How to tell a gravastar from a black hole (also when it is rotating...)

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Where is an ergoregion possible?For a given pointin the (µ, δ)plane it ispossible todetermine thecritical angularvelocity abovewhich anergoregion ispresent.G*’s with angular velocities smaller than the critical one do nothave an ergoregion and are therefore expected to be stable.WKB analysis or calculation of eigenfrequencies will then establishwhether unstable g*’s are so in a Hubble time...
Conclusionso g*’s are an ingenious solution of the Einstein eqs: can be arbitrarily compact, withouter surface just outside the horizon of BH of same masso Still unclear processes leading to formation of g*’s and if they exist at all. If exist,heat capacity of the surface high enough to avoid heating and black-body emissiono We have constructed a simple but general class of g*’s with the basic features ofthe thin-shell model but allows for a stability analysis.o Two basic questions have two basic answers: g*’s are stable (axial perturbations):can be discerned from a BH through the emission of GWs via QNM oscillationso Rotating g*’s could be subject to an “ergoregion instability” (Cardoso et al. 2007). Sofar, found the space of parameters that allows the existence of an ergoregion.o Interesting new result: an ergoregion is not present for all g*’s and hence somerotating g*’s could be ergoregion-stable.o Future work: compute eigenfrequencies of unstable g*’s and assess if instabilityoccurs on timescales shorter than Hubble time.
Page 10: Axial-Perturbations analysisWe use
Page 14: Rotating gravastarsds 2 = −e ν(r
Page 18 and 19: Size of the ergoregionThe size of t
Page 22: Where is an ergoregion possible?We

How to tell a gravastar from a black hole (also when it is rotating...)

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