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On the study of superradiance in general relativity


In our research project we want to study the propagation of waves in the geometry of a black hole. One particular problem of interest is super-radiance for rotating black holes. This phenomenon arises for fields of integral spin. Such fields admit no positive definite conserved quantity and this makes it possible to extract energy from an annular region around the black hole called ergosphere. The important question is whether fields can extract an infinite amount of energy. A scattering theory would be the most natural and complete answer to this question but the usual techniques do not apply here because the evolution is not unitary on any Hilbert space. A possible path towards the construction of wave operators consists in using a precise integral representation for the propagator.
This program has been achieved in the case of Dirac fields (spin 1/2) by Finster and Batic. We aim to study the scattering operator for the wave equation in the Kerr geometry. The integral representation for the corresponding propagator, recently obtained by Finster et al, combined with spectral methods such as Mourre theory, should allow us to construct wave operators, prove their completeness and obtain propagation estimates. We also would like to study equations of integral spin with richer structures such as Maxwell and Bianchi.
The idea is to study the ``master'' equation derived by Teukolski: a scalar equation depending on the spin s, and that reduces to the wave equation when s equals zero. Integral representations for t he propagators as well as scattering results could be investigated in Schwarzschild black holes first and then in Kerr black holes. An interesting point is to see whether we can obtain scattering results for Maxwell and Bianchi equations from the knowledge of the corresponding results for the Teukolski equation.

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