Ergoregion instability of ultracompact astrophysical objects

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.