Big Bang Nucleosynthesis and the Helium Isotope Ratio
  The conventional approach to search for departures from the standard model of
physics during Big Bang Nucleosynthesis involves a careful, and subtle
measurement of the mass fraction of baryons consisting of helium. Recent
measurements of this quantity tentatively support new physics beyond the
standard model but, historically, this method has suffered from hidden
systematic uncertainties. In this letter, I show that a combined measurement of
the primordial deuterium abundance and the primordial helium isotope ratio has
the potential to provide a complementary and reliable probe of new physics
beyond the standard model. Using the recent determination of the primordial
deuterium abundance and assuming that the measured pre-solar 3He/4He meteoritic
abundance reflects the primordial value, a bound can be placed on the effective
number of neutrino species, Neff(BBN) = 3.01 (+0.95 -0.76, with 95 per cent
confidence). Although this value of Neff supports the standard model, it is
presently unclear if the pre-solar 3He/4He ratio reflects the primordial value.
New astrophysical measurements of the helium isotope ratio in near-pristine
environments, together with updated calculations and experimental values of
several important nuclear reactions (some of which are already being
attempted), will lead to much improved limits on possible departures from the
standard model. To this end, I describe an analysis strategy to measure the 3He
I flux emitted from nearby low metallicity H II regions. The proposed technique
can be attempted with the next generation of large telescopes, and will be
easier to realize in metal-poor H II regions with quiescent kinematics.
