David Pereniguez Rodriguez "Gravitational Duality at the Lightring, QNM isospectrality, and Ringdown Nonlinearities from Penrose Limits"

Wednesday 4 Mar 2026, 12:30 → 14:30 Europe/Rome

Sala Lauree (Marconi Building) (La Sapienza)

At high frequencies, the physics near a black hole’s lightring can be captured by Penrose-limit plane waves adapted to the photon sphere. In this talk, I will solve exactly the equations for gravitational perturbations in this limit using Penrose’s 2-spinor formalism, and I will show that the resulting solutions correspond to quasinormal modes (QNMs) in the eikonal regime. 
I will then prove that gravitational electric–magnetic duality holds on-shell for perturbations at the lightring. This symmetry has long been understood to hold on-shell only for perturbations about flat spacetime. The lightring therefore provides a distinct setting in which it reemerges. In addition, QNM isospectrality follows as an immediate consequence. 
I will next extend the analysis to nonlinear gravitational fluctuations, where the duality is broken, and compute the excitation of quadratic QNMs, which are particularly relevant given recent prospects for their detection. I will conclude by discussing further directions in which Penrose limits can be used to sharpen our understanding of lightring physics.