Gravitational wave signatures of the absence of an event horizon. I. Nonradial oscillations of a thin-shell gravastar

TL;DR

This paper investigates how gravitational wave signals from ultra-compact objects like gravastars differ from black holes, focusing on their oscillation modes to identify potential observational signatures of the absence of an event horizon.

Contribution

It introduces a thin-shell gravastar model and analyzes its nonradial oscillation modes, revealing distinct quasinormal mode spectra from black holes, which could help differentiate these objects observationally.

Findings

Quasinormal mode spectra differ significantly from black holes.

Polar modes depend on the shell's equation of state.

Low-compactness gravastars may be unstable with high sound speed.

Abstract

Gravitational waves from compact objects provide information about their structure, probing deep into strong-gravity regions. Here we illustrate how the presence or absence of an event horizon can produce qualitative differences in the gravitational waves emitted by ultra-compact objects. In order to set up a straw-man ultra-compact object with no event horizon, but which is otherwise almost identical to a black hole, we consider a nonrotating thin-shell model inspired by Mazur and Mottola's gravastar, which has a Schwarzschild exterior, a de Sitter interior and an infinitely thin shell with finite tension separating the two regions. As viewed from the external space-time, the shell can be located arbitrarily close to the Schwarzschild radius, so a gravastar might seem indistinguishable from a black hole when tests are only performed on its external metric. We study the linearized dynamics of the system, and in particular the junction conditions connecting internal and external gravitational perturbations. As a first application of the formalism we compute polar and axial oscillation modes of a thin-shell gravastar. We show that the quasinormal mode spectrum is completely different from that of a black hole, even in the limit when the surface redshift becomes infinite. Polar QNMs depend on the equation of state of matter on the shell and can be used to distinguish between different gravastar models. Our calculations suggest that low-compactness gravastars could be unstable when the sound speed on the shell vs/c>0.92.