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 to an accuracy better than 10%, based on high-precision Li abundances for metal-poor halo stars, and a recent model of post-BBN (Big Bang Nucleosynthesis) chemical evolution that provides a quantitative explanation of the detected gentle ascent of the Spite Plateau for stars with metallicities [Fe/H]$>-3$. Our maximum likelihood analysis obtains an estimate for the primordial Li abundance of $A(\mathrm{L}\mathrm{i}{)}_p=2.07_{-0.04}^{+0.16}$, after taking into 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 the universe) based on this estimate is more consistent with that derived from the set of reported low He'' + high D’’ from extragalactic sites than that derived from reported high He'' + low D’’ measurements. Since, within current models of stellar depletion processes, it is difficult to account for the observed very small scatter of Li abundance in metal-poor stars, our estimate of $A(\mathrm{L}\mathrm{i}{)}_p$ should be taken as an independent constraint on the baryonic mass density parameter in the universe, giving $\Omega_b h^2=(0.64-1.4)\times 10^{-2}$with$h=H_0/100$kms$^{-1}$Mpc$^{-1}$.