Black hole photo: What’s next for Event Horizon Telescope after its black hole pictures?

Now that the Event Horizon Telescope collaboration has released its image of the Milky Way’s black hole, the team is focusing on making movies of the two photographed black holes and finding other distant black holes large enough to study

By Leah Crane

On 12 May, the Event Horizon Telescope (EHT) revealed the first close-up picture of the black hole at the centre of the Milky Way. Now that it has taken images of both that black hole, called Sagittarius A* (Sgr A*), and the one at the centre of the M87 galaxy, known as M87*, it is time for the collaboration behind the images to move on to new scientific pursuits. So, what is next?

First, the researchers will have to examine the data they have already collected. The images of Sgr A* and M87* were both assembled from data gathered in 2017, but there have since been two more observation periods, with extra telescopes added to the collaboration’s original network of eight.

“Data does exist. We have taken data in 2018 with one additional telescope, 2022 with three additional telescopes, and we are working very, very hard to get that to you… as soon as we possibly can, but I can’t make any promises about when,” said EHT researcher Lia Medeiros at the Institute for Advanced Study in New Jersey during a 12 May press event. It will probably take years before the results of that analysis are released, she said.

One thing this work is expected to clarify is the structure of the material around Sgr A*, particularly the three bright “knots” of light seen in the new image. Because of the way the image was made, the bright spots could just be artefacts. “Those knots tend to line up with the directions in which we have more telescopes,” said EHT researcher Feryal Özel at the University of Arizona during the press event. “Even though it’s natural in theory to expect these brighter spots, we don’t trust them in our data that much yet.”

While the images are consistent with Albert Einstein’s general theory of relativity so far, deeper analysis may give us another check on how that theory might break down in the extreme areas around black holes. “It should give us a hint, at some point, of maybe something different than how we formulate gravity with the theory of general relativity right now,” said Özel. “We don’t see a crack in that theory yet.”

Finally, another major goal of the EHT collaboration is to make videos of Sgr A* and M87* as the material around them moves and changes over time. “We tried to use the data that we got to try to recover a movie,” said EHT researcher Katie Bouman at the California Institute of Technology during the press event. Although they do have some data they could use, there currently isn’t enough to make movies of the black holes, she said.

The additional telescopes recently added to the array should help with that. These will collect data in multiple wavelengths, which will increase the resolution of the images and could produce colour pictures – the images that have been released up to now have had colour added to indicate brightness.

So far, these two black holes are the only ones we know of that can be imaged by EHT with high enough resolution to see their silhouettes against the light of the hot plasma around them – Sgr A* because of its proximity to Earth, and M87* because of its colossal size. Work is ongoing to spot other supermassive black holes the researchers could observe and compare to these two. Based on statistical studies, there should be other black holes that are enormous enough and not too distant for the EHT to resolve, but researchers haven’t found them yet.