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

Abstract

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 $ϵ\sim 10^{-9}$. 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 $ϵ\stackrel{<}{\sim }10^{-9}$.

Comments: about 10 pages, matches published version