Black hole shadow and chaos bound violation in $f(T)$ teleparallel gravity

We show that the so-called chaos bound, proposed by Maldacena, Shenker, and Stanford, can be violated in $f(T)$ teleparallel gravity. In particular, it is possible to select a new gravitational Lyapunov parameter, controlling chaotization of circular trajectories, in black hole photo-sphere, that can exceed the Maldacena, Shenker and Stanford thermal bound. This feature alters the standard quasi-normal modes and ringdown phase after black hole merging with intriguing implications for future gravitational wave detections and black hole shadow measurements. It is a general characteristic for several gravitational theories beyond standard General Relativity.