also shown is the Pinsonneault et al. (2002 PSWN) primordial lithium abundance estimate adopted here (filled triangle) along with the Ryan et al. (2000 RBOFN) estimate (filled circle). 

There is another alternative to SBBN which, although currently less favored, does have a venerable history BBN in the presence of a background of degenerate neutrinos. First, a brief diversion to provide some perspective. In the very early universe there were a large number of particle-antiparticle pairs of all kinds. As the baryon-antibaryon pairs 

FIGURE 15. The BBN-predicted primordial 4He mass fraction Y as a function of the BBN-predicted primordial Deuterium abundance (by number relative to Hydrogen) for three choices of N),. Also shown by the filled circle and error bars are the primordial abundances adopted here ( 5). 

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