the unique supergravity that can be constructed in the highest dimension in which a consistent supergravity can be constructed. It can accommodate the bosonic part of the Standard Model with minimal supersymmetry. However, these theories are anomalous and it is impossible to obtain the chiral structure of the Standard Model by compactification on *smooth* manifolds [@Witten:1983ux]. The vacuum of these theories was arbitrarily chosen to recover the Standard Model. The arbitrariness in the choice of vacuum replaces that of the choice of Higgs field and potential (and gauge interactions, dimensionality...). This makes these theories, conceptually, far superior, but raises to a very prominent place the vacuum selection problem.

    These problems and the advent of String Theory, in particular the Heterotic Superstring [@Gross:1985fr], which is anomaly-free and has chiral fermions, killed these theories, although they have been resurrected again by the same theory that killed them.

10. [**Superstring Theories**]{}

    In these theories, all quantum particles are different vibration states of a single physical entity: the superstring. All known interactions could be described in this way. At low energies, one recovers an anomaly-free supergravity theory. However, there are still some problems:

    -   They are 10-dimensional, and require compactification. At low energies we 