Reexamining the constraint on the helium abundance from the CMB

Abstract

We revisit the constraint on the primordial helium mass fraction ${Y}_{p}$ from observations of cosmic microwave background (CMB) alone. By minimizing ${\ensuremath{\chi}}^{2}$ of recent CMB experiments over 6 other cosmological parameters, we obtained rather weak constraints as $0.17\ensuremath{\le}{Y}_{p}\ensuremath{\le}0.52$ at $1\ensuremath{\sigma}$ 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 ${Y}_{p}$ 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.