Abstract
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 impacts 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 present a measurement of interstellar 7Li in the low-metallicity gas of
the Small Magellanic Cloud (SMC), a nearby galaxy with one quarter of the solar
metallicity. The present-day SMC 7Li abundance is nearly equal to the BBN
predictions, severely constraining the amount of post-BBN 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.
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