Topic: General properties of light rings and quasi-black holes

Speaker:  Prof. Sijie Gao

Coodinates: PCFT C1124, 16:00, Thursday, Sep. 30

Abstract：Light rings (LRs)  are circular photon orbits which play an important role in gravitational wave observations and black hole photographs. In this talk, we first prove the general existence of LRs in black hole spacetimes. We also find that LRs always appear in pairs in horizonless spacetimes. Only some natural and generic assumptions are used in our proof. The results are applicable to general relativity as well as most modified theories of gravity. Then we investigate the relationship between LRs and stability of ultracompact objects. We revisit the quasi-black hole solutions, a family of horizonless spacetimes whose limit is the extremal Reissner-Nordstrom black hole. We find a critical parameter at which the light rings just appear. We then calculate the quasinormal modes of the quasi-black holes. Both the WKB result and the numerical result show that long-live modes survive for the range where light rings exist, indicating that horizonless spacetimes with light rings are unstable. This result provides a strong and explicit example that light rings could be direct observational evidence for black holes.