Constraints in the ω_b– plane from Planck and Planck+BAO, compared to helium abundance measurements. Here 68% and 95% contours are plotted for the CMB(+BAO) data combinations when is allowed to vary as an additional parameter to base ΛCDM. The horizontal band shows observational bounds on ⁴He compiled by Aver et al. (2013) with 68% and 95% errors, while the dashed line at the top of the figure delineates the conservative 95% upper bound inferred from the Solar helium abundance by Serenelli & Basu (2010). The green stripe shows the predictions of standard BBN for the primordial abundance of ⁴He as a function of the baryon density. Both BBN predictions and CMB results assume N_eff = 3.046 and no significant lepton asymmetry.