Gravastars must have anisotropic pressures

@article{Cattoen2005GravastarsMH,
  title={Gravastars must have anisotropic pressures},
  author={C'eline Cattoen and Tristan Faber and Matt Visser},
  journal={Classical and Quantum Gravity},
  year={2005},
  volume={22},
  pages={4189 - 4202}
}
One of the very small number of serious alternatives to the usual concept of an astrophysical black hole is the ‘gravastar’ model developed by Mazur and Mottola, and a related phase-transition model due to Laughlin et al. We consider a generalized class of similar models that exhibit continuous pressure—without the presence of infinitesimally thin shells. By considering the usual TOV equation for static solutions with negative central pressure, we find that gravastars cannot be perfect fluids… 

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