These are heady times for black hole researchers. A century after physicists first realized that Einstein’s theory of general relativity predicts the possibility of black holes, new tools and technologies are enabling astronomers to almost literally hear and see black holes, in ways that previous generations could only dream of. In 2015, the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment directly detected the very first gravitational waves — ripples in space-time — which emanated from the merger of two black holes. And in April 2019, people around the world gazed in wonder at the first image of a black hole, released by the Event Horizon Telescope after two years of processing data from eight observatories.
Even after a century’s worth of research into black holes, in some ways the field is just getting started, says Chiara Mingarelli, associate research scientist at the Flatiron Institute’s Center for Computational Astrophysics (CCA). “There will likely be many strange-looking events we can’t immediately explain,” she says. “Then things will get really interesting.”
Flatiron Institute researchers are at the vanguard of this new era of black hole research. The CCA, created in 2016, has quickly become the focal point of astrophysics research in the greater New York City area. “The density of ideas, the level of cross-talk and interaction, is much larger than anywhere else I’ve been,” says Will Farr, the CCA’s group leader for gravitational wave astronomy and associate professor at Stony Brook University.
Farr and his colleagues have been studying the spectrum of distinct tones in the gravitational waves emitted when two black holes merge. Einstein’s theory of general relativity implies that the frequencies and decay rates of these tones are completely determined by the black holes’ mass and spin, which means that measuring these tones would provide an independent test of general relativity. But until recently, most researchers believed that precise measurements of these tones would have to wait another decade at least, until more sensitive detectors could be built.
But Farr — together with Maximiliano Isi, a Flatiron affiliate at the Massachusetts Institute of Technology, and other researchers — figured out how to reanalyze LIGO data to tease out the frequencies and decay rates of two different tones emitted by a pair of merging black holes. The researchers used one of these tones to calculate the merger’s mass and spin, then used that information to calculate general relativity’s prediction for the second tone’s frequency and decay rate. The predicted and observed values matched closely, offering fresh validation for the theory of general relativity.
The researchers next plan to look for similar phenomena among another 10 black hole mergers LIGO has found in recent years, and, further down the road, another 30 mergers that LIGO has detected but not yet cataloged. “This will rapidly become a precision test of general relativity,” Farr says.
As black holes go, the ones LIGO can detect are fairly petite — only about 30 or 40 times the mass of the sun. Mingarelli, meanwhile, studies supermassive black holes, which can be a million or even billion times the mass of the sun. When two such black holes merge, they produce the loudest gravitational waves in the universe, easily a million times louder than those LIGO detects. And unlike weaker gravitational waves, which decay rapidly, these loud waves can linger for 25 million years, collectively forming a gravitational wave background against which all other signals are overlaid.
But just because these waves are strong doesn’t mean they are easy to detect. An experiment like LIGO, which measures the wobble when a gravitational wave passes through two hanging mirrors, is good at picking up the high-frequency waves emitted when two small black holes merge. But the waves emitted by merging supermassive black holes are about 10 orders of magnitude lower in frequency than what LIGO can detect, with wavelengths that are multiple light-years long. To measure these, Mingarelli says, “you need an experiment that’s the size of the whole galaxy.”
Fortunately, the universe itself provides the perfect gravitational wave detectors: rotating neutron stars called pulsars that emit flashing radio waves with such regularity that they are like “radio lighthouses,” Mingarelli says. “They’re so regular that if you notice a 100-nanosecond change in the arrival of these flashes over 10 years, it’s enough to tell you that a gravitational wave is changing the distance between you and the pulsar.”
Mingarelli and her team of global collaborators have been monitoring 65 pulsars scattered through the sky. “We think we’ll need 15 years of data, and right now we have 14,” she says. “So we’re very close to making the first detection of the gravitational wave background.”
Although a black hole inexorably consumes all energy and particles that fall within its ‘event horizon,’ jets of particles sometimes escape from just outside the event horizon. These jets are thought to gush forth from a plasma cloud that surrounds and feeds the black hole, but how the particles become energized enough to escape has not been fully understood.
Astrophysicists have had a theoretical framework for jet launching for more than four decades, but until Flatiron researchers recently turned their attention to the problem, there was no way to effectively simulate the plasma and test the theoretical model. That’s because plasma simulations traditionally rely on treating the plasma as a unified fluid, but the plasma around a black hole doesn’t behave like a fluid: It is ‘collisionless,’ meaning that its density is so low that its particles don’t typically collide with each other.
Now, the Flatiron Institute’s Alexander Philippov and his collaborators have developed a computational method for simulating collisionless plasma and black hole jets. “Before we started working, there were no algorithms to merge general relativity and plasma physics,” Philippov says. “The equations for general relativity and electromagnetic fields are all well known, but the numerical modeling had not been developed, so we had to come up with the algorithms.”
The Flatiron Institute, whose mission is to tackle grand scientific challenges from a computational framework, is uniquely positioned to help astrophysicists advance the current understanding of black holes. “The in-house supercomputing facilities and computational expertise are resources that don’t exist anywhere else in academia,” Mingarelli says.
Black hole research is likely to move forward rapidly in the coming years, Philippov predicts. “There are so many new observational windows that were not available before,” he says. “There are tremendous perspectives ahead of us.”