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BBN Light-Element Abundances

What you are seeing: the primordial 4^4He mass fraction YpY_p, deuterium-to-hydrogen ratio D/HD/H, and lithium-to-hydrogen ratio 7^7Li/H/H, all as a function of the baryon-to-photon ratio η10=1010nb/nγ\eta_{10} = 10^{10} n_b / n_\gamma.

Figure 1. Primordial light-element abundances vs η10\eta_{10}.
η₁₀6.10

WHAT TO TRY

  • Slide the baryon-to-photon ratio eta_10: the helium fraction Y_p creeps up while deuterium D/H falls steeply, since a denser baryon soup burns more deuterium into helium. The three abundance curves move in opposite senses.
  • Park eta_10 at the Planck value marked on the plots: Y_p and D/H both land on their observed bands, the concordance that pins down the cosmic baryon density independently of the CMB.
  • Read the lithium curve: the predicted 7Li/H sits well above the observed value at the same eta. That stubborn gap is the unsolved primordial-lithium problem.