Collaborative Research: Experimental General Relativity using Radio Interferometry of a Black Hole Photon Ring

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In April 2019, the Event Horizon Telescope (EHT) collaboration released the first images of a black hole, a breakthrough result that has since prompted a flurry of new theoretical studies. These have revealed that black hole images carry distinctive relativistic signatures, which are embedded within a bright, narrow ring encircling the black hole. A research collaboration between the Smithsonian Institution Astrophysical Observatory and Vanderbilt University will explore the fundamental properties of this “photon ring,” will show how it can be studied using radio interferometry, and will determine the properties of a black hole’s spacetime that can be probed with observations. These researchers have already established the photon ring as a universal prediction of General Relativity: it is a matter-independent effect caused by the extreme bending of light near the event horizon of the black hole. Indeed, Einstein’s theory predicts that Kerr black holes can deflect light rays to such a degree that they circumnavigate the black hole—possibly several times—before eventually reaching distant observers. Black hole imaging is a nascent field with an already global reach and intense public engagement. As part of this project, the investigators will engage the public with this research through the development of new classroom activities for neurodiverse and special education students as well as novel art exhibits conveying the elegance and mysteries hidden within black hole images.During this project, scientists will study the information that the orbiting light encodes about the geometry of the black hole’s spacetime, as well as the astrophysical conditions of the surrounding plasma and electromagnetic fields. This will combine the pursuit of two parallel yet interconnected lines of work: 1) On the theory side, analysis of simple models will build intuition and provide insight into interferometric properties of the photon ring, while development of time- dependent, “slow-light” photon ring models with variability informed by plasma astrophysics will clarify how the simple models relate to the complex structure in a realistic accretion flow model; and 2) On the observational side, the development of progressively more elaborate statistical inference tools and numerical simulations will utilize these models to extract increasingly precise knowledge of supermassive black hole astrophysics from interferometric data, culminating in formal posteriors for the mass, spin, and inclination of both M87* and Sgr A* from EHT data.Two major research outputs of this project will be the comprehensive characterization of the unique photon ring signatures that can be targeted with VLBI (very-long-baseline interferometry), and model-agnostic estimates of mass, spin, and inclination for M87* and Sgr A* using current EHT data.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.