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A culmination of 15 years of data, observed by over 190 scientists from the US and Canada, has resulted in a remarkable discovery: the first evidence for gravitational waves at very low frequencies. UF astrophysicist Laura Blecha and graduate student Analis Evans contributed their expertise to the continent-wide collaboration, and the results were published today as a set of papers in The Astrophysical Journal Letters.

With the support of the National Science Foundation, the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Physics Frontiers Center uses a network of radio telescopes to transform a collection of millisecond pulsars into a galaxy-scale gravitational-wave detector.

“The NSF NANOGrav team created, in essence, a galaxy-wide detector revealing the gravitational waves that permeate our universe,” says NSF Director Sethuraman Panchanathan. “The collaboration involving research institutions across the U.S. shows that world-class scientific innovation can, should, and does reach every part of our nation.”

Much like the harmonious blend of musical notes, gravitational waves create a cosmic rhythm. Their cosmic vibrations shake the fabric of space and time as they traverse the vast expanse of the Universe. From the birth of stars to supernovae explosions, the symphony of the cosmos resounds, carrying crucial insights into cosmic events. Astronomers have listened for the telltale signs of these waves, just as musicians listen for the subtleties in a concerto, using sophisticated instruments that help capture these elusive vibrations.

Astrophysicists around the globe have been busy chasing this gravitational-wave signal. Several papers released today by the Parkes Pulsar Timing Array in Australia, the Chinese Pulsar Timing Array, and the European Pulsar Timing Array/Indian Pulsar Timing Array report hints of the same signal in their data. Through the International Pulsar Timing Array consortium, regional collaborations are working together to combine their data to better characterize the signal and search for new types of sources.

At the heart of this cosmic symphony are pulsars, compact stars that emit regular pulses of radio waves at precise intervals. These celestial metronomes, renowned for their reliable timekeeping, play a crucial role in detecting and analyzing deviations in the arrival times of pulsar intervals.

According to Einstein’s theory of general relativity, gravitational waves have a specific impact on pulsar signals: They distort the fabric of space in predictable shifts, altering the timing of pulses by causing some to be delayed and others more advanced. Timing deviations for any two pulsars also depend on how far apart the two stars appear in the sky.

“NANOGrav’s 15-year data set reveals evidence for a gravitational-wave signal consistent with Einstein’s theory,” said Blecha.

Blecha, a theoretical astrophysicist and active NANOGrav member, has contributed to the astrophysical interpretation of pulsar timing array data by focusing on modeling supermassive black holes and galaxy evolution. In addition to analyzing the 15-year data set, she played a key role as a writer of the astrophysical interpretation paper released today. As the NANOGrav Equity and Climate Committee co-chair, she actively contributes to the organization’s efforts in addressing equity-related matters.

Using large simulations of galaxies and the cosmos, Blecha’s research group advances our understanding of black hole formation and growth, their interactions with the galaxies they inhabit, and the cosmic choreography that unfolds when galaxies merge. To model the evolution of supermassive black hole binaries emitting gravitational waves, the NANOGrav collaboration developed a simulation software called “holodeck.” Graduate student Analis Evans conducted tests on the software and built models based on cosmological simulations, which will enhance the software’s predictive capabilities.

“Interpreting this signal is one giant puzzle,” said Evans. “A lot of science is packed into fitting a theoretical gravitational wave background spectrum to the observational evidence.”

While the gravitational-wave background unveiled in the 15-year data set could stem from several physical processes, Blecha believes that a promising candidate is a population of supermassive black hole binaries scattered throughout the cosmos. These pairs arise when galaxies merge. So far, it’s been difficult to find evidence of these objects with the limitations of existing telescopes. However, the team’s analysis suggests that the observed result aligns with a gravitational-wave background generated by a population of these binaries.

“If future data can confirm that the signal comes from supermassive black hole binaries, it would be the first conclusive proof of their existence,” said Blecha.

Moreover, according to Blecha, if these binaries are the signal’s origin, the analysis indicates that these celestial duos may be more numerous or massive than previously thought. As researchers continue to enhance the sensitivity of data collection, more detailed insights will unlock new avenues for discovery.

The University of Florida has a rich legacy of involvement in gravitational-wave science, with significant contributions that shaped the field. Professor Steve Detweiler proposed the idea of using pulsars as detectors in 1979, setting the first limits on the gravitational-wave background. His profound impact on gravitational-wave science endures as UF’s quest to unravel the mysteries of black holes and gravitational waves continues.

“The era of low-frequency gravitational-wave astrophysics is just beginning, and there’s lots of exciting science to come from pulsar timing arrays,” said Blecha.

Read more about the results here. The researchers, including Blecha, will host a live briefing about their discovery on Thursday at 1 p.m. EST.