Mirror dark matter cosmology - predictions for $N_{eff}[CMB]$ and $N_{eff}[BBN]$

Mirror dark matter interacting with ordinary matter via photon-mirror photon kinetic mixing can explain the DAMA, CoGeNT and CRESSTII direct detection experiments. This explanation requires kinetic mixing of strength $\epsilon \sim 10^{-9}$. Such kinetic mixing will have important implications for early Universe cosmology. We calculate the additional relativistic energy density at recombination, $\delta N_{eff}[CMB]$. We also calculate the effects for big bang nucleosynthesis, $\delta N_{eff}[BBN]$. Current hints that both $\delta N_{eff} [CMB]$and$\delta N_{eff} [BBN]$ are non-zero and positive can be accommodated within this framework if $ϵ\approx few\times {10}^{-9}$. In the near future, measurements from the Planck mission will either confirm these hints or constrain $ϵ{10}^{-9}$.