Ergoregion instability of ultracompact astrophysical objects

@article{Cardoso2008ErgoregionIO,
  title={Ergoregion instability of ultracompact astrophysical objects},
  author={Vitor Cardoso and Paolo Pani and Mariano Cadoni and Marco Cavagli{\`a}},
  journal={Physical Review D},
  year={2008},
  volume={77},
  pages={124044}
}
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 $M=1\ensuremath{-}{10}^{6}{M}_{\ensuremath{\bigodot}}$ and angular momentum $Jg0.4{M}^{2}$. This provides a strong indication that ultracompact objects with large rotation are black… 
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