PhD defense by Haopeng Yan

Observational signatures of near-extremal rotating black hole

We are entering an exciting new era of imaging black holes with the help of the Event Horizon Telescope (EHT). This has stimulated many theoretical works predicting the signals that EHT may possibly observe and examining the type of properties of gravity that the signals can inform us. While these signals may in general depend on a complex nearby environment of a black hole, it is possible to expect some universal and striking signals for the case of near-extremal rotating black holes due to the existence of an enhanced conformal symmetry in the near-horizon region of such black holes.

These particular signals may serve as a typical signature for identifying a near-extremal rotating black hole in the Universe. Moreover, the enhanced symmetry supplies powerful tools which enable one to do analytical computations for these interesting signals. From a practical perspective, astronomical observations have suggested that plenty of supermassive black holes are rotating very rapidly (i.e., they are in the near-extremal limit). Therefore, this thesis will focus on the optical observational signatures of high-spin black holes. In particular, we study the images of a point-like orbiting emitter (referred to as a “hot spot”) near the Innermost Stable Circular Orbit (ISCO) of a high-spin black hole.

Images of such an emitter may reveal important features of the black hole event horizon since the emitter resides in the near-horizon region, thus the images can further inform us of the properties of the underlying gravity theory. We analytically compute the shadow of a near-extremal rotating black hole and the optical observables of a near-ISCO hot spot. A key feature of the optical appearance of such an orbiting hot spot is that there are many images of it moving on a vertical portion of the black hole shadow and having a rich structure. The computations rely on the geometric properties of the black hole spacetime and the motion of massive particles and photons in it.

Many studies on black hole imaging are based on the assumption that the underlying gravity theory is general relativity (GR) and the motion of lights follows geodesic equations in the spacetime. Here we study alternative possibilities: a) we compute the influence of a plasma on the observational signature by taking into account its interactions with photons; b) we compute the observational signature based on gravity theories that go beyond GR, in particular the Scalar-Tensor-Vector (STVG) modified gravity (MOG) and the heterotic string theory. The obtained results may not only provide other possible templates for the EHT to test, but also propose a new way to distinguish different gravity theories.