  If all three neutrino oscillation data were to be confirmed in the near
future, it is probable that one might need a sterile neutrino, in addition to
the three active ones. This sterile neutrino, nu_S, would be very light with
mass m_{nu_S} less than or equal to 1 eV or even with m_{\nu_S} approximately
10^{-3} eV according to some scenarios. Why would it be so light? On another
front, recent cosmological observations and analyses appear to indicate that
the present universe is flat and accelerating, and that the present energy
density is dominated by a ``dark variety'', with rho_V approximately
(10^{-3}eV)^4. Is it a constant? Is there a link between these apparently
unrelated phenomena?
