"For the past 10,000 years, astronomers have been studying the light side of the universe. We have now just begun to study the dark side of the universe."
Illustration of a black hole in front of the Large Magellanic Cloud, a galaxy nearby our own Milky Way. Image: Alain r/CC-by-2.5
Black holes, by nature, are tricky to spot. These regions of spacetime have such forceful gravitational fields that even light cannot escape them, rendering the vast majority of them functionally invisible from our perspective on Earth.
Because of these stealth invisibility cloaks, only a few dozen black holes have ever been identified. These rare observations are the result of black holes interacting with other objects—for instance, the radiant death of a star as it is torn apart by a black hole's tidal forces, or the gravitational ripple created by the collision of two black holes.
Scientists estimate that there are at least 100 million stellar-mass black holes in the Milky Way galaxy alone, so evidently there's a large gap between the theoretical population of black holes and those we can actually observe.
New research published on Thursday in the Astrophysical Journal proposes an inventive new way to bridge that divide, and thus exponentially boost discoveries of black holes. The study was led by cosmologist Avery Broderick and PhD student Mansour Karami, who are jointly based at the University of Waterloo and the Perimeter Institute for Theoretical Physics. The team's conclusions, profiled in the below video, show that radio-wave "movies" of gravitational microlensing events could be the secret to detecting and characterizing isolated black holes.
Microlensing occurs when a large object, like a black hole, passes in front of a background light source, which distorts and sometimes amplifies the background object's light from our perspective. This cosmic "zoom in" effect has often been used to study very distant stars and planets that seem magnified by the foreground object.
Microlensing-focused observatories hunt for telltale flashes of light in the optical bands of the spectrum, but Broderick and Karami's technique would focus on "radio-bright" background sources, like distant quasars, in order to build higher resolution models.
"When you look at the same event using a radio telescope—interferometry—you can actually resolve more than one image," said Karami in a statement. "That's what gives us the power to extract all kinds of parameters, like the object's mass, distance, velocity."
The team intends to partner with sophisticated interferometer facilities, like the Very Long Baseline Array in New Mexico, in seeking out microlensing events that involve single black holes. Broderick and Karami estimate that this technique would lead to the discovery of about ten black holes per year, "doubling the current number of black hole detections within two years, and unlocking the galactic history of black holes in over a decade," according to a press release.
"For the past 10,000 years, astronomers have been studying the light side of the universe," said Broderick. "We have now just begun to study the dark side of the universe with gravitational waves. This project brings the two of those together."
Correction: This article originally stated that Avery Broderick and Mansour Karami were based at the Perimeter Institute for Theoretical Physics. It has been updated to reflect that they have joint appointments with Perimeter Institute and the University of Waterloo.
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