Avoiding BBN constraints on mirror models for sterile neutrinos

We point out that in models that explain the Los Alamos neutrino scintillation detector result for neutrino oscillations using the mirror neutrinos, the big bang nucleosynthesis constraints on the number of sterile neutrinos can be avoided by using the late time phase transition that only helps to mix the active and the sterile neutrinos. The main idea is to have a standard model singlet scalar field that mixes the visible and the mirror sector and has a low vacuum expectation value ($\sim 100\mathrm{k}\mathrm{e}\mathrm{V}$) so that the active and sterile neutrinos remain unmixed at the big bang nucleosynthesis epoch. The model predicts different effective number of neutrinos at the epoch of recombination compared to the standard model and is therefore testable in experiments that measure cosmic microwave spectrum with a higher precision.