  The abundance of lithium-7 confronts cosmology with a long lasting problem
between the predictions of standard big bang nucleosynthesis and the baryonic
density determined from the cosmic microwave background observations. This
article investigates the influence of the existence of a mirror world, focusing
on models in which neutrons can oscillate into mirror neutrons. Such a
mechanism allows for an effective late time neutron injection, which induces an
increase of the destruction of beryllium-7, due to an increase of the neutron
capture, and then a decrease of the final lithium-7 abundance. Big bang
nucleosynthesis sets constraints on the oscillation time between the two types
of neutron and the possibility for such a mechanism to solve, or alleviate, the
lithium problem is emphasized.
