The final abundance should also depend on nB, the number density of baryons (particles such as neutrons and protons that participate in nuclear reactions) and ny, the number density of photons.

Even though each of these densities decreases as the universe expands, they each go as 1/R3, so their ratio doesn't change. (Remember the num-

Fig 21.17.

How the abundances of certain light elements depend on the density of the universe. On the horizontal axis, we plot the density of baryons (particles which can undergo nuclear reactions), expressed as the baryon density relative to the closure density. On the vertical axis, we plot the abundances of the various isotopes.The shaded area represents the best estimate of density from the abundance data. [© Edward L.Wright, used with permission]

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