Prof Martin Asplund

During the first ~15min after the Big Bang, the temperatures and densities were high enough to sustain nuclear reactions. During this Big Bang nucleosynthesis the isotopes of H, He and Li were produced in exact proportions dictated by the cosmic baryon-to-photon ratio. Measuring the abundances of these isotopes in the oldest objects in the Universe thus allows to determine the baryon-to-photon ratio which can be compared with alternative methods, most notably using the cosmic microwave background radiation. The agreement is overall very good, except for Li where the oldest stars seem to contain 3-5 times less 7Li than predicted by standard models of Big Bang nucleosynthesis. Furthermore, it has been argued that these stars also contain >10 times more 6Li than expected. Is Big Bang wrong? Or do we not understand stars as well as we thought? Can improved nuclear reaction measurements resolve these cosmological Li problems? Or do we need to invoke the effects of non-standard physics to modify Big Bang nucleosynthesis by e.g. annihilation or decay of supersymmetric particles?