Texas Tech University

Scientists use Exotic Stars, Tune into Hum from Cosmic Symphony

Doug Hensley

June 28, 2023

Texas Tech scientists are part of a group who discovered additional evidence of gravitational waves.

Astrophysicists using large radio telescopes to observe a collection of cosmic clocks in the galaxy have found evidence for gravitational waves that oscillate with periods of years to decades, according to a set of papers published today (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. 


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 the signal is consistent with slowly undulating gravitational waves passing through the galaxy.

The NANOGrav team includes Joseph Romano, professor in Texas Tech University's Department of Physics and Astronomy, and Nipuni Palliyaguru, a post-doctoral research fellow in the department.

“We have strong evidence for a gravitational wave signal, possibly produced by many supermassive black hole binary systems orbiting one another,” Romano said. “This is a qualitatively different type of gravitational-wave signal than what has been seen by other gravitational-wave detectors.”

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 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.

Graphic illustrating the positions in the galaxy of pulsars included in NANOGrav's 15-year data set. Pulsars are indicated by blue stars, while the position of the Earth is the yellow star. For some pulsars, locations are estimated, as they are too far and too faint for precise distances to be measured.

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

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 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. 

“It's very important in terms of its implications,” Palliyaguru said. “Discoveries like this allow us to continue learning about the universe. NANOGrav is an incredible collaboration with dedicated scientists who are committed to this work, and I'm happy to see these results coming out.”

Over the course of 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. 

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. 

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.