Big bang nucleosynthesis with a new neutron lifetime

We show that the predicted primordial helium production is significantly reduced when new measurements of the neutron lifetime and the implied enhancement in the weak reaction rates are included in big-bang nucleosynthesis. Therefore, even if a narrow uncertainty in the observed helium abundance is adopted, this brings the constraint on the baryon-to-photon ratio from BBN and the observed helium into better accord with the independent determination of the baryon content deduced from the WMAP spectrum of power fluctuations in the cosmic microwave background and measurements of primordial deuterium in narrowline quasar absorption systems at high redshift.

Figure 1

Predicted BBN light element abundances vs the baryon-to-photon ratio ${\eta }_{10}$ in units of ${10}^{-10}$. These are compared with the observationally inferred [16] primordial abundances (horizontal lines) and the independent determination of ${\eta }_{10}$ from the WMAP results (light shaded region). The top box shows the primordial helium abundances. The inset shows an expanded view of $Y_p$ near the allowed region. The banded regions indicate the range of predicted $Y_p$ due to the neutron lifetime uncertainty. The upper lines are based upon the previous world average ${\tau }_n=885.7\pm 0.8\mathrm{s}\mathrm{e}\mathrm{c}$. The lower lines are based upon the new measured value of ${\tau }_n=878.5\pm 0.8\mathrm{s}\mathrm{e}\mathrm{c}$. The previously allowed ${\eta }_{10}$ values (shown by the dashed open box) shifts to the dark shaded box if the new neutron lifetime is adopted.