Speaker: Pedro Cunha (Universidade de Aveiro)
Title: Backreaction of perturbations around a stable Light Ring
Abstract: Light rings (LRs) - closed circular orbits of null geodesics - are key features of both black holes
and horizonless ultracompact objects. While unstable LRs are relevant for the observation of black
hole images, stable LRs have been suspected to trigger instabilities, namely in exotic compact
objects that could mimic black holes. The underlying mechanism behind this instability remains
poorly understood, but a key missing piece is how the backreaction of a perturbation around the
stable LR modifies the surrounding spacetime. In this talk, some progress in this direction is
discussed by examining a conceptually simple, yet instructive, toy model: continuum-shell stars,
supported solely by tangential pressures. Using both analytical and numerical methods, we show
how perturbations around the stable LR deepen the geodesic potential and shifts its location inward,
potentially amplifying any instability associated with the LR. By then extending the analysis to more
general stars with nonzero radial pressure, we find that the same phenomenon can be expected to
persist under reasonable assumptions.
Room: Sala de Reuniões e Seminários (2-8.3, 2nd Floor of Physics Building)