Massive and Massless Black Holes

TL;DR

This paper classifies extreme black-hole solutions in N=8 supergravity, identifying possible parameters, unique solutions under dualities, and exploring massless black holes as composite objects or alternative vacua.

Contribution

It provides a complete classification of certain extreme black holes in supergravity and discusses the nature of massless black holes as potential composite states or vacua.

Findings

Unique solutions for each parameter value under U duality

Existence of a non-supersymmetric Reissner-Nordström black hole at a=0

Massless black holes may be composite objects or alternative vacua

Abstract

We review some recent work on the existence and classification of extreme black-hole-type solutions in N=8 supergravity. For the black holes considered (those that are also solutions of N=4 supergravity and of the Einstein-Maxwell-dilaton theory with coupling a) a complete classification is achieved: the only possible values of a are \sqrt{3},1,1/\sqrt{3},0. Up to U duality transformations there is only one solution for each of those values. The exception is a=0 for which an additional extreme but non-supersymmetric Reissner-Nordstr\"om black hole solution exists. We also study the so-called massless black-hole solutions. We argue that they can be understood as composite objects. At least one of the components would have ``negative mass''. We also argue that these states, being annihilated by all the generators in the supersymmetry algebra, could also constitute alternative vacua of the supergravity theory.