Most of the properties of black holes can be mimicked by horizonless compact objects such as gravastars and boson stars. We show that these ultracompact objects develop a strong ergoregion instability when rapidly spinning. Instability time scales can be of the order of 0.1 seconds to 1 week for objects with mass M=1-106M⊙ and angular momentum J > 0.4M2. This provides a strong indication that ultracompact objects with large rotation are black holes. Explosive events due to ergoregion instability have a well-defined gravitational-wave signature. These events could be detected by next-generation gravitational-wave detectors such as Advanced LIGO or LISA.
V. Cardoso et al., "Ergoregion Instability of Ultracompact Astrophysical Objects," Physical Review D - Particles, Fields, Gravitation and Cosmology, vol. 77, no. 12, American Physical Society (APS), Jun 2008.
The definitive version is available at https://doi.org/10.1103/PhysRevD.77.124044
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