  We analyze the boson mixing in curved spacetime and compute the expectation
value of the energy-momentum tensor of bosons on the flavor vacuum in spatially
flat Friedmann-Lemaitre-Robertson-Walker metrics. We show that the
energy-momentum tensor of the flavor vacuum behaves as the effective
energy-momentum tensor of a perfect fluid. Assuming a fixed de Sitter
background, we show that the equation of state can assume values in the
interval [-1, 1], and, in the flat spacetime limit has a value -1, which is the
one of the dark energy. The results here presented show that vacuum of mixed
bosons like neutrino super-partners, can represent a possible component of the
dark energy of the Universe.
