Mirror dark matter cosmology – predictions for Neff[CMB] and Neff[BBN]

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