Bounds on 3+1 active-sterile neutrino oscillations in very low reheating scenarios

In the 3+1 neutrino scheme with an additional state, we consider the thermalisation of neutrinos in the early Universe in the so-called very low reheating scenarios. This process could be incomplete due to the lack of interactions, leading to a reduced contribution of neutrinos to the cosmological energy density of radiation. We calculate this contribution, usually measured in terms of the parameter effective number of neutrino species (N-eff), taking into account the full 4 x 4 neutrino mixing matrix, as a function of the reheating temperature T-rh.