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Exciting Stars, Exciting Times, Exciting Research

Photo by Cassie Carlson

David neilsen and eric hirschmann have spent years studying binary partnerships that are millions, even billions, of light years away in the universe—and those binary partnerships echo their personal partnership, both as friends and as research colleagues.

Neilsen and Hirschmann, who both teach in the Department of Physics and Astronomy at BYU, have researched together for over a decade. It all began with their love for physics and stargazing.

“I had a high school physics teacher who maybe tried to destroy that love!” Neilsen said. “But it wasn’t until I took my first modern physics class, where we learned about quantum mechanics and relativity, that I really fell in love with it and decided that this is what I really want to do.”

For Hirschmann, deciding to pursue an education and a career in physics was not as straightforward, even though his father was a physicist and an engineer at NASA.

“There was an unspoken encouragement as well as real life encouragement,” Hirschmann said. “It wasn’t something that I felt obligated to do, but it was something that was always available to me.”

Despite an early interest in philosophy at BYU, Hirschmann eventually studied physics (although he minored in philosophy) and then received a doctorate in theoretical physics at the University of California, Santa Barbara.  Neilsen received both his bachelor’s and master’s degrees in physics at BYU and then a doctorate at the University of Texas at Austin. Though the two have researched together for a long time now, their paths crossed years before they delved into learning about black holes and neutron stars, including at Texas, where Hirschmann was doing a post-doc while Neilsen was a student.

Neilsen and Hirschmann study general relativity, and their research focuses on black holes, neutron stars, and binaries of the two, such as two neutron stars or a neutron star and a black hole. Neutron stars contain the densest matter in the universe and are created when a star’s core runs out of fuel for nuclear fusion and gravity overcomes the natural stability of atoms. The nucleus disintegrates and the electrons react with the newly free protons to create a star comprised almost solely of neutrons.

When two neutron stars collide together, they can become a hypermassive star, which exists for short times when hot and rapidly rotating. A black hole is formed when a hypermassive star cools and friction slows it down. When black hole and neutron star binaries merge together, they produce gravitational radiation. Neilsen and Hirschmann write computer codes and run simulations to model binary mergers of black holes and neutron stars and the gravitational radiation that they emit.

“We’re currently working on a new way of solving the equations—new numerical techniques for solving the equations that will make it faster, more efficient, and allow us to do larger runs,” Neilsen said. 

Given the recent announcement of the first detections of gravitational waves that were made last year, the two professors hope to learn more about gravitational wave observations and how to more readily detect them in the future.

“We would like to learn more about neutron stars, what they’re made of, what their structure is, and we hope to learn more about that with gravitational wave observations,” Neilsen said. “Depending on what the star is made of and depending on how stiff the matter in the star is, the neutron stars can disrupt as they start to merge, and that’s something that we hope to be able to detect.”

Neutron stars and black holes are difficult to detect because they are infrequent and relatively quiet.

“If I know that a car might be approaching me from very far away, if I know something about the car, it’s easier to identify as a car,” Hirschmann said. “What we want to do is identify or detect this radiation, and if we know something about it beforehand, it becomes easier to actually measure or detect.”

With last year being the centennial celebration of general relativity and with this year’s announcement of the first detections of gravitational waves, Neilsen and Hirschmann still have a lot of work to do and work that they want to do.

“It’s a historical time. During my entire career in relativity people have been talking about when LIGO [Laser Interferometer Gravitational-Wave Observatory] will detect gravitational waves—now it’s actually happening,” Neilsen said.        

“It’s going to open up a new way for us to understand what’s going on.”

Neilsen and Hirschmann hope that their research will lead to more studies in the area of gravitational radiation.

“We really have no idea how many black holes are out there,” Neilsen said. “By detecting their mergers, we’ll be able to pin that number down—learn how many black holes there are, what sizes they are. We will also learn about neutron stars, and the nuclear matter in the stars. I hope it’s groundbreaking!”

The two professors’ passion for studying neutron stars and black holes is ardent, and they already have research projects in the works for the future.

“One thing that I started to dabble in is a project that relates to the merger of these compact objects—black holes for instance—but doing so in an alternative description of gravity,” Hirschmann said. “Having now seen such objects merging together, say two black holes colliding, and seeing the gravitational radiation . . .  it’s natural to ask, is there more?”

For Hirschmann, studying the merger of black holes will lead to a better and more thorough understanding of gravity.

“Is there, not just the radiation, but are there other things about this event that can teach us about gravity?” Hirschmann said. “We think we have a good description of gravity, but are there other facts that are lurking beneath the surface? For instance, we don’t believe that we have a quantum theory of gravity . . . might these events lead us or help give us evidence for better ideas that might lead us to a quantum theory of gravity?”

Hirschmann thinks that spiraling black holes might give scientists a way of discovering more about gravitational effects, so he and Neilsen have plenty of research to conduct and questions to answer. At the end of the day, Neilsen and Hirschmann love to study compact objects that are billions of light years away. If they did not love it, they would not be scientists.

“If you don’t enjoy it, it’s not worth doing,” Neilsen said.

Note: This article was originally written in our published science magazine Frontiers. Read the magazine here.