Perturbative and non-perturbative N=8 supergravity

We study extremal black holes, their ADM mass and area of the horizon in N=8 supergravity. Contrary to intuition gained from N=2,3,4 theories, in N=8, as well as in N=5,6, supergravity BPS states may become massless only at the boundary of moduli space. We show that stringy states described in Green, Ooguri, Schwarz (2007) [10], which have no mass gap and survive in toroidal compactifications in addition to the massless states of perturbative N=8 supergravity, display a null singularity in four-dimensional space-time, when viewed as solutions of the N=8 version of Einstein equations. We analyze known methods of resolving such singularities and explain why they all fail in D=4, N=8 supergravity. We discuss possible implications for the issue of UV finiteness of perturbative N=8 supergravity and the plausibility of a non-perturbative completion that exclude singular states.