The cosmological density of baryons from observations of 3He+ in the Milky Way

Primordial nucleosynthesis after the Big Bang can be constrained by the abundances of the light elements and isotopes (2)H, (3)He, (4)He and (7)Li (ref. 1). The standard theory of stellar evolution predicts that (3)He is also produced by solar-type stars(2), so its abundance is of interest not only for cosmology, but also for understanding stellar evolution and the chemical evolution of the Galaxy. The (3)He abundance in star-forming (H II) regions agrees with the present value for the local interstellar medium(3), but seems to be incompatible(4-6) with the stellar production rates inferred from observations of planetary nebulae(7), which provide a direct test of stellar evolution theory(8). Here we develop our earlier observations(9,10), which, when combined with recent theoretical developments in our understanding of light-element synthesis and destruction in stars(11-14), allow us to determine an upper limit for the primordial abundance of (3)He relative to hydrogen: (3)He/H = (1.1 +/- 0.2) x 10(-5). The primordial density of all baryons determined from the (3)He data is in excellent agreement with the densities calculated from other cosmological probes. The previous conflict is resolved because most solar-mass stars do not produce enough (3)He to enrich the interstellar medium significantly.