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 $\epsilon \approx few \times 10^{-9}$. In the near future, measurements from the Planck mission will either confirm these hints or constrain $\epsilon \stackrel{<}{\sim} 10^{-9}$.