Calculations Predict Intricate Substructure in Black Hole Images

In April 2019, the Event Horizon Telescope (EHT) revealed the first image of a black hole that generated a great deal of excitement worldwide.

The image of a black hole has a bright ring of emission surrounding a “shadow” cast by the black hole. This ring is composed of a stack of increasingly sharp subrings that correspond to the number of orbits that photons took around the black hole before reaching the observer. Image Credit: George Wong (UIUC) and Michael Johnson (CfA).

Currently, a research team has published the latest calculations that predict a complex and remarkable substructure within the images of black holes from extreme bending of gravitational light.

The image of a black hole actually contains a nested series of rings. 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.

Michael Johnson, Astrophysicist, Center for Astrophysics, Harvard and Smithsonian

Black holes are capable of capturing any number of photons that reach their event horizon and, as a result, they cast a shadow on their vivid surrounding emission produced by hot infalling gas. This shadow, which is generated from light, is encircled by a “photon ring”. The light is concentrated by the powerful gravity close to the black hole.

The fingerprint of the black hole is carried by the photon ring—its shape and size encode the “spin” or rotation and mass of the black hole. Now, using the images captured by the EHT, black hole scientists have a novel tool at their disposal to analyze these unusual objects.

This is an extremely exciting time to be thinking about the physics of black holes. Einstein’s theory of general relativity 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.

Daniel Kapec, Member, School of Natural Sciences, Institute for Advanced Study

Kapec continued, “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.”

Astrophysicists, theoretical physicists, and observational astronomers were part of the research team.

Bringing together experts from different fields enabled us to really connect a theoretical understanding of the photon ring to what is possible with observation,” observed physics graduate student George Wong from the University of Illinois at Urbana-Champaign.

Wong eventually created software to create simulated and higher resolutions images of the black hole than that are calculated earlier and to disintegrate these images into the expected sequence of sub-images.

What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits,” Wong added.

In addition, the scientists discovered that the image substructure of the black hole offers new opportunities to view black holes.

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.

Michael Johnson, Astrophysicist, Center for Astrophysics, Harvard and Smithsonian

Johnson continued, “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.”

Black hole physics has always been a beautiful subject with deep theoretical implications, but now it has also become an experimental science,” stated 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.”

The study results were published in the Science Advances journal and are available online.

The study was financially supported by grants from the United States Department of Energy, the National Science Foundation, NASA, the Jacob Goldfield Foundation, the John Templeton Foundation, and the Gordon and Betty Moore Foundation.

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. Video Credit: Center for Astrophysics, Harvard & Smithsonia.


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