Gravity seminar - Jake Shipley Seminar

Abstract: An exciting era for gravitational astronomy is underway. In 2015, the LIGO–Virgo collaboration announced the first direct detection of gravitational waves from merging black holes (BHs). More recently, the Event Horizon Telescope (EHT) began observing nearby galactic centres, with the principal aim of capturing the BH shadow and other signatures of strong-field gravitational lensing. Motivated by these two advances in gravitational astronomy, recent work has focussed on what the shadow of a pair of BHs would look like, both for dynamical binaries and imitative models. In this talk, we analyse the lensing dynamics of a pair of BHs using the Majumdar–Papapetrou (MP) model, which describes a pair of extremal BHs in static equilibrium. We demonstrate that binary BHs are natural exemplars of chaotic scattering. There exists a set of unstable perpetual photon orbits which are responsible for the existence of fractal structures in MP binary BH shadows. Moreover, in a certain parameter regime, the MP spacetime admits stable photon orbits (SPOs). We will analyse the existence and phenomenology of SPOs in the Weyl family of stationary axisymmetric (electro)vacuum spacetimes.