This makes this particular black hole, discovered at the center of galaxy Abell 1201 by researchers from Durham University, led by Dr. James Nightingale, one of the largest currently known to science. But It’s not the size of this black hole that makes the work of Dr. Nightingale and his colleagues significant. It’s how they found it.

For the first time, gravitational lensing was used to detect and measure a black hole — and there are major implications for the future of astronomy.

To understand gravitational lensing we need to understand one of Albert Einstein’s major insights. According to his theory of general relativity, space and time are not two separate entities, but a single four-dimensional manifold called spacetime. And spacetime — often called the “fabric” of our universe — warps and distorts in the presence of massive objects. Gravity, in this picture, is simply a function of the way this “warping” affects the movement of objects as they travel along these distorted spacetime paths. 

On smaller scales these distortions give us the orbits of planets, for example. But on huge scales other strange phenomena emerge.

Gravitational lensing occurs when a massive object curves spacetime to such an extent that light emitted from behind the object bends around it. Often this lensing produces a magnifying effect, allowing astronomers to see distant galaxies in more detail. And we can use the lensing effect to calculate the mass of the distorting object itself, since the amount of curvature produced is in proportion to the object’s mass. There are many forms this lensing effect can take. It can double an image, or form what is called a ‘gravitational arc,’ and in the presence of a particularly large mass the distorted light forms a ring, called an Einstein Ring.  

The team at Durham wanted to understand the source of a gravitational arc in the galaxy Abell1201, first identified with the Hubble Space Telescope in 2003. The distortion was unaccounted for by the visible matter in Abell1201, which meant the sources of the lensing were either a concentration of dark matter, or a black hole. If a black hole is inactive — that is, it is not actively accreting — then it has to be detected indirectly. This is, however, difficult for distant objects.

The application of gravitational lensing thus overcomes some of the major obstacles in the study of black holes, as lead researcher Dr. James Nightingale tells Supercluster. “Techniques for determining black hole masses face limitations, as they are confined to nearby galaxies with restricted mass ranges (e.g., through the velocities of stars,) or are applicable only to "active" black holes, which represent less than 1% of all galaxies, and restrict the range of masses that can be studied.”

This poses a question. If gravitational lensing lets us study a whole range of black holes normally outside detection and in the far reaches of the galaxy, why is this the first time it has been used? The answer lies in the particular structure of Abell1201. 

According to Dr. Nightingale: “The reason gravitational lensing has not previously facilitated black hole detection is simply because we had not encountered a lens system where the lensed background source's light passed close enough to the black hole at the center of a lens galaxy. The occurrence of such a situation, as in the case of Abell 1201, relies on a series of rare and fortuitous circumstances, including a precise alignment of the lens galaxy relative to the source.”

In 2019 the world was captivated by the first published images of a black hole, at the center of the galaxy 87 Messier, by the Event Horizon Telescope.   

Dr. Nightingale clarified for us that while these images were also aided by gravitational lensing, his team’s approach is unique. “One thing to note is that the [87 Messier] images are technically “gravitational lensing,” as the light of material behind the black holes being imaged is being lensed to form these images. Abell 1201 is the first use of gravitational lensing where two galaxies, separated by billions of light years, are used.”

To test their theory the team at Durham University took the image of the gravitational arc and ran several simulations where they varied the mass of black holes. The simulations also tested the likelihood the arc was caused solely by dark matter. After ruling out other scenarios, the simulations were able to prove that an ultramassive blackhole was the source of the gravitational arc in Abell 1201.

Gravitational lensing was first experimentally confirmed in 1979. Dennis Walsh, Robert F. Carswell and Ray J. Weymann looked at a double Quasar called  Q0957+561 A and Q0957+561 B. They showed that it was actually one quasar, whose image had been doubled by a gravitational lens. This not only showed that gravitational lenses were real, but highlighted their use in helping us develop an accurate picture of the universe.

Gravitational lensing was also critical to the discovery of dark matter. Since it is mass that dictates how much light will bend, if there is a discrepancy between a lensing effect and the visible mass, we know there must be an invisible source of mass. As NASA summarizes: “dark matter acts like a cosmic magnifying glass, bending and amplifying the light from distant galaxies behind it.”

In addition to the work of Dr. Nightingale's team at Durham, there are ongoing efforts to use gravitational lensing to map the distribution of dark matter in the universe. There is also an current research project by University Chicago and Fermilab scientists to examine light from just after the big bang, light which they can examine and analyze by looking for gravitational lensing effects.

The Durham team’s research has indicated ways to locate distant inactive black holes using gravitational lensing. This point they highlight in their article’s conclusion: “This would enable the masses of non-active black holes to be measured at high redshifts…With over 100,000 strong lenses set to be observed in the next decade, it is inevitable that more SMBH measurements via strong lensing will be made.”

The door is now open to studying the largest black holes in the universe. Dr. Nightingale told Supercluster that “While gravitational lensing events are rare, they are most prevalent in the most massive galaxies that host the largest black holes.” This means, he explained, that “lensing effectively "selects for" the largest black holes, enabling us to focus our investigations on these astronomical giants.”

This allows us to study the formation and details of one of the universe’ most intriguing objects and give us deeper insight into the early formation of the universe’s galaxies. As a series of massive telescopic projects — Euclid, LSST, SKA — go online in the coming years, our ability to detect and study a variety of non luminous cosmological objects will be of increasing importance for enhancing our understanding of the universe.

“The future of this methodology holds the potential to greatly enhance our understanding of galaxy formation in the early universe … providing valuable insights into the nature and characteristics of these cosmic behemoths."