Reexamining the constraint on the helium abundance from the CMB

We revisit the constraint on the primordial helium mass fraction Yₚ from observations of cosmic microwave background (CMB) alone. By minimizing χ² of recent CMB experiments over 6 other cosmological parameters, we obtained rather weak constraints as 0.17≤Yₚ≤0.52 at 1σ C.L. for a particular data set. We also study the future constraint on cosmological parameters when we take account of the prediction of the standard big bang nucleosynthesis (BBN) theory as a prior on the helium mass fraction where Yₚ can be fixed for a given energy density of baryon. We discuss the implications of the prediction of the standard BBN on the analysis of CMB.