4.2. Nonstandard Expansion Rate: S 1 ( N 0)

The excellent agreement between the SBBN-predicted baryon density inferred from the primordial-D abundance and that derived from the CBR and large scale structure (Spergel et al. 2003), and also the agreement between predicted and observed D and ³He suggest that the tension with ⁴He, if not observational or astrophysical in origin, may be a sign of new physics. As noted earlier, Y_P is sensitive to the early-Universe expansion rate (while D, ³He, and ⁷Li are less so). A faster expansion (S > 1, N ;> 0) leads to a higher predicted primordial abundance of ⁴He, and vice versa for S < 1 ( N < 0). In Figure 12 is shown the same Y_P versus y_D band as for SBBN in Figure 10, along with the corresponding bands for the nonstandard cases of a faster expansion ( N = 4) and a slower expansion ( N = 2). It can be seen that the data "prefer" a slower than standard early-Universe expansion rate. If both and N are allowed to be free, it is possible (not surprisingly) to accommodate the adopted primordial abundances of D and ⁴He (see Fig. 2). Given the similar effects of N 0 on the BBN-predicted D, ³He, and ⁷Li abundances, while it is possible to maintain the good agreement (from SBBN) for ³He, the tension between ⁷Li and D cannot be relieved. In Figure 13 are shown the 1-, 2-, and 3- BBN contours in the - N plane derived from the adopted values of y_D and Y_P. Although the best-fit point is at N = -0.7 (and ₁₀ = 5.7), it is clear that SBBN (N = 3) is acceptable.

Figure 12. As in Figure 10 for N = 2, 3, 4, which correspond to S = 0.915, 1, 1.078.

Figure 13. The 1-, 2-, and 3- contours in the - N plane for BBN and the adopted D and 4He abundances.

The CBR temperature anisotropy spectrum and polarization are also sensitive to the early-Universe expansion rate (see, e.g., Barger et al. 2003a, and references therein). There is excellent overlap between the - N confidence contours from BBN as shown in Figure 13 and from the CBR (Barger et al. 2003a). In Figure 14 are shown the confidence contours in the - N plane for a joint BBN - CBR fit (Barger et al. 2003a). Again, while the best fit value for N is negative (driven largely by the adopted value for Y_P), N = 0 is quite acceptable.

Figure 14. The 1- and 2- contours in the - N plane for the joint BBN - CBR (WMAP) fit (Barger et al. 2003a).