In April 2019, the first image of a black hole was published, marking a powerful confirmation of Albert Einstein’s theory of gravity, or general relativity. The theory not only explains how matter bends spacetime, but it also predicts the presence of black holes, including the scale of a black hole’s shadow cast on the bright disk of material that swirls around some of the dense objects. The shadow of the supermassive black hole at the core of galaxy M87, which is around 55 million light-years away, closely matched general relativity’s estimates of its size (SN: 4/10/19). To put it another way, Einstein was right — once again.
The Event Horizon Telescope Collaboration announced that this result addressed one question: Is the size of M87’s black hole compatible with general relativity? However, EHT team member Dimitrios Psaltis of the University of Arizona in Tucson says, “it is very difficult to address the opposite question: How much can I tweak general relativity and still be compatible with the [black hole] measurement?” The issue is crucial since another theory of gravity might still explain the universe when posing as general relativity near a black hole.
Psaltis and colleagues used the shadow of M87’s black hole in a report published in Physical Review Letters on October 1 to take a significant step toward ruling out certain alternate theories. The researchers used the black hole’s size to conduct a “second-order” test of general relativity in order to improve confidence in the result. According to EHT team member Lia Medeiros of the Institute for Advanced Study in Princeton, N.J., this “can’t really be achieved in the solar system” because the gravitational field is too small. When the researchers conducted this second-order test, they discovered that relativity was doing well so far.
The findings are comparable to those of gravitational wave studies such as the Advanced Laser Interferometer Gravitational-Wave Observatory, which has observed ripples in spacetime from black hole mergers smaller than M87’s (SN: 9/16/19). However, according to physicist Emanuele Berti of Johns Hopkins University, who was not involved in the latest research, “it’s the first attempt at constraining a [second-order] effect via a black hole observation.”
Despite being scrutinized by researchers for more than a century, Albert Einstein’s theory of general relativity, which states that gravity is the warping of space and time, has once again proven to be right. Researchers from the Event Horizon Telescope collaboration recently revealed that Einstein’s theory holds up under some of the universe’s most intense conditions.
Researchers from the EHT collaboration analyzed photographs of the supermassive black hole at the center of the Messier 87 galaxy in a report published on Thursday in the journal Physical Review Letters to put Einstein’s theory to the test once more. The black hole, M87*, is the same one that the EHT team used to build the first-ever picture of a black hole in 2019. According to Lia Medeiros, a co-author on the study and an astrophysicist at the Institute for Advanced Study, “this is only just the beginning,” adding that “they have now shown that it is possible to use a picture of a black hole to test the theory of gravity.”
The images of M87*, which is about 6.5 billion times more massive than the sun, enabled the researchers to examine how the massive black hole bends spacetime more closely. According to the researchers, a black hole does not emit light, but it is surrounded by a hot disk of accreting gas across the event horizon, or point of no return. They went on to say that the black hole’s shadow is cast by heavy, bright gas. When researchers took the first picture, M87* was visible against a bright orange background, confirming predictions made by general relativity theory.
Initially, the research team attempted to disprove general relativity theory in the new study. The scientists continued to search for ways to demonstrate that it could break down, but when they look at the scale of the black hole shadow, it holds up. The latest finding demonstrates that general relativity holds up in the presence of a black hole, even under the most extreme circumstances.
The Event Horizon Telescope is a network of telescopes located around the world that have been focused on two supermassive black holes, M87* and Sagittarius A*, at the Milky Way’s core. When photographs of Sgr A* become available, researchers will have yet another chance to bring Einstein’s theory to the test.