Lithium-6: A Probe of the Early Universe

I consider the synthesis of ${}^6\mathrm{Li}$ due to the decay of relic particles, such as gravitinos or moduli, after the epoch of big bang nucleosynthesis. The synthesized ${}^6\mathrm{Li}/\mathrm{H}$ ratio may be compared to ${}^6\mathrm{Li}/\mathrm{H}$ in metal-poor stars which, in the absence of stellar depletion of ${}^6\mathrm{Li}$, yields significantly stronger constraints on relic particle densities than the usual consideration of overproduction of ${}^3\mathrm{He}$. Production of ${}^6\mathrm{Li}$ during such an era of nonthermal nucleosynthesis may also be regarded as a possible explanation for the relatively high ${}^6\mathrm{Li}/\mathrm{H}$ ratios observed in metal-poor halo stars.