Shadows and strong gravitational lensing: a brief review

TL;DR

This review discusses how Fundamental Photon Orbits influence gravitational lensing and black hole shadows, offering insights into testing the Kerr black hole hypothesis in both GR and alternative theories.

Contribution

It provides a concise overview of the role of FPOs in gravitational lensing and shadows, highlighting their importance in understanding ultra compact objects and testing gravity theories.

Findings

FPOs are crucial in analyzing black hole shadows.

Illustrates non-trivial lensing effects using specific models.

Provides insights into physics beyond Kerr black holes.

Abstract

For ultra compact objects (UCOs), Light Rings (LRs) and Fundamental Photon Orbits (FPOs) play a pivotal role in the theoretical analysis of strong gravitational lensing effects, and of BH shadows in particular. In this short review, specific models are considered to illustrate how FPOs can be useful in order to understand some non-trivial gravitational lensing effects. This paper aims at briefly overviewing the theoretical foundations of these effects, touching also some of the related phenomenology, both in General Relativity (GR) and alternative theories of gravity, hopefully providing some intuition and new insights for the underlying physics, which might be critical when testing the Kerr black hole hypothesis.