Relativity Seminar

Tue, 05/05/2009 12:00	Pieter Blue (Edinburgh)	Relativity Seminar	L3
	Hidden symmetries and decay for the wave equation outside a Kerr black hole
	The Kerr solutions to Einstein's equations describe rotating black holes. For the wave equation in flat-space and outside the non-rotating, Schwarzschild black holes, one method for proving decay is the vector-field method, which uses the energy-momentum tensor and vector-fields. Outside the Schwarzschild black hole, a key intermediate step in proving decay involved proving a Morawetz estimate using a vector-field which pointed away from the photon sphere, where null geodesics orbit the black hole. Outside the Kerr black hole, the photon orbits have a more complicated structure. By using the hidden symmetry of Kerr, we can replace the Morawetz vector-field by a fifth-order operator which, in an appropriate sense, points away from the photon orbits. This allows us to prove the necessary Morawetz estimate. From this we can prove a decay estimate of almost $t^{-1}$ for fixed $r$ and the corresponding decay rates at the event horizon and null infinity. The major innovation in this result is that, by using the hidden symmetries with the energy-momentum, we can avoid taking Fourier tranforms in time. This is joint work with Lars Andersson.

Tue, 02/06/2009 12:00	Spyridon Alexakis (MIT)	Relativity Seminar	L3
	A black hole uniqueness theorem.
	I will discuss recent joint work with A. Ionescu and S. Klainerman on the black hole uniqueness problem. A classical result of Hawking (building on earlier work of Carter and Robinson) asserts that any vacuum, stationary black hole exterior region must be isometric to the Kerr exterior, under the restrictive assumption that the space-time metric should be analytic in the entire exterior region. We prove that Hawking's theorem remains valid without the assumption of analyticity, for black hole exteriors which are apriori assumed to be "close" to the Kerr exterior solution in a very precise sense. Our method of proof relies on certain geometric Carleman-type estimates for the wave operator.

Tue, 16/06/2009 12:00	Stefan Hollands (Cardiff)	Relativity Seminar	L3
	From the geometry of spacetime to the geometry of numbers
	One of the major open challenges in general relativity is the classification of black hole solutions in higher dimensional theories. I will explain a recent result in this direction in the context of Kaluza-Klein spacetimes admitting a sufficient number N of commuting U(1)-symmetries. It turns out that the black holes in such a theory are characterized by the usual asymptotic charges, together with certain combinatorical data and moduli. The combinatorial data characterize the nature of the U(1)^N-action, and its analysis is closely related to properties of integer lattices and questions in the area of geometric number theory. I will also explain recent results on extremal black holes which show that such objects display remarkable “symmetry enhancement” properties

Forthcoming Seminars