Primordial helium (Y_P) and deuterium (y_DP) abundances as a function of baryon density (ω_b), from Planck Collaboration IV (2020), assuming N_eff = 3.046. The shaded, horizontal bands show the 68% CL on the measured values from Aver et al. (2015) and Cooke et al. (2018). The red vertical band is the 68% CL on ω_b from the ΛCDM model fit to the Planck temperature and polarization data, while the other bands show the predictions from the theory of nucleosynthesis (including uncertainties in nuclear reaction rates and neutron lifetime; see Planck Collaboration IV 2020 for more details). The excellent agreement on the inferred value of ω_b from processes in the first 3 mins of the Universe’s history with that from the CMB at 380 000 years after the big bang is one of our best demonstrations of the universality of the laws of physics.