Acknowledgements

This work benefited greatly from discussions over the years with many colleagues. In particular, we thank E. Berti, R. Brito, J. C. Degollado, R. Emparan, P. Figueras, D. Hilditch, P. Laguna, L. Lehner, D. Mateos, A. Nerozzi, H. Okawa, P. Pani, F. Pretorius, E. Radu, H. Reall, H. Rúnarsson, M. Sampaio, J. Santos, M. Shibata, C. F. Sopuerta, H. Witek and M. Zilhão for very useful comments and suggestions. This work was partially funded by the NRHEP 295189 FP7-PEOPLE-2011-IRSES and PTDC/FIS/116625/2010 grants and the CIDMA strategic project UID/MAT/04106/2013. V.C. acknowledges partial financial support provided under the European Union’s FP7 ERC Starting Grant “The dynamics of black holes: testing the limits of Einstein’s theory” grant agreement no. DyBHo–256667. L.G. acknowledges partial financial support from NewCompStar (COST Action MP1304). C.H. is funded by the FCT-IF programme. U.S. acknowledges support from FP7-PEOPLE-2011-CIG Grant No. 293412 CBHEO, STFC GR Roller Grant No. ST/L000636/1, NSF XSEDE Grant No. PHY-090003 and support by the Cosmos system, part of DiRAC, funded by STFC and BIS under Grant Nos. ST/K00333X/1 and ST/J005673/1. This research was supported in part by Perimeter Institute for Theoretical Physics. Research at Perimeter Institute is supported by the Government of Canada through Industry Canada and by the Province of Ontario through the Ministry of Economic Development.