Primordial Lithium Abundance as a Stringent Constraint on the Baryonic Content of the Universe

We have refined the estimate of the primordial level of $^7$Li abundance toan accuracy better than 10%, based on high-precision Li abundances formetal-poor halo stars, and a recent model of post-BBN (Big BangNucleosynthesis) chemical evolution that provides a quantitative explanation ofthe detected gentle ascent of the Spite Plateau for stars with metallicities[Fe/H]$ > -3$. Our maximum likelihood analysis obtains an estimate for theprimordial Li abundance of $A({\rm Li})_p=2.07^{+0.16}_{-0.04}$, after takinginto account possible systematic errors in the estimation of Li abundances,with the exception of a still-controversial issue regarding stellar depletion.The inferred value of $\eta$ (the baryon-to-photon number-density ratio in theuniverse) based on this estimate is more consistent with that derived from theset of reported ``low He'' + ``high D'' from extragalactic sites than thatderived from reported ``high He'' + ``low D'' measurements. Since, withincurrent models of stellar depletion processes, it is difficult to account forthe observed very small scatter of Li abundance in metal-poor stars, ourestimate of $A({\rm Li})_p$ should be taken as an independent constraint on thebaryonic mass density parameter in the universe, giving $\Omega_bh^2=(0.64-1.4)\times 10^{-2}$ with $h=H_0/100$ km s$^{-1}$Mpc$^{-1}$.Comment: 9 pages, 2 figures (1 color figure), ApJ in pres