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A year after scientists unveiled the first-ever image of the shadow of the Messier 87 (M87*) black hole, Harvard astrophysicists have made yet another discovery — the crescent-like shadow of that black hole appears to be wobbling.
In a report published on Sept. 23, the group — led by Black Hole Initiative Postdoctoral Fellow Maciek Wielgus — analyzed data collected by the Event Horizon Telescope (EHT) over the past decade. The researchers combined this archival data with mathematical models to simulate the changes in the flow of matter around M87* over the timescale of months and years. They observed a ring of light surrounding the black hole that varied in brightness at different points, creating a wobbling motion over time.
Sheperd S. Doeleman of the Center for Astrophysics, Founding Director of the EHT and co-author of the paper, likened the method to doing “archaeology through [their] records” and “finding treasure twice.”
The wobbling is unrelated to any motion of M87* itself, but rather the effect of turbulence that may be caused by the motion of gas or plasma falling in. As matter enters, factors like density and temperature cause changes in appearance.
This discovery can help scientists narrow down the possible theoretical models describing accretion, or the accumulation of matter pulled in by the black hole’s powerful gravitational attraction.
Doeleman explained that it would be unusual if there were no variation, as that would be “like going into the ocean and seeing waves come in only at one spot.”
Wielgus elected to use data collected since 2009 due to the slow-moving nature of objects as massive as black holes.
“You don’t expect much to change in the image of a black hole during one week,” he said.
The group also observed that the black hole’s shadow formed a crescent shape, formed by plasma orbiting M87*, and that the diameter of this crescent is consistent with Einstein’s theory of general relativity.
“The matter that’s coming toward us is emitting light, and that’s brighter than the light received from matter which is moving away from us,” Doeleman said, describing why the swirling plasma looked like a crescent. “The size we saw was exactly what Einstein predicted, to within the precision of our measurements.”
University of Birmingham Astrophysics professor Alberto Vecchio described the work as “phenomenal” and of great importance for both theoretical physics and astrophysics research.
“The most extreme objects of [Einstein’s theory of general relativity] are black holes,” he said. “On the fundamental physics side, the importance is to test whether these objects behave as Einstein predicted.”
Understanding the dynamics of M87* might also help predict the behavior of other black holes, including the Sagittarius A* black hole at the center of our galaxy.
“Astrophysicists are always trying to make some more universal theory that can describe a breadth of objects, instead of just trying to look at them one on one,” Lindy Blackburn — a data scientist and radio astronomer at the EHT — said.
Researchers working with the EHT have begun analysis on data collected in 2018 and await further data in 2021.
“In a couple of years, with several high quality data sets, we will be able to really quantify this amount of variation that the system experiences,” Wielgus said.
Scientists have also started work on the next-generation Event Horizon Telescope, which will enable them to create videos of black holes by doubling the number of radio dishes and placing them in optimized locations.
“A decade ago, we predicted that we'd be able to make the first image of a black hole," Doeleman said. "And now in 2020, we're predicting that by the end of this decade, we'll have the first movie of a black hole."
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