Observation of interstellar lithium in the low-metallicity Small
  Magellanic Cloud
  The primordial abundances of light elements produced in the standard theory
of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to
photons, a quantity inferred from observations of the microwave background. The
predicted primordial 7Li abundance is four times that measured in the
atmospheres of Galactic halo stars. This discrepancy could be caused by
modification of surface lithium abundances during the stars' lifetimes or by
physics beyond the Standard Model that affects early nucleosynthesis. The
lithium abundance of low-metallicity gas provides an alternative constraint on
the primordial abundance and cosmic evolution of lithium that is not
susceptible to the in situ modifications that may affect stellar atmospheres.
Here we report observations of interstellar 7Li in the low-metallicity gas of
the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's
metallicity. The present-day 7Li abundance of the Small Magellanic Cloud is
nearly equal to the BBN predictions, severely constraining the amount of
possible subsequent enrichment of the gas by stellar and cosmic-ray
nucleosynthesis. Our measurements can be reconciled with standard BBN with an
extremely fine-tuned depletion of stellar Li with metallicity. They are also
consistent with non-standard BBN.
