Mirror matter is a promising self-collisional dark matter candidate. Here we study the evolution of thermodynamical quantities in the early Universe for temperatures below $\sim 100\mathrm{MeV}$ in presence of a hidden mirror sector with unbroken parity symmetry and only with gravitational interactions. This range of temperatures is interesting for primordial nucleosynthesis analyses, therefore we focus on the temporal evolution of number of degrees of freedom in both sectors. Numerically solving the equations, we obtain the interesting prediction that the effective number of extra-neutrino families raises for decreasing temperatures before and after big bang nucleosynthesis; this could help solving the discrepancy in this number computed at nucleosynthesis and cosmic microwave background formation epochs.

• Received 30 July 2008

DOI:https://doi.org/10.1103/PhysRevD.78.123003