On the uniqueness of extremal vacuum black holes

TL;DR

This paper proves the uniqueness of extremal vacuum black holes in four and five dimensions with specific symmetries, establishing conditions under which these solutions are unique and static.

Contribution

It extends black hole uniqueness theorems to extremal cases in higher dimensions with specific symmetries, including necessary and sufficient conditions for staticity.

Findings

Uniqueness of extremal Kerr in 4D for single extremal horizon.

At most one extremal vacuum black hole in 5D with given symmetries and horizon structure.

Conditions for staticity of extremal and non-extremal black holes.

Abstract

We prove uniqueness theorems for asymptotically flat, stationary, extremal, vacuum black hole solutions, in four and five dimensions with one and two commuting rotational Killing fields respectively. As in the non-extremal case, these problems may be cast as boundary value problems on the two dimensional orbit space. We show that the orbit space for solutions with one extremal horizon is homeomorphic to an infinite strip, where the two boundaries correspond to the rotational axes, and the two asymptotic regions correspond to spatial infinity and the near-horizon geometry. In four dimensions this allows us to establish the uniqueness of extremal Kerr amongst asymptotically flat, stationary, rotating, vacuum black holes with a single extremal horizon. In five dimensions we show that there is at most one asymptotically flat, stationary, extremal vacuum black hole with a connected horizon, two commuting rotational symmetries and given interval structure and angular momenta. We also provide necessary and sufficient conditions for four and five dimensional asymptotically flat vacuum black holes with the above symmetries to be static (valid for extremal, non-extremal and even non-connected horizons).