Topology-Induced Anisotropies in Black Hole Photon Rings

We report a falsifiable deviation from the Kerr prediction for black hole photon rings. We show that a non-factorizable causal topology induces anisotropic broadening of the photon ring with angular harmonics forbidden(i.e., exponentially suppressed at observable levels) in General Relativity (GR). The effect is parameterized by a single dimensionless amplitude ϵ, producing characteristic multipole components beyond those generated by spin, lensing, or plasma effects. We argue that current and near-future Event Horizon Telescope (EHT) observations can directly constrain or exclude this signal.