Within the last decade, our notions of the cosmos have been radically altered by precision<br /><br />
observations of the light from distant Type Ia supernovae and, separately, the power spectrum of the cosmic microwave background radiation [CMBR]. These show that the expansion of the<br /><br />
universe is actually accelerating, and that the Universe is overwhelmingly composed of dark<br /><br />
energy (75%) and dark matter (21%), with only 4% of the total being baryonic (“normal”) matter.<br /><br />
Determining the amount and characteristics of dark matter, dark energy, and normal matter is one of the most compelling problems in astrophysics today. A complementary and independent<br /><br />
approach to determine the baryonic matter density is to compare the predictions of the<br /><br />
abundances of “primordial” light elements (H, He, Li) formed three minutes after the Big Bang with observations of these elements in the interstellar medium and on the surface of very old stars.<br /><br />
“Big Bang Nucleosynthesis” (BBN) calculations require, as input, thermonuclear reaction rates at the high temperatures characteristic of the early universe. BBN estimates of the 2H, 4He, and 7Li abundances imply a baryonic matter density that, respectively, agrees, marginally agrees, and disagrees with the density from other approaches.<br /><br />
The precision of the BBN constraint on the baryonic matter density depends on the uncertainties<br /><br />
in observational determinations of primordial 2H, 4He, and 7Li abundances, as well as on<br /><br />
uncertainties in BBN abundance predictions derived from input nuclear physics uncertainties [1].<br /><br />
We have performed new Monte Carlo BBN simulations wherein all input reaction rate<br /><br />
uncertainties are reduced to their smallest reasonable limit in order to determine the “ultimate”<br /><br />
precision of the BBN baryonic matter constraint given current observational uncertainties: 21%,<br /><br />
41%, and 39%, using (respectively) 2H, 4He, and 7Li. The uncertainties in abundance<br /><br />
determinations must be significantly reduced to give the BBN constraint a precision approaching that of the 3.5% precision derived from WMAP observations of the CMBR [2]. Comparisons of the constraints from these two complementary methods are important because they can indicate, and constrain, potential new physics.<br /><br />
We have also performed simulations where individual rate uncertainties are systematically<br /><br />
reduced to determine the impact that future nuclear physics measurements would have on the<br /><br />
abundance uncertainties and the baryonic matter constraint. We find that the neutron lifetime and the rates of a number of reactions all deserve further scrutiny. Results of the simulations and their implications for cosmology and for future nuclear physics measurements will be presented. These calculations were all performed with the new online suite of codes freely available at <br /><br />
bigbangonline.org.<br /><br />
[1] M.S. Smith, L.H. Kawano, R.A. Malaney, Astrophys. J. Suppl. 85 (1993) 219.<br /><br />
[2] D.N. Spergel et al., Astrophys. J. Suppl. 170 (2007) 377.<br /><br />
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