Strongest evidence yet for low-frequency gravitational waves

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

“This is key evidence for gravitational waves at very low frequencies,” said Vanderbilt University’s Dr 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 our sector of the Milky Way 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 it 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 Dr 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) 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 Dr Xavier Siemens, Co-Director of the NANOGrav PFC.

In 2004, a small group of astronomers carried out the first set of pulsar observations that would form the foundation for this work. For nearly two decades, the group has been growing in the number of people and diversity of expertise needed to perform this complex gravitational-wave search. Along the way, the NANOGrav collaboration took form.

Initially, pulsar instrumentation was not precise enough to achieve the sensitivity needed for this experiment. The team worked to develop next-generation instrumentation for both the Arecibo and Green Bank telescopes. They scoured known pulsars to find those precise enough to enable the search for low-frequency gravitational waves and added them to the pulsar timing array. In parallel, there were advances in theory and breakthroughs in data-analysis techniques that are tuned and optimised for modern computing architectures.

Along the way, NANOGrav found many uses for their rich pulsar timing data, addressing a wide range of intriguing astrophysical puzzles. The data and NANOGrav’s methodologies are described in companion papers.

“This marks the first time we’ve released the software used to produce our data set alongside the data products themselves,” said Dr Joseph Swiggum of Lafayette College, who led the pulsar timing paper. “All the tools necessary to reproduce our results are now public, making it easier for other scientists to get involved.”

In 2020, with just over 12 years of data, NANOGrav scientists began to see hints of a signal, an extra ‘hum’ that was common to the timing behaviour of all pulsars in the array, and that careful consideration of possible alternative explanations could not eliminate. The collaboration felt confident that this signal was real, and becoming easier to detect as more observations were included. But it was still too faint to show the gravitational-wave signature predicted by general relativity. Now, their 15 years of pulsar observations are showing the first evidence for the presence of gravitational waves, with periods of years to decades.

“Now that we have evidence for gravitational waves, the next step is to use our observations to study the sources producing this hum,” said Dr Sarah Vigeland from the University of Wisconsin-Milwaukee, who is spearheading NANOGrav’s effort to determine the source of the signal. “One possibility is that the signal is coming from pairs of supermassive black holes, with masses millions or billions of times the mass of our Sun. As these gigantic black holes orbit each other, they produce low-frequency gravitational waves.”

Supermassive black holes are believed to reside at the centres of the largest galaxies in the universe. When two galaxies merge, the black holes from each wind up sinking to the centre of the newly combined galaxy, orbiting each other as a binary system long after the initial galaxy merger. Eventually, the two black holes will coalesce. In the meantime, their slow inspiral stretches and squeezes the fabric of space-time, generating gravitational waves that propagate away from their origin galaxy like ripples in a pond, eventually reaching our own.

Gravitational-wave signals from these gigantic binaries are expected to overlap, like voices in a crowd or instruments in an orchestra, producing a background ‘hum’ that imprints a unique pattern in pulsar timing data. This pattern is what NANOGrav scientists have been seeking, and what they have demonstrated in the suite of newly published papers.

Detailed analysis of the background hum is already providing insights into how supermassive black holes grow and merge. Given the strength of the signal NANOGrav sees, the population of extremely massive black hole binaries in the universe must number in the hundreds of thousands, perhaps even millions.

“At one point, scientists were concerned that supermassive black holes in binaries would orbit each other forever, never coming close enough together to generate a signal like this,” said Dr Luke Kelley of University of California, Berkeley, and Chair of NANOGrav’s astrophysics group. “But now we finally have strong evidence that many of these extremely massive and close binaries do exist. Once the two black holes get close enough to be seen by pulsar timing arrays, nothing can stop them from merging within just a few million years.”

Future investigation of this signal will feed into scientists’ understanding of how the universe evolved on the largest scales, providing information about how often galaxies collide, and what drives black holes to merge. In addition, gravitational ripples of the Big Bang itself may make up some fraction of the signal, offering insight into how the universe itself was formed. These results even have implications at the smallest scales, placing limits on what kind of exotic particles may exist in our universe.

Over time, NANOGrav expects to be able to pick out the contributions of relatively nearby, individual supermassive black hole binaries. As noted by the National Radio Astronomy Observatory’s Dr Scott Ransom, “We’re using a gravitational-wave detector the size of the galaxy that’s made out of exotic stars, which just blows my mind.

“Our earlier data told us that we were hearing something, but we didn’t know what. Now we know that it’s music coming from the gravitational universe. As we keep listening, we'll likely be able to pick out notes from the instruments playing in this cosmic orchestra. Combining these gravitational-wave results with studies of galaxy structure and evolution will revolutionise our understanding of the history of our universe.”

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 in order to better characterise the signal and search for new types of sources.

“Our combined data will be much more powerful,” said Vanderbilt University’s Dr Stephen Taylor, who co-led the search and is the current Chair of the NANOGrav collaboration. “We’re excited to discover what secrets they will reveal about our universe.”

Image caption: The supermassive black hole binaries at the cores of galaxies produce electromagnetic waves at radio to gamma-ray wavelengths that can be detected by telescopes on Earth and in space. They also produce gravitational waves that can be studied through their effects on an array of radio pulsars. These dual electromagnetic and gravitational wave messengers provide extremely valuable insights that cannot be gleaned from either type of observation alone. Image credit: Olena Shmahalo.