Solar neutrino spectrum, sterile neutrinos, and additional radiation in the Universe

Recent results from the SNO, Super-Kamiokande, and Borexino experiments do not show the expected upturn of the energy spectrum of events (the ratio RN₎₁ₒ/Nₒₒ₌) at low energies. At the same time, cosmological observations testify for the possible existence of additional relativistic degrees of freedom in the early Universe: N₄₅₅=1--2. These facts strengthen the case of a very light sterile neutrino, ₒ, with m₀₁^2 (0. 7--2) 10^-5 eV^2, which mixes weakly with the active neutrinos. The ₒ mixing in the mass eigenstate ₁ characterized by sin^2210^-3 can explain an absence of the upturn. The mixing of ₒ in the eigenstate ₃ with sin^20. 1 leads to production of ₒ via oscillations in the Universe and to additional contribution N₄₅₅0. 7--1 before the big bang nucleosynthesis and later. Such a mixing can be tested in forthcoming experiments with the atmospheric neutrinos, as well as in future accelerator long baseline experiments. It has substantial impact on conversion of the supernova neutrinos.