Pseudospectrum and the instability horizonless ultracompact objects

In this talk I will present resutls obtained from the pseudospectrum of linear perturbations on horizonless black hole mimickers. I will briefly introduce the notions of transient growth and pseudoresonance in dissipative systems, measured by the pseudospectrum of their evolution operator. I will then apply these notions to an analysis of potential (nonlinear) instabilities in ultracompact horizonless objects.