Tristan Pitre, Berend Schneider, Eric Poisson

We establish the dynamical instability of a static, spherically symmetric, and infinitesimally thin shell in general relativity. The shell is made up of a perfect fluid with a barotropic equation of state, and it produces a Schwarzschild spacetime in its exterior and a Minkowski spacetime in its interior. We reveal the existence of two modes with a purely imaginary frequency, one negative (which describes stable oscillations), the other positive (which describes an exponential growth); these modes occur for all sampled values of the shell's compactness and adiabatic index, and all sampled values of the multipolar order $\ell \geq 2$, in the even-parity sector of the perturbation. All other quasinormal modes describe damped oscillations. This study complements a recent analysis by Yang, Bonga, and Pen, which also concluded in a dynamical instability, but was limited by an eikonal approximation to small angular scales ($\ell \gg 1$); our treatment applies to all angular scales. The eigenvalue problem for the mode frequencies is formulated by introducing a perturbation of Minkowski spacetime, a perturbation of Schwarzschild spacetime, and a perturbation of the shell matter. The metric perturbations are governed by the Einstein field equations, and they are matched across the shell with the help of Israel's junction conditions. The matter perturbation is governed by the equations of fluid mechanics, and it produces a source term in the junction conditions. All calculations are carried out in full general relativity, but we also examine a nonrelativistic formulation of the problem; we show that a Newtonian shell also is necessarily unstable to a time-dependent perturbation. Our conclusion suggests that a compact object that features a thin shell at its surface will be dynamically unstable; this makes it nonviable as a model of black-hole mimicker.