Black hole pairs and supergravity domain walls

We examine the pair creation of black holes in the presence of supergravity domain walls with broken and unbroken supersymmetry. We show that black holes will be nucleated in the presence of non- extreme, repulsive walls which break the supersymmetry, but that as one allows the parameter measuring deviation from extremality to approach zero the rate of creation will be suppressed. In particular, we show that the probability for creation of black holes in the presence of an extreme domain wall is identically zero, even though an extreme vacuum domain wall still has repulsive gravitational energy. This is consistent with the fact that the supersymmetric, extreme domain wall configurations are BPS states and should be stable against quantum corrections. We discuss how these walls arise in string theory, and speculate about what string theory might tell us about such objects.Comment: 21 pages LaTeX, special style files (psfrag.sty, efsf_psfrag.sty, a4local.sty, epsf.tex), minor revisions and amended reference

Nucleating Black Holes via Non-Orientable Instantons

We extend the analysis of black hole pair creation to include non- orientable instantons. We classify these instantons in terms of their fundamental symmetries and orientations. Many of these instantons admit the pin structure which corresponds to the fermions actually observed in nature, and so the natural objection that these manifolds do not admit spin structure may not be relevant. Furthermore, we analyse the thermodynamical properties of non-orientable black holes and find that in the non-extreme case, there are interesting modifications of the usual formulae for temperature and entropy.Comment: 27 pages LaTeX, minor typos are correcte