2.2. Deuterium - The Ideal Baryometer

As may be seen in Fig. 1, the deuterium abundance (the ratio, by number, of deuterium to hydrogen: hereinafter, (D/H)_P y_2P) is a monotonic, rapidly decreasing function of the baryon abundance . The reason for this behavior is easily understood. Once BBN begins in earnest, when the temperature drops below ~ 80 keV, D is rapidly burned to ³H, ³He and ⁴He. The higher the baryon abundance, the faster the burning and the less D survives. For ₁₀ in the ``interesting'' range 1 - 10, y_2P decreases with the ~ 1.6 power of . As a result, a 10% error in y_2P corresponds to only a 6% error in . This strong dependence of y_2P on ₁₀, combined with the simplicity of the evolution of D/H in the epochs following BBN, is responsible for the unique role of deuterium as a baryometer [2]. Because almost all the relevant reaction cross sections are measured in the laboratory at energies comparable to those of BBN, the theoretical uncertainties in the BBN-predicted abundance of deuterium is quite small, 8 - 10% for most of the interesting range shown in Fig. 3.

Figure 3. The predicted D/H abundance (solid curve) and the 2 theoretical uncertainty [3]. The horizontal lines show the range indicated by the observational data for both the high D/H (upper two lines ) and low D/H (lower two lines).