The synthesis of heavier elements started with the formation of deuterium (n+p -> D+γ). Deuterium has a binding energy BD = 2.22MeV, so we might expect that deuterium forms when kT ~ BD, since a typical photon wouldn't be able to dissociate a deuterium nucleus at that temperature. However, there is an enormous number of photons per baryon, so photons on the high-energy tail of the black body distribution are common enough to split deuterium nuclei. A rough (but more accurate) approximation is that deuterium is synthesised when e^(-BD/kBT) ≈ η, where η is the baryon to photon ratio. Using this formula and a baryon density ΩB*h2 = 0.02, estimate the temperature TD and time tD at which deuterium synthesis happens.
