Pioneering Tests of General Relativity with Black Hole Photon Rings
Our group has been at the forefront of testing General Relativity (GR) using observations in the electromagnetic spectrum, particularly through black hole imaging. In 2010, we identified a critical feature in black-hole images, which we called the photon ring, that allows probing their strong-field spacetimes. Unlike other observational signatures, the photon ring is minimally influenced by the complexities of astrophysical accretion, making it a robust tool for exploring black hole properties. For a Kerr black hole, the photon ring’s size is tightly constrained to within ±4%, providing a direct measure of the gravitational field’s strength near the event horizon. Its nearly perfect circularity serves as a stringent test of the no-hair theorem, with any measurable deviation signaling a breakdown of this foundational aspect of GR. We have applied these ideas to the first black hole images obtained with the EHT for the M87 and Sgr A* black holes.
Review Article on Testing GR with the Event Horizon Telescope
Probing Black Hole Spacetimes with Multi-Messenger Observations
Our group has developed innovative methods to probe black hole spacetimes and quantify potential deviations from the Kerr metric predicted by General Relativity. Beyond black hole images, we explored leveraging diverse electromagnetic observations such as quasi-periodic oscillations and Iron-K lines that will enable precise constraints on black hole properties across various scales. Additionally, we have combined tests of GR from images of supermassive black holes with those using gravitational wave signals from the coalescence of stellar-mass black holes. This multi-messenger approach addresses a fundamental question: does the spacetime of a black hole scale universally with its mass, or do deviations emerge in different regimes? Our work provides a comprehensive framework for testing the universality of black hole spacetimes.
