Measurement of the primordial helium abundance from the intergalactic medium

Almost every helium atom in the Universe was created just a few minutes after the Big Bang through a process commonly referred to as Big Bang nucleosynthesis(1,2). The amount of helium that was made during Big Bang nucleosynthesis is determined by combining particle physics and cosmology(3). The current leading measures of the primordial helium abundance (Y-p) are based on the relative strengths of H-I and He-I emission lines emanating from star-forming regions in local metal-poor galaxies(4-7). As the statistical errors on these measurements improve, it is essential to test for systematics by developing independent techniques. Here we report a determination of the primordial helium abundance based on a near-pristine intergalactic gas cloud that is seen in absorption against the light of a background quasar. This gas cloud, observed when the Universe was just one-third of its present age (z(abs) =1.724), has a metal content around 100 times less than that of the Sun, and at least 30% less metal content than the most metal-poor H-II region currently known where a determination of the primordial helium abundance is possible. We conclude that the helium abundance of this intergalactic gas cloud is Y= 0.250(-0.025)(+0.033) which agrees with the standard model primordial value(8-10), Y-p = 0.24672 +/- 0.00017. Our determination of the primordial helium abundance is not yet as precise as that derived using metal-poor galaxies, but our method has the potential to offer a competitive test of physics beyond the standard model during Big Bang nucleosynthesis.