Speaker
Description
In Einstein’s general relativity, extremely strong gravity can trap light. In a spacetime admitting a singularity, we say that light (or a “photon”) is trapped if it neither escapes to spatial infinity nor falls into the singularity. Null geodesics govern the trajectories of light. In the Schwarzschild spacetime with positive mass $M$, there exist (unstable) circular orbits of trapped photons at the Schwarzschild radius $r = 3M$, outside the black hole horizon at $r = 2M$. These orbits fill a three-dimensional submanifold of topology $S^2\times\mathbb{R}$ called the photon sphere of the Schwarzschild spacetime. In general, a region in spacetime that is a union of all trapped null geodesics is called the Trapped Photon Region (TPR) of spacetime. In this talk, we will consider the Kerr-Newman and Kerr-Sen spacetime and see that, unlike the TPR of Schwarzschild spacetime, the TPR in such spacetimes is not a submanifold of the spacetime in general. However, the lift of TPR in the phase space is a five-dimensional submanifold. This result has applications in various problems in mathematical relativity (This work is an extension of a similar result but in Kerr spacetime by Cederbaum and Jahns- 2019).
