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 v _ { s } / c \gtrsim 0.92 .