Published: June 28, 2023

Illustration showing two merging black holes creating undulations in the fabric of space and time

Artist's depiction of two merging black holes generating gravitational waves, or ripples in the fabric of space and time. (Credit: NANOGrav)

This story was adapted from a version published by the NANOGrav collaboration. Read the original story here.

Astrophysicists using large radio telescopes to observe a collection of cosmic clocks in our galaxy have found evidence for gravitational waves that oscillate with periods of years to decades, according to a set of papers published today 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, a collaboration of more than 190 scientists from the U.S. and Canada who use pulsars to search for gravitational waves. The collaboration includes several researchers from CU Boulder. 

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.

“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." 

Joseph Simon, a postdoctoral researcher in the Department of Astrophysical and Planetary Sciences (APS) at CU Boulder, said the collaboration’s discoveries could give scientists a new look at the evolution of the cosmos—including the lifecycle of what researchers call “supermassive” black holes.

“These massive black holes lie at the center of galaxies and form binaries following galaxy mergers,” said Simon, a member of the NANOGrav team and co-author of the new studies. “Little is known about the delicate dance that these colossal objects take over hundreds of millions of years spiraling ever closer together until they eventually merge.”

Julie Comerford, professor in APS, is working on using the gravitational wave background that NANOGrav detected to study the physics of how supermassive black hole binaries combine.

Maggie Huber, a doctoral student in APS who has contributed to the NANOGrav findings, added:

“It’s amazing to be part of a huge, collaborative research effort making major discoveries in astrophysics,” Huber said. “The gravitational wave community is full of incredible researchers, and I would definitely keep my eyes peeled for their insights on this discovery.”

Unlike the fleeting high-frequency gravitational waves seen by ground-based instruments like the Laser Interferometer Gravitational-wave Observatory (LIGO), this continuous low-frequency signal could be perceived only with a detector much larger than the Earth. To meet this need, astronomers turned our 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.

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 the collaboration observes.

“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,” said Maura McLaughlin of West Virginia University and co-Director of the NANOGrav PFC. “These results are made possible through the National Science Foundation’s (NSF’s) continued commitment to these exceptionally sensitive radio observatories.”