Primordial lithium and Big Bang nucleosynthesis

4 problems

• What explains the threefold lithium-7 deficit in old halo stars?

  Open

  Old metal-poor stars in the Milky Way halo show a constant lithium level, but it is about three times below the Big Bang prediction. Either the stars destroyed lithium, or the early universe made less of it.

• Is the primordial helium abundance consistent with standard nucleosynthesis?

  Contested

  The fraction of mass in helium after the Big Bang is predicted to about one part in a thousand. One survey of tiny star-forming galaxies found less helium than predicted, which could point to an asymmetry between neutrinos and antineutrinos.

• Can stellar transport deplete lithium uniformly by a factor three?

  Open

  Inside stars, elements slowly sink and mix, which can carry lithium down to where it burns. The question is whether this happens by the same factor in every old star, as the flat plateau requires.

• What is the lithium abundance in gas never processed by stars?

  Open

  Measuring lithium in interstellar gas of a nearly pristine galaxy would avoid any doubt about stars destroying it. Such measurements are difficult because the lithium absorption line is very weak.