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 R equivalent to N(obs)/N(SSM)) at low energies. At the same time, cosmological observations testify for the possible existence of additional relativistic degrees of freedom in the early Universe: Delta N(eff) 1-2. These facts strengthen the case of a very light sterile neutrino, v(s), with Delta m(01)(2) similar to (0.7-2) X 10(-5) eV(2), which mixes weakly with the active neutrinos. The v(s) mixing in the mass eigenstate v(1) characterized by sin(2)2 alpha similar to 10(-3) can explain an absence of the upturn. The mixing of v(s) in the eigenstate v(3) with sin(2)beta similar to 0.1 leads to production of v(s) via oscillations in the Universe and to additional contribution Delta N(eff) approximate to 0.7-1 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.831