When two black holes collide and merge, they don’t go quietly. Instead, the resulting mega black hole rings like a struck bell, radiating energy outward through space-time in the form of gravitational waves.
The gravitational wave signal produced when two black holes merge is called the ringdown, and for physicist ’05, G’11, and his doctoral student Alex Correia, studying the ringdown may hold the key to rewriting our understanding of the universe.
Capano, research associate professor in the , and Correia, a third-year Ph.D. student, are part of a growing field of gravitational wave astronomy, a discipline that was barely possible a decade ago and is now producing results that could one day surpass even Einstein’s greatest work.
“We’re hoping to prove that Einstein was wrong,” says Capano, whose research has been funded by grants from the National Science Foundation (NSF). “We’re hoping to find some deviation from Einstein’s theory of general relativity, because that would point the way to a better, deeper understanding of our universe.”
Through black hole spectroscopy, a technique for analyzing how black holes merge, Capano and Correia want to learn if there are any discoverable abnormalities that could lead to significant breakthroughs.
Helping Us ‘Hear’ the Universe Better
Gravitational waves are invisible, high-speed ripples in the fabric of spacetime, and they were first recorded on Sept. 14, 2015, a landmark moment that confirmed Einstein’s prediction, made within his theory of general relativity, that gravitational waves exist.
But Einstein’s theories don’t just predict that black holes exist. They describe black holes with specific properties that can essentially be “heard,” detected through the damping energy produced when a black hole rings. That ringing, Capano explains, produces distinct modes or notes, much like striking a key on a piano.
“The question is, can you see more than one of these frequencies, or more than one of these notes, in the ringdown?” Capano says. “Is there a C note and an E note, or is there just a C note? That’s an important question because if you can see more than one note, then you can do some of these advanced tests of general relativity to see if the signal is consistent with it or if something could point to some new physics.”
Searching for an Answer in the Noise
The ringdown signal Capano and Correia are tracking fades almost instantly, swallowed by noise in milliseconds. Extracting meaningful measurements requires cutting-edge computational methods.
“A lot of what I’m working on is trying to figure out efficient ways of cutting out the earlier part of the signal because we’re only interested in the ending part, the ringdown,” says Correia, who has published several papers with Capano highlighting their findings. “We have a signal and we want to extract the actual parameters of the black holes merging, their masses, their angular momentums and their frequency.”
Capano and Correia developed a method to explore whether these colliding black holes were producing one note or multiple notes. Initially, their findings couldn’t prove either outcome conclusively, but then, last year, there was a detection that was three times stronger than the initial discovery of gravitational waves.
Before the results were made public, Capano and Correia ran simulations to project what the ringdown waveform would look like, and when the findings were released six months later, they matched what Capano and Correia had predicted.
“We found strong evidence in favor of seeing at least one of those notes in the signal,” Correia says. “With that strong signal, it seems to suggest that yes, you can clearly see more than one note; you can see two notes in the ringdown.”
Strong Bond Forged Between Mentor and Mentee
For Correia, the path to Syracuse ran through the University of Massachusetts at Dartmouth, where he first began working with Capano as a master’s student.
When Capano joined the Syracuse faculty, Correia followed along, drawn by both continuing to work with his mentor and the strength of the gravitational waves research at the University.
“Alex is a very good student and he’s methodical with his research,” Capano says. “He has a good handle on both the theories and the computational, day-to-day work that drives this research forward.”
The work behind their research can be grueling, spending month after month “grinding away at problems without knowing why they happen,” Correia says.
That dedication earned Correia a trip to Scotland for the preeminent gathering of scientists working in relativity and gravitational waves, the combined International Conference on General Relativity and Gravitation and Edoardo Amaldi Conference on Gravitational Waves.
“That’s the most rewarding part, sharing this groundbreaking research with people and getting insight from other researchers,” says Correia, whose trip was funded by Capano’s NSF grant.
Every major physics breakthrough began with an experiment that revealed a crack in the prevailing theory. Capano and Correia are hunting for that crack.
“We’re hoping gravitational waves will turn up experimental evidence that shows the current paradigm doesn’t explain everything,” Capano says. “Once we have that experimental evidence, someone will be able to turn that into new theories, which is exciting.”
