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 ^{3}He suggest that the tension with ^{4}He, 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, ^{3}He, and ^{7}Li are less so). A faster expansion (S > 1, N_{} ;> 0) leads to a higher predicted primordial abundance of ^{4}He, 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 ^{4}He (see Fig. 2). Given the similar effects of N_{} 0 on the BBN-predicted D, ^{3}He, and ^{7}Li abundances, while it is possible to maintain the good agreement (from SBBN) for ^{3}He, the tension between ^{7}Li 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 ^{4}He 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). |