Slow Decay of Waves in Gravitational Solitons

We consider a family of globally stationary (horizonless), asymptotically flat solutions of five-dimensional supergravity. We prove that massless linear scalar waves in such soliton spacetimes cannot have a uniform decay rate faster than inverse logarithmically in time. This slow decay can be attributed to the stable trapping of null geodesics. Our proof uses the construction of quasimodes which are time periodic approximate solutions to the wave equation. The proof is based on previous work to prove an analogous result in Kerr-AdS4$$\text{ Kerr-AdS}_4$$ black holes (Holzegel and Smulevici in Anal PDE 7(5):1057–1090, 2014). We remark that this slow decay is suggestive of an instability at the nonlinear level.