Zhen Zhong "Instability and backreaction of massive spin-2 fields around black holes"

Wednesday 1 Apr 2026, 12:30 → 14:30 Europe/Rome

Sala Lauree (Marconi Building) (La Sapienza)

Ultralight bosonic fields are compelling dark matter candidates. In particular, massive spin-2 fields can not only extract energy from rotating black holes via superradiant instabilities but also trigger a unique monopolar instability—specifically, the axisymmetric (azimuthal number m = 0) instability—which can even affect non-rotating black holes. This instability dominates in most of the parameter space and could potentially influence all black holes, from stellar-mass to supermassive. A thorough understanding of its nonlinear evolution and final state is therefore of great importance. In this work, based on ghost-free massive gravity theories (dRGT and the Hassan–Rosen bigravity theory), we employ time-domain evolution methods to systematically investigate the nonlinear dynamics of this instability in a spherically symmetric black hole background. Our results show that the final outcome of the evolution depends on the model parameters: the system can either evolve into a stable "hairy" black hole or experience a breakdown of the theory itself due to nonlinear effects. This study provides important insights into the nonlinear dynamics of massive gravity theories and paves the way for testing ultralight spin-2 dark matter models through astronomical observations.