Gravastars have been recently proposed as potential alternatives to explain the astrophysical phenomenology traditionally associated with black holes, raising the question of whether the two objects can be distinguished at all. Leaving aside the debate about the processes that would lead to the formation of a gravastar and the astronomical evidence in their support, here we address two basic questions: is a gravastar stable against generic perturbations? If it is stable, can an observer distinguish it from a black hole of the same mass? To answer these questions we construct a general class of gravastars and determine the conditions they must satisfy in order to exist as equilibrium solutions of the Einstein equations. For such models we perform a systematic stability analysis against axial perturbations, computing the real and imaginary parts of the eigenfrequencies. Overall, we find that gravastars are stable to axial perturbations, but also that their quasi-normal modes differ from those of a black hole of the same mass and thus can be used to discern, beyond dispute, a gravastar from a black hole.

The interferometric LIGO detectors have recently measured the first direct gravitational-wave signal from what has been interpreted as the inspiral, merger and ringdown of a binary system of black holes. The signalto-noise ratio of the measured signal is large enough to leave little doubt that it does refer to the inspiral of two massive and ultracompact objects, whose merger yields a rotating black hole. Yet, the quality of the data is such that some room is left for alternative interpretations that do not involve black holes, but other objects that, within classical general relativity, can be equally massive and compact, namely, gravastars. We here consider the hypothesis that the merging objects were indeed gravastars and explore whether the merged object could therefore be not a black hole but a rotating gravastar. After comparing the real and imaginary parts of the ringdown signal of GW150914 with the corresponding quantities for a variety of gravastars, and notwithstanding the very limited knowledge of the perturbative response of rotating gravastars, we conclude it is not possible to model the measured ringdown of GW150914 as due to a rotating gravastar.PACS numbers: 04.25. Dm, 04.25.dk, 04.30.Db, 04.40.Dg, 95.30.Lz, 95.30.Sf, 97.60.Jd Introduction. Gravastars were proposed in 2004 by Mazur and Mottola [1] as an ingenious alternative to the end state of stellar evolution for very massive stars, that is, as an alternative to black holes. The name gravastar comes from "gravitational vacuum condensate star" and it was proposed to be almost as compact as a black hole, but without an event horizon or a central singularity. This object would be formed as gravitational collapse brought the stellar radius very close to its Schwarzschild radius and as a phase transition would form a de Sitter core. This "repulsive" core stabilises the collapse, while the baryonic mass ends as a shell of stiff matter surrounding the core. Despite their uncertain and rather exotic origin, gravastars are perfectly acceptable solutions of the Einstein equations within classical general relativity.Considerable effort has been dedicated to study gravastars, for instance exploring different possibilities for its structure [2,3], generalising the solution [4-6] and investigating possible observational signatures [7][8][9]. As alternatives almost indistinguishable from a black hole in terms of electromagnetic radiation, gravastars have attracted the attention of those who wished for a spacetime solution without the issues brought by the existence of singularities and event horizons. Work was also done in order to assess its viability, in particular looking for instabilities in the solutions. Hence, there have been studies on the stability against radial oscillations [2,10] and axial and polar gravitational perturbations [11][12][13]. For slowly rotating gravastars, scalar perturbations in the context of the ergoregion instability were also studied [14,15]. None of these works has pointed out to a response that would allow one to discard...

The increasing richness of data related to cold dense matter, from laboratory experiments to neutron-star observations, requires a framework for constraining the properties of such matter that makes use of all relevant information. Here, we present a rigorous but practical Bayesian approach that can include diverse evidence, such as nuclear data and the inferred masses, radii, tidal deformabilities, moments of inertia, and gravitational binding energies of neutron stars. We emphasize that the full posterior probability distributions of measurements should be used rather than, as is common, imposing a cut on the maximum mass or other quantities. Our method can be used with any parameterization of the equation of state (EOS). We use both a spectral parameterization and a piecewise polytropic parameterization with variable transition densities to illustrate the implications of current measurements and show how future measurements in many domains could improve our understanding of cold catalyzed matter. We find that different types of measurements will play distinct roles in constraining the EOS in different density ranges. For example, better symmetry energy measurements will have a major influence on our understanding of matter somewhat below nuclear saturation density but little influence above that density. In contrast, precise radius measurements or multiple tidal deformability measurements of the quality of those from GW170817 or better will improve our knowledge of the EOS over a broader density range.

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.