Classification of Extremal Black Holes

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General relativity (GR) is our theory to study gravitation. It has led to a very accurate model of our Universe at large scales. Its boldest prediction is the existence of black holes, massive objects that form a region of space from which nothing can escape. It is now accepted that black holes really exist in our Universe - a striking example of how abstract theory can have profound physical significance. Despite its successes, GR has resisted attempts to make it consistent with quantum mechanics, which is equally adept at describing the basic constituents of Nature. Such a theory of `quantum gravity' is needed to answer big questions, like how our Universe formed. Luckily, black holes offer a unique laboratory to study this difficult problem - their gravitational fields are so intense that effects from both quantum mechanics and GR come into play. My research focusses on black holes in higher dimensions. We are all familiar with four dimensions - an event has four labels, three for 'where' and one for `when' it occurred. But string theory, a leading candidate for a theory of quantum gravity, requires more than four dimensions. At large scales, it reduces to GR, impelling us to investigate black holes in dimensions greater than four. Much is known about equilibrium black holes in four dimensions. They are shaped like the surface of a ball, and they are uniquely `labelled' by their mass and spin. These remarkable properties no longer hold in higher dimensions. The recent discovery of black rings - donut shaped black holes - proves not only that many shapes are possible, but that uniqueness is lost. The central goals of my work are to classify black holes in higher dimensions, examine the extent of black hole non-uniqueness, and determine how this issue is resolved within quantum gravity. To achieve these challenging objectives, my strategy is to target a special type of black hole which I have shown are simplest to analyse. My research will contribute significantly to a deeper understanding of quantum gravity, and train students with unique quantitative skills valued in other fields. Like most basic research, it is unlikely to immediately benefit society. But it could help us understand Nature at a fundamental level, which I believe is a worthwhile aim.