Black Holes Devour Light — Except When They Create it, Finds Physics Undergrad

This article was originally published by IMPACT Magazine. By Elana Roldan To picture a black hole is to picture violence: Stars are mangled, planets are ground to dust, and time itself is wrung out and slowed by these super-sized whirlpools of gravity. They are immense, indiscriminate and the ultimate destroyers of the universe. Then they merge. […]

July 8, 2024

This article was originally published by IMPACT Magazine.

By Elana Roldan

To picture a black hole is to picture violence: Stars are mangled, planets are ground to dust, and time itself is wrung out and slowed by these super-sized whirlpools of gravity. They are immense, indiscriminate and the ultimate destroyers of the universe. Then they merge.

One such turbulent event is what Honors College physics senior Phia Morton spent last summer studying in Pisa, Italy. She returned to Oregon State as the first author of a paper correlating the merger with two different signals — a flare and a gravitational wave — making it the first strongly supported instance of a black hole merger emitting light. This discovery not only deepens our understanding of these astronomical objects but can also provide a new lens to view the gradual stretch of the universe.

“Any time you take on a research project, you’re going to be full of questions and uncertainties,” Morton said. “Research is nice because it’s about the journey, not the destination. You’re doing something new, something that hasn’t been done before.”

Morton was able to present her work at the Conference for Undergraduate Women in Physics at Stanford University, where she will pursue a Ph.D. this fall. She also presented at the American Astronomical Society conference in New Orleans this past January. With the infinite realm of the cosmos as a muse, new discoveries are the beating heart of astrophysics that continues to pull her in.

Phia Morton discusses her research experience with NANOGrav at the OSU President’s Dinner in Portland, Oregon.

The ripple before the flare

Morton’s journey with black holes began amidst the similarly chaotic pandemic. A first-year student at Oregon State, she sought out ways to connect with the physics department in a time of isolation. Her search led her to Xavier Siemens, a physics professor and co-director of the NANOGrav Physics Frontier Center.

NANOGrav, the North American Nanohertz Observatory for Gravitational Waves, recently released nearly two decades’ worth of research to international acclaim. Its work upheld the existence of low-frequency gravitational waves and enhanced current knowledge of how cosmic structures are pieced together. With several Oregon State scientists leading the project, Morton was able to reach out and work alongside groundbreaking researchers during this process.

“NANOGrav has been super supportive of everything that I’ve done here at OSU and elsewhere,” she said.

The foundational knowledge of gravitational waves she gained from NANOGrav was critical to working on the black hole merger. Gravity is more than what made the apple fall — it moves as invisible waves across the universe, wrinkling and stretching space itself. As Morton explains, spacetime (the fabric of space) can be described as a flat trampoline. When a heavy object is set in the middle of the trampoline, the fabric warps inward like a funnel. If two heavy objects are placed on the mat, they circle each other down this funnel, sending out ripples onto the trampoline as they move. Gravitational waves are these ripples in space. In the case of Morton’s research, the objects are two black holes with one inevitable destination at the funnel’s end: each other.

Morton stands in front of the LIGO detector — a facility used to measure gravitational waves — in Livingston, Louisiana while attending an American Astronomical Society conference.

If orbiting black holes cause ripples, their collisions create tsunamis. Black holes and neutron stars, the incredibly dense cores of collapsed stars, have been observed in merger events. While both are signaled by monstrous gravitational waves, mergers between neutron stars are always paired with an electromagnetic pulse. In other words, they create light.

“Black hole mergers traditionally do not give off light, so we can only see that gravitational wave — the caveat being my research over the summer,” Morton explained.

In 2019, the LIGO-Virgo-KAGRA collaboration detected waves from a binary black hole merger. It was unique for a number of reasons, including its currently upheld position as the largest event of its kind discovered by the collaboration. Thirty-four days after the merger’s already remarkable detection, a flare was spotted in the same sliver of sky. Yet, the two events were ultimately determined too far away to be related.

Enter Morton.

“At OSU, I’d already gotten to learn a bit more about gravitational waves. That led me to apply for a Research Experience for Undergraduates,” she said. The application took her to the University of Pisa where she was given a project with an all but certain answer.

“We were expecting to rerun this analysis with a new model and get a result that confirmed these two events are likely not connected. Instead, when we tested whether or not this merger between two black holes and this light, this flare we were seeing, were related at all, we found that it’s likely they are connected,” she explained.

Morton visits Cinque Terre, Italy during her research at the University of Pisa.

Morton’s research presents the first strong candidate for a black hole merger emitting light. In her first-author paper published in Physical Review D, she proposes the collision occurred around a supermassive black hole in an area called an active galactic nucleus. Such regions are filled with dust and gas that shine vividly as they fall into the central black hole. For the merger to emit light, she suggests it moved through and heated up gas within the AGN, creating the flare.

These results rattle astrophysics on several levels. For one, it could present a new way to detect black holes.

Like the inky fingerprints of constellations, black holes are scattered across the universe, marking where a star met the explosive end of its life. Just as it would be difficult to see dark ink on dark paper, physicists must find these light-eating masses against the backdrop of space. Being able to correlate them with flares could widen our ability to locate them and their mergers.

Another exciting prospect is the potential for a new measure of the universe’s expansion. The exact value of the Hubble constant, the rate at which the universe is growing, is currently debated, with two known options as of now.

The first measurement comes from observing supernovae — how far they are from Earth and how fast they are moving away. The second is based on residual radiation from the Big Bang called the cosmic microwave background. This fossilized light is used to find tiny deviations in the early universe that explain its growth.

What complicates things is that the values from these methods disagree. Physicists aren’t sure which is correct or if either is correct, but Morton’s research offers a fresh way to tackle the mystery.

“Gravitational waves — when paired with light — give a third, outside opinion on what this Hubble constant is. We can use that gravitational wave to determine how far away something is, and then we can use the light to determine the redshift, or how fast it’s moving away from us. It’s another opinion that tells us about cosmology, how our universe is evolving and how it’s expanding,” she explained.

As she nears graduation, Morton is grateful for her undergraduate experiences in science. From her time at NANOGrav to her summer abroad doing critical research, she’s placed pieces of the universal puzzle before she’s turned the graduation tassel. Whatever lies ahead, Morton knows there will be more to discover.

“I want to keep doing research because I think I’ll never stop having questions, and that’s the only way I’ll get answers,” she said. “For me, astronomy is one of the reasons that life is wonderful. We get to look up at the sky and wonder.”

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