Using big bang nucleosynthesis in cosmological parameter extraction from the cosmic microwave background: a forecast for PLANCK

Data from future high precision cosmic microwave background measurements will be sensitive to the primordial helium abundance Y-p. At the same time, this parameter can be predicted from big bang nucleosynthesis (BBN) as a function of the baryon and radiation densities, as well as a neutrino chemical potential. We suggest using this information to impose a self-consistent BBN prior on Yp and determining its impact on parameter inference from simulated planck data. We find that this approach can significantly improve bounds on cosmological parameters compared to an analysis which treats Yp as a free parameter, if the neutrino chemical potential is taken to vanish. We demonstrate that. xing the helium fraction at an arbitrary value can seriously bias parameter estimates. Under the assumption of degenerate BBN (i.e., letting the neutrino chemical potential. vary), the BBN prior's constraining power is somewhat weakened, but nevertheless allows us to constrain. with an accuracy that rivals that for bounds inferred from present data on light element abundances.