Baryon-to-Photon Emergent

The baryon-to-photon ratio η = n_b/n_γ ≈ 6.1 × 10⁻¹⁰ is one of the most precisely determined numbers in cosmology, fixed by the agreement between BBN predictions and primordial light-element abundances, and independently confirmed by the CMB acoustic peak structure. In ΛCDM, η is simply an initial condition: the model takes it as given and makes no attempt to explain why there are roughly one billion photons for every baryon. This ratio is not predicted by any sector of the Standard Model; it is a boundary condition imposed on the cosmological calculation.

The baryon-to-photon ratio encodes the history of baryogenesis — the physical process that generated the matter-antimatter asymmetry in the early universe. ΛCDM has no baryogenesis mechanism; it simply assumes the asymmetry exists as an initial condition. The specific value η ~ 6 × 10⁻¹⁰ must emerge from particle physics processes at temperatures above ~100 GeV, none of which are specified by ΛCDM. A complete cosmological theory should predict η from first principles. That ΛCDM cannot do so — and instead fits it as a free parameter — represents a genuine incompleteness, separate from but related to the baryon asymmetry problem.