On the ergoregion instability in rotating gravastars

TL;DR

This paper investigates the ergoregion instability in rotating gravastars, showing that some models remain stable even with high angular momentum, challenging the idea that all ultra-compact rotating objects are black holes.

Contribution

It demonstrates that stable rotating gravastars can exist with J/M^2 near 1, providing new insights into their potential viability despite ergoregion instability concerns.

Findings

Not all rotating gravastars are unstable.

Stable models can have J/M^2 ~ 1.

Ergoregion instability does not rule out gravastars.

Abstract

The ergoregion instability is known to affect very compact objects that rotate very rapidly and do not possess a horizon. We present here a detailed analysis on the relevance of the ergoregion instability for the viability of gravastars. Expanding on some recent results, we show that not all rotating gravastars are unstable. Rather, stable models can be constructed also with J/M^2 ~ 1, where J and M are the angular momentum and mass of the gravastar, respectively. The genesis of gravastars is still highly speculative and fundamentally unclear if not dubious. Yet, their existence cannot be ruled out by invoking the ergoregion instability. For the same reason, not all ultra-compact astrophysical objects rotating with J/M^2 <~ 1 are to be considered necessarily black holes.