Axion Dark Matter and Cosmological Parameters

We observe that photon cooling after big bang nucleosynthesis but before recombination can remove the conflict between the observed and theoretically predicted value of the primordial abundance of ${}^7\mathrm{Li}$. Such cooling is ordinarily difficult to achieve. However, the recent realization that dark matter axions form a Bose-Einstein condensate provides a possible mechanism because the much colder axions may reach thermal contact with the photons. This proposal predicts a high effective number of neutrinos as measured by the cosmic microwave anisotropy spectrum.

Figure 1

Values of ${\eta }_{10,\mathrm{BBN}}$ inferred from the abundances of ${}^7\mathrm{Li}$, D, ${}^3\mathrm{He}$ and ${}^4\mathrm{He}$, and the predicted values in the standard cosmological model (WIMP) and in our proposal (axion). The data inferred values are taken from Refs. 4, 25, 26. The error bars indicate the ${\eta }_{10,\mathrm{BBN}}$ values consistent with the estimated $1\mathrm{\text{−}}\sigma$ uncertainties in the observations.