Astrophysicists using large radio telescopes to observe a collection of cosmic clocks in the Milky Way Galaxy –- including SUNY Oswego faculty member and planetarium director Natalie Lewandowska –- have found evidence for gravitational waves that oscillate with periods of years to decades, according to a set of papers published June 28 in The Astrophysical Journal Letters. 

The gravitational-wave signal was observed in 15 years of data acquired by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Physics Frontiers Center (PFC), a collaboration of more than 190 scientists from the U.S. and Canada who use pulsars to search for gravitational waves. International collaborations using telescopes in Europe, India, Australia and China have independently reported similar results.

While earlier results from NANOGrav uncovered an enigmatic timing signal common to all the pulsars they observed, it was too faint to reveal its origin. The 15-year data release demonstrates that the signal is consistent with slowly undulating gravitational waves passing through the galaxy that includes Earth.

“This is key evidence for gravitational waves at very low frequencies,” said Vanderbilt University’s Stephen Taylor, who co-led the search and is the current chair of the collaboration. “After years of work, NANOGrav is opening an entirely new window on the gravitational-wave universe." 

Unlike the fleeting high-frequency gravitational waves seen by ground-based instruments like LIGO (the Laser Interferometer Gravitational-wave Observatory), this continuous low-frequency signal could be perceived only with a detector much larger than the Earth. To meet this need, astronomers turned this sector of the Milky Way Galaxy into a huge gravitational-wave antenna by making use of exotic stars called pulsars. NANOGrav’s 15-year effort collected data from 68 pulsars to form a type of detector called a pulsar timing array.

As a researcher and co-chair of the noise budget working group, Lewandowska’s role primarily involves understanding and separating white noise –- noise that is not time-correlated with the signal from a pulsar –- from red noise, which is time correlated with the signal from a pulsar. 

Lewandowska specifically examines the noise contributions from pulsars which emit radio giant pulses, a very bright and irregular form of radio emission. The search for low-frequency gravitational waves that Albert Einstein predicted is essentially a search for a signal in the noise. She compared it to being somewhere and trying to separate out music playing in the background from conversational chatter.

“The gravitational wave signal is buried in noise, and you need to model and reduce it so that what remains after that is the gravitational wave signal,” Lewandowska said. “The role of our working group is to assess sources of noise and analyze them, characterizing the sources of noise. We collaborate  with other working groups in NANOGrav and the International Pulsar Timing Array (IPTA) in order to understand and implement the sources of noise into the search for gravitational waves.” 

With regard to the recent release of the 15 year data results of the NANOGrav Collaboration Lewandowska primarily worked on “The NANOGrav 15-Year Data Set: Detector Characterization and Noise Budget” paper which was released on June 28 in the Astrophysical Journal Letters from the American Astronomical Society.

“This is a paper that describes all the known sources of noise in the NANOGrav data,” Lewandowska said. Because of the interdependent nature of their findings, NANOGrav is a member of the IPTA, an international consortium of pulsar timing array collaborations, which also released their findings at the same time. 

She also co-chairs the collaboration’s education and public outreach group within NANOGrav and the IPTA, helping to disseminate these findings to the public and promote understanding of why the research is important.

Tracking pulsars

A pulsar is the ultra-dense remnant of a massive star's core following its demise in a supernova explosion. Pulsars spin rapidly, sweeping beams of radio waves through space so that they appear to “pulse” when seen from the Earth. The fastest of these objects, called millisecond pulsars, spin hundreds of times each second. Their pulses are very stable, making them useful as precise cosmic timepieces. 

Over 15 years of observations with the Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico, NANOGrav has gradually expanded the number of pulsars they observe.

“Pulsars are actually very faint radio sources, so we require thousands of hours a year on the world’s largest telescopes to carry out this experiment,” Maura McLaughlin of West Virginia University and co-director of the NANOGrav PFC explained.  “These results are made possible through the National Science Foundation’s continued commitment to these exceptionally sensitive radio observatories.”

Einstein’s theory of general relativity predicts precisely how gravitational waves should affect pulsar signals. By stretching and squeezing the fabric of space, gravitational waves affect the timing of each pulse in a small but predictable way, delaying some while advancing others. These shifts are correlated for all pairs of pulsars in a way that depends on how far apart the two stars appear in the sky. 

“The large number of pulsars used in the NANOGrav analysis has enabled us to see what we think are the first signs of the correlation pattern predicted by general relativity,” said Oregon State University’s Xavier Siemens, co-director of the NANOGrav PFC.

NANOGrav’s most recent dataset shows growing evidence for gravitational waves with periods of years to decades. These waves could arise from orbiting pairs of the most massive black holes in the entire Universe: billions of times more massive than the Sun, with sizes larger than the distance between the Earth and the Sun. Future studies of this signal will open a new window on the gravitational-wave universe, providing insight into titanic black holes merging in the hearts of distant galaxies, among other exotic sources.

Support from the National Science Foundation (NSF) has been critical to NANOGrav’s success by providing support for scientific work through the Physics Frontiers Center program and through access to multiple world-class radio telescopes. Future NANOGrav results will incorporate data from Canada’s CHIME telescope, added to the project in 2019. 

"The NSF NANOGrav team created, in essence, a galaxy-wide detector revealing the gravitational waves that permeate our universe," said 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.”

Astrophysicists around the globe have been busy chasing this gravitational-wave signal. Several papers released June 28 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.

“Our combined data will be much more powerful,” Taylor said. “We’re excited to discover what secrets they will reveal about our Universe.” 

About the NANOGrav collaboration

The NANOGrav collaboration receives support from National Science Foundation Physics Frontiers Center award numbers 1430284 and 2020265, the Gordon and Betty Moore Foundation, NSF AccelNet award number 2114721, a Natural Sciences and Engineering Research Council of Canada Discovery Grant, and the Canadian Institute for Advanced Research. The Arecibo Observatory is a facility of the National Science Foundation operated under cooperative agreement (#AST-1744119) by the University of Central Florida (UCF) in alliance with Universidad Ana G. Méndez and Yang Enterprises, Inc. The Green Bank Observatory and The National Radio Astronomy Observatory are facilities of the National Science Foundation operated under cooperative agreements by Associated Universities, Inc.

SUNY Oswego faculty member and planetarium director Natalie Lewandowska had the opportunity to meet Nobel Prize recipient and physicist Kip Thorne

SUNY Oswego faculty member and planetarium director Natalie Lewandowska had the opportunity to meet Nobel Prize recipient and physicist Kip Thorne at a June 29 news conference on the results of the North American Nanohertz Observatory for Gravitational Waves Physics Frontiers Center collaboration findings at the National Science Foundation.