The Event Horizon Telescope (EHT) provided astronomers with the first-ever images of supermassive black holes: the one at the center of galaxy M87 and our friendly neighborhood one, Sagittarius A*, which sits at the center of the Milky Way. Those incredible images captured for the first time the shadows of black holes. Now, astronomers have a new goal in mind.
Most of what we can see in those first images is from light that was deflected by the incredible gravity of the black hole. But very close to these supermassive objects, photons can actually orbit them. The closer they are, the more times the photons orbit the supermassive black hole, and the more orbits they perform, the sharper these photon rings are. And that is what astronomers are hoping to reveal next.
Different theoretical approaches have been taken to extract the photon ring signal from the current EHT data. The observations conducted in the last few years will further refine this measurement. But to truly be able to see the photon ring, the telescope needs to be made sharper. And for that, you need to go to space.
The EHT is a truly unique telescope. A neat trick in radio astronomy allows us to improve the resolution of images by having two telescopes looking at the same object at the same time. The combined observatory has an effective diameter as wide as the distance between the two telescopes. In the case of the EHT, eight radio telescopes were spread all over the world from Europe through the Americas all the way to Antarctica to create an Earth-sized telescope.
To make a bigger telescope, we need to go to space. That’s where the proposal for the $300 million Event Horizon Explorer (EHE) comes in: a spacecraft that can make the telescope even bigger and allow researchers to probe the photon ring.
“What we are trying to do now is launch a space mission that would improve the sharpness of the EHT images by a factor of 10,” one of the mission promoters Michael Johnson, an astrophysicist at the Center for Astrophysics, Harvard and Smithsonian (CfA), said in a statement.
In a recent meeting, 70 researchers in different disciplines looked at the feasibility of such a mission. It will have to be extremely sensitive and have very precise timing to be able to coordinate with the other telescopes. It needs to be on stable orbits but not too far away because it will have to transfer a lot of data down to Earth.
“Detecting the photon ring requires recording huge volumes of data on the spacecraft. We plan on using laser light to beam the information equivalent of the entire Library of Congress down to Earth,” added Peter Galison, Joseph Pellegrino University Professor in the History of Science and Physics and director of Harvard’s Black Hole Initiative.
Technologies such as laser communication are emergent, with some already tested and more underway. Based on the consideration of current and near-future technologies, the team realized that this mission is not farfetched in the short term.
“We were trying to figure out if there were any showstoppers. Was there any reason that we can’t launch this within the next 10 years? And the exciting thing was that there weren’t,” said Janice Houston, a systems engineer at the CfA. “We think that we can keep our foot on the gas and actually get this built within the next decade.”
Observing the photon ring will allow us to better understand the properties of these incredible celestial objects, and also test the fundamental theories we have of the universe, and maybe find what lies beyond.
“If a black hole is spinning, it would distort the shape of the photon ring, squeezing it into an oval,” Galison added. “[If the EHE is able to measure the photon ring] that will be a rock-solid measurement of the effects of the rotating black hole to bend the path of light itself.”
Before the EHE is launched, we should get new results from the EHT including the long-awaited first video of a supermassive black hole – because the shadow we have seen is far from static.