Extremely metal-poor gas at a redshift of 7
  In typical astrophysical environments, the abundance of heavy elements ranges
from 0.001 to 2 times the solar concentration. Lower abundances have been seen
in select stars in the Milky Way's halo and in two quasar absorption systems at
redshift z=3. These are widely interpreted as relics from the early universe,
when all gas possessed a primordial chemistry. Before now there have been no
direct abundance measurements from the first Gyr after the Big Bang, when the
earliest stars began synthesizing elements. Here we report observations of
hydrogen and heavy element absorption in a quasar spectrum at z=7.04, when the
universe was just 772 Myr old (5.6% its present age). We detect a large column
of neutral hydrogen but no corresponding heavy elements, limiting the chemical
abundance to less than 1/10,000 the solar level if the gas is in a
gravitationally bound protogalaxy, or less than 1/1,000 solar if it is diffuse
and unbound. If the absorption is truly intergalactic, it would imply that the
universe was neither ionized by starlight nor chemically enriched in this
neighborhood at z~7. If it is gravitationally bound, the inferred abundance is
too low to promote efficient cooling, and the system would be a viable site to
form the predicted but as-yet unobserved massive population III stars in the
early universe.
