The Cheshire cap

A key role in black hole dynamics is played by the inner horizon; most of the entropy of a slightly nonextremal charged or rotating black hole is carried there, and the covariant entropy bound suggests that the rest ‘floats’ in the region between the inner and outer horizon. An attempt to match this onto results of the microstate geometries program suggests that a ‘Higgs branch’ of underlying long string states of the configuration space realizes the degrees of freedom on the inner horizon, while the geometrical ‘Coulomb branch’ describes the black hole exterior; the inter-horizon region has excitations from both branches. Support for this proposal comes from an analysis of the way singularities develop in microstate geometries, and their close analogy to corresponding structures in fivebrane dynamics. These singularities signal the opening up of the long string degrees of freedom of the theory, which are partly visible from the geometry side. A conjectural picture of the black hole interior is proposed, wherein the long string degrees of freedom resolve the geometrical singularity on the inner horizon, yet are sufficiently nonlocal to communicate information to the outer horizon and beyond