Cosmic breakthrough uncovering evidence of low-frequency gravitational waves

More than two dozen researchers with ties to West Virginia University have helped unearth evidence of ripples in spacetime that have never been observed before now.

Gravitational waves travel outwards from a source at light speed, stretching and squeezing the very fabric of spacetime — for instance, making the length of a ruler longer or shorter, or making time tick a little faster or slower as the wave passes. The first evidence for these ripples at very low frequencies was identified by a cohort of nearly 200 scientists from the United States and Canada. These low-frequency oscillations happen with periods of years to decades and were recognized through high-precision timing of cosmic radio clocks called pulsars.

This result emerged from 15 years of data acquired by the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav, which includes many researchers from the WVU Department of Physics and Astronomy and the Center for Gravitational Waves and Cosmology.

Of the 95 authors documenting this evidence in newly published papers, 30 are associated with WVU, including 11 authors currently at the University and 19 from past years, generally those trained as undergraduate students, graduate students or postdoctoral researchers.

Maura McLaughlin, Eberly Distinguished Professor in the Department of Physics and Astronomy, and director of the Center for Gravitational Waves and Cosmology, was a founding member of the NANOGrav collaboration and serves as co-director of the NANOGrav Physics Frontier Center. She is also a member of the international team of scientists collaborating globally on the International Pulsar Timing Array project.

“NANOGrav and its international partners have discovered the first evidence for a background hum of gravitational waves,” McLaughlin said. “The universe is not static and never changing — we are awash in a space that is constantly stretching and squeezing.”

A series of papers on NANOGrav’s findings and observations has been published in The Astrophysical Journal Letters.

“The papers describe the evidence as arising from a ‘gravitational wave background,’ which refers to the indiscernible signals coming from many sources of gravitational waves in the universe,” said Sarah Burke-Spolaor, assistant professor at WVU, who founded NANOGrav’s Astrophysics Working Group in 2011.

“This can be understood by thinking about a huge orchestra tuning up their instruments: you know it’s an orchestra, and know there are many instruments, but can’t necessarily make one out specifically. We don’t yet know what is giving rise to this signal, but our publications discuss several possibilities: supermassive black holes in galaxy mergers, cosmic strings and relic echoes of the big bang.”

One popular theory is that pairs of supermassive black holes orbiting one another could be the dominant source of these low-frequency gravitational waves. The black holes are estimated to be billions of times the mass of the sun.

Supermassive black holes are believed to reside at the centers of the largest galaxies in the universe, although they have never been directly detected. When two galaxies merge, black holes from each wind up sinking to the center of the newly combined galaxy, orbiting each other as a binary system. Ultimately, the two black holes also merge. In the meantime, they stretch and squeeze the fabric of spacetime, generating gravitational waves that propagate away from their origin galaxy like ripples in a pond.

This isn’t the first gravitational wave breakthrough involving WVU. Sean McWilliams, associate professor of physics and astronomy, was part of the team in 2015 that detected gravitational waves for the first time, confirming Albert Einstein’s general theory of relativity. That finding was made possible by Laser Interferometer Gravitational-Wave Observatory, or LIGO, detectors.

But unlike the fleeting high-frequency gravitational waves seen by ground-based instruments like LIGO, the low-frequency signal observed by NANOGrav could be perceived only with a detector much larger than the Earth. To meet that need, astronomers turned a sector of the Milky Way galaxy into a gravitational wave antenna by making use of pulsars, the ultra-dense remnants of a star’s core following a supernova explosion. When observed from Earth, pulsars appear to “pulse,” making them useful as precise cosmic timepieces. NANOGrav’s effort collected data from 68 pulsars to form a type of detector called a pulsar timing array.

“Gravitational waves were not directly detected until LIGO,” said Emmanuel Fonseca, assistant professor of astronomy and NANOGrav member who was involved in the new discovery. “But it could only observe gravitational waves within a certain part of the spectrum, like seeing gravitational wave versions of optical light when there are gravitational wave versions of X-rays. What NANOGrav did was confirm the existence of gravitational waves at a completely different part of the spectrum.”

The team relied heavily on the Green Bank Telescope located in Pocahontas County — the world’s largest fully steerable radio telescope — for observation and data collection. The Arecibo Observatory in Puerto Rico and the Very Large Array in New Mexico were also utilized.

“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,” McLaughlin said. “These results are made possible through the National Science Foundation’s continued commitment to these exceptionally sensitive radio observatories.”

Burke-Spolaor said this newest discovery is a testament to the University’s expertise and dedication to the space sciences.

“WVU is one of maybe two or three universities that serve as a major hub for all branches of science contributing to NANOGrav as a galaxy-sized detector that is beginning to detect the gravitational universe,” she said. “NANOGrav involves a unique detector — essentially using pulsars as a GPS system — and requires extensive work by experts on stellar evolution, gravity, fundamental physics, galaxy evolution and black holes. We have contributors at WVU who span that range. The University’s close involvement with Green Bank Telescope has contributed a strong West Virginia-based network of scientists.”

Burke-Spolaor also applauded the skill and dedication of University students and postdoctoral researchers who contributed to the project.

Graduate student Andrew Kaiser, of Fayetteville, Arkansas, has spent much of his research experience exploring and characterizing sources of noise and signals in gravitational wave detectors. For this discovery, Kaiser conducted statistical analyses included in the published papers.

“I analyzed specific pulsars by looking at noise and timing in a very myopic way,” he said. “With that information, we can combine it with the statistics we use in our detections.”

After graduating with a bachelor’s degree from the University of Arkansas, Kaiser wound up at WVU because of its opportunities in the field of gravitational waves.

“There were really big exciting things happening in pulsar timing,” he said. “Coming to WVU to witness the triumphs of pulsar timing firsthand has been one of the biggest things for me.”

“Since coming here and joining NANOGrav, I’ve spent a lot of time thinking about using gravitational waves and multi-messenger observations to search for and study sources,” said Tingting Liu, a postdoctoral researcher originally from Nanjing, China.

Liu explained that light is a messenger of astrophysical information, while gravitational waves are another messenger. When two or more messengers combine, that produces multi-messenger observations.

The team said future studies will enable scientists to view gravitational waves through a new window along with how the universe evolved on the largest scales, providing information about how often galaxies collide and what drives black holes to merge.

Researchers also said they believe gravitational ripples of the Big Bang itself may make up a fraction of this new evidence, offering insight into how the universe was formed.

Journal: The Astrophysical Journal Letters
DOI: 10.3847/2041-8213/acdac6
Article Title: The NANOGrav 15 yr Data Set: Evidence for a Gravitational-wave Background
Article Publication Date: 29-Jun-2023

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