The positivity of the matter-energy density in the Universe, defined from the energy-momentum tensor as \rho \equiv T00 --- one of the initial assumptions in the positive-energy conjecture of Arnowitt et al. --- is related to the Lorentzian signature of space-time, assumed to be spatially flat, via the Friedmann equation, by applying the Faddeev (Newton--Wigner) propagator K for the cosmological Schrödinger equation in the semi-classical approximation, the corresponding Euclidean propagator, which allows negative \rho , decaying on the Planck time-scale. A corollary of this result is that the masses of all elementary particles, and hence of all astrophysical bodies and black holes, are positive semi-definite.
PACS numbers: 03.65.Sq, 03.75.Dg, 04.60.Gw, 06.30.Dr
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