In this series of posts, we sit down with a few of the keynote speakers of the 247th AAS meeting to learn more about them and their research. You can see a full schedule of their talks here, and read our other interviews here!”
When I ask Xavier Siemens what he enjoys about analyzing gravitational wave data, he responds with a wonderful confidence, “It’s like listening to the universe! The universe is telling you something, and the data analysis is the process by which we try our best at figuring out what that message is.” He and Maura McLaughlin are receiving the Bruno Rossi Prize on behalf of the NANOGrav collaboration “for finding evidence of the stochastic gravitational wave background, the first direct indication of the existence of binary supermassive black holes.” We spoke about his path from theoretical work on cosmic strings to hands-on gravitational-wave data analysis to being a leader of the NANOGrav collaboration, as well as his advice to younger researchers in the field.
Career in Two Acts: From Cosmic Strings to Gravitational Waves
Now a Professor at Oregon State University, Siemens’ career journey was not a straight line. “My PhD topic was on cosmic strings,” he said, “but I focused on gravitational waves from cosmic strings and gravitational wave effects on cosmic strings.” His pivot from theory to data came when he accepted a postdoctoral position with LIGO in 2002. “I wasn’t 100% sure that I was going to like it. I took the job, and it turns out I loved working on data analysis.” His early work with LIGO concerns continuous-wave searches, hunting for the hum of “bumps on neutron stars that emit continuous waves in the LIGO [frequency] band.” Later, he moved on to detector calibration, a crucial step in making the LIGO instruments’ measurements trustworthy. He also led LIGO’s first search for cosmic strings. As the LIGO collaboration grew from a few hundred members to well over a thousand, Siemens broadened his interests across the gravitational wave spectrum. During his early faculty years, he learned about pulsar timing arrays (PTAs), in which millisecond pulsars serve as galactic-scale clocks that can be used to measure gravitational waves at low frequencies. “Nature has been nice enough to give us these neutron stars, these millisecond pulsars, and all we have to do to make a gravitational wave detector is point radio telescopes. It’s an amazing thing,” he said. He saw an opportunity to adapt lessons from LIGO’s data analysis for PTAs, though he would learn that the details could be very different.
PTAs and the Art of the Fit
If LIGO taught him how to listen, PTAs taught him how to disentangle. Every PTA observation is a time-of-arrival (TOA) measurement of a pulsar’s radio pulse. Those TOAs must be compared against a detailed timing model that accounts for the pulsar’s sky position, spin, binary motion, and more, along with numerous noise processes. “The data analysis process in pulsar timing is significantly different and significantly more complicated. Every data point has a different error bar. The data has gaps. We need to fit a timing model. The operation of subtracting a best-fit timing model also does something complicated to the data that you need to take into account,” Siemens explained.
Siemens eventually moved to Oregon State University and, together with Maura McLaughlin, became co-directors of NANOGrav when the collaboration received its Physics Frontier Center grant in 2015. In 2023, NANOGrav, along with other PTA collaborations worldwide, announced the observation of a gravitational-wave background at nanohertz frequencies, the long-suspected underlying rumble from a population of supermassive black hole binaries across the universe. Siemens emphasizes the collaborative depth at NANOGrav that led to this breakthrough. “This [AAS] prize is for NANOGrav. Maura and I are there to receive it, but it is the culmination of the hard work of a lot of people over two plus decades.”
His group at Oregon State University’s specific contributions centered on model comparison and noise characterization. “In my group, we were doing a lot of what we call the hyper model or product space sampling methods, where we compare one Bayesian model to another Bayesian model,” Siemens said. The team also explored alternate correlations and contributed to NANOgrav’s gravitational wave background paper and noise budget paper.
The Rich Future of Gravitational Waves
Siemens is clear about the magnitude of this discovery and the excitement it brings. “It’s the first time we see a stochastic background of gravitational waves. It’s the first time we see gravitational waves in the nanohertz band. It’s the second observational window that we’ve opened onto the gravitational wave spectrum.” Looking ahead, his sights are set on the spectrum’s shape and what it can reveal. “The background is almost certainly due to a population of supermassive black hole binaries. When we confirm that, that will also be the first direct observation of a supermassive black hole binary.”
To him, the future of gravitational wave research is rich: advanced ground-based detectors now and in the future, PTAs developing richly over the next decade, LISA mid-next decade, and third-generation ground-based observatories after that. “There’s at least another three decades’ worth of work,” he said, adding that it is more valuable now than ever for students to prepare to work across experiments as the most interesting questions evolve over time.
The Joy of Science
Siemens’s advice to early-career researchers is both practical and humane. One line that has stuck with him throughout his career: “If you work close to the data, the data never goes out of fashion.” He encourages students to learn resilience by wrangling real measurements and to cultivate the broader skills that make science, and many other careers, possible. “In the process of getting a PhD, I think you pick up a lot of really amazing skills: programming, teamwork, and of course, how to finish a thing.” He also underscored the value of public speaking: “This skill of standing up in front of a bunch of people and explaining yourself is something that serves you no matter what you do.”
Equally important is recognizing the diversity of career paths. “It’s a hard career path to be an academic,” Siemens told me, recalling NANOGrav postdocs who found fulfilling work in industry. “I learned from them that there are many ways to succeed in your life and to be happy in your life. You can work at a place that is not a university, and be working on a very interesting problem surrounded by very intelligent people who are really fun to work with. Universities are not the only place where that happens.” At the center of his group’s expectations is a simple aspiration: “You should be having fun. Research doesn’t always work, and it almost never works quickly, at least in my experience, but you should be having fun as you’re doing it, at least on average.”
Above all, Siemens returns to the joy of doing science collaboratively. “I feel so very fortunate to be working on a really fun and interesting problem with a group of extremely talented humans,” he said. As the AAS meeting approaches, Siemen’s and McLaughlin’s plenary will hold a field-spanning tour of how PTAs turn the galaxy into a detector and how NANOGrav’s discovery opens a new gravitational-wave window. It will also be a chance to hear, through data and through story, what the universe has been saying, and to meet the community of kind, talented, and curious people working to translate it into our deeper understanding of the universe.
To learn more about the NANOGrav Collaboration and their results, be sure to attend the talk by Dr. McLaughlin and Dr. Xavier Siemens at 11:40 am MST on Thursday January 8th at #AAS245!
Astrobite edited by: Neev Shah
Photo credit: Xavier Siemens
