Last April, the Event Horizon Telescope (EHT) sparked international excitement when it unveiled the first image of a black hole. 
© George Wong (UIUC) and Michael Johnson (CfA) | Research team discovers a path to razor-sharp black hole images

Today, a team of researchers has published new calculations that predict a striking and intricate substructure within black hole images from extreme gravitational light bending.
"The image of a black hole actually contains a nested series of rings," explains Michael Johnson of the Center for Astrophysics, Harvard and Smithsonian (CfA). "Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we've caught just a glimpse of the full complexity that should emerge in the image of any black hole."
Because black holes trap any photons that cross their event horizon, they cast a shadow on their bright surrounding emission from hot infalling gas. A "photon ring" encircles this shadow, produced from light that is concentrated by the strong gravity near the black hole. This photon ring carries the fingerprint of the black hole its size and shape encode the mass and rotation or "spin" of the black hole. With the EHT images, black hole researchers have a new tool to study these extraordinary objects.
"This is an extremely exciting time to be thinking about the physics of black holes," says Daniel Kapec from the Institute for Advanced Study. "Einstein's  makes a number of striking predictions for the types of observations that are finally coming within reach, and I think we can look forward to lots of advances in the coming years. As a theorist, I find the rapid convergence between theory and experiment especially rewarding, and I hope we can continue to isolate and observe more universal predictions of general relativity as these experiments become more sensitive."


Black holes cast a shadow on the image of bright surrounding material because their strong gravitational field can bend and trap light. The shadow is bounded by a bright ring of light, corresponding to photons that pass near the black hole before escaping. The ring is actually a stack of increasingly sharp subrings, and the n-th subring corresponds to photons that orbited the black hole n/2 times before reaching the observer. This animation shows how a black hole image is formed from these subrings and the trajectories of photons that create the image. Credit: Center for Astrophysics, Harvard & Smithsonian

The research team included observational astronomers, , and astrophysicists.
"Bringing together experts from different fields enabled us to really connect a theoretical understanding of the photon ring to what is possible with observation," notes George Wong, a physics graduate student at the University of Illinois at Urbana-Champaign. Wong developed software to produce simulated black hole images at higher resolutions than had previously been computed and to decompose these into the predicted series of sub-images. "What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits."

Black hole team discovers path to razor-sharp black hole images
Reference: Universal Interferometric Signatures of a Black Hole's Photon Ring. Credit: Michael D. Johnson (CfA), Simulation: George Wong (UIUC)
The researchers also found that the black hole's image substructure creates new possibilities to observe . "What really surprised us was that while the nested subrings are almost imperceptible to the naked eye on images even perfect images they are strong and clear signals for arrays of telescopes called interferometers," says Johnson. "While capturing black hole images normally requires many distributed telescopes, the subrings are perfect to study using only two telescopes that are very far apart. Adding one space telescope to the EHT would be enough."
"Blackhole physics has always been a beautiful subject with deep theoretical implications, but now it has also become an experimental science," says Alex Lupsasca from the Harvard Society of Fellows. "As a theorist, I am delighted to finally glean real data about these objects that we've been abstractly thinking about for so long."
This article was originally published by the Institute for Advanced Study.
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