On the Primordial Helium Content: Cosmic Microwave Background and Stellar Constraints

We present the results of a joint investigation aimed at constraining the primordial He content (Y_P) on the basis of both the cosmic microwave background (CMB) anisotropy and two stellar observables, namely, the tip of the red giant branch (TRGB) and the luminosity of the zero-age horizontal branch (ZAHB). Current baryon density estimates based on CMB measurements cover a wide range of values 0.009 ≲ Ω_bh² ≲ 0.045, which according to big bang nucleosynthesis models would imply 0.24 ≲ Y_P ≲ 0.26. We constructed several sets of evolutionary tracks and horizontal-branch (HB) models by adopting Y_P = 0.26 and several metal contents. The comparison between theory and observations suggests that ZAHB magnitudes based on He-enhanced models are 1.5 σ brighter than the empirical ones. The same outcome applies for TRGB bolometric magnitudes. This finding somewhat supports a Y_P abundance close to the canonical 0.23-0.24 value. More quantitative constraints on this parameter are hampered by the fact that the CMB pattern shows a sizable dependence on both Y_P and the baryon density only at small angular scales, i.e., at high l in the power spectrum (l ≳ 100). However, this region of the power spectrum could be still affected by deceptive systematic uncertainties. Finally, we suggest using the UV upturn to estimate the He content on Gpc scales. In fact, we find that a strong increase in Y_P causes a decrease in the UV emission in metal-poor, hot HB structures.