SEMICLASSICAL STABILITY OF SUPERGRAVITY VACUA

We discuss the existence of instantonic decay modes which would indica te a semiclassical instability of the vacua of 10- and 11-dimensional supergravity theories. Decay modes whose spin structures are incompati ble with those of supersymmetric vacua have previously been constructe d, and we present generalizations including those involving nontrivial dilaton and antisymmetric tensor fields. We then show that the requir ement that any instanton describing supersymmetric vacuum decay should admit both a zero momentum hypersurface from which we describe the su bsequent Lorentzian evolution and a spin structure at infinity compati ble with the putative vacuum excludes all such decay modes, except: th ose with unphysical energy-momentum tensors which violate the dominant energy condition.