Scientists use exotic stars to tune into hum from cosmic symphony
Astrophysicists including a University of Michigan researcher have found evidence for gravitational waves that oscillate with periods of years to decades, according to a set of papers published in The Astrophysical Journal Letters.
The researchers made the discovery using large radio telescopes to observe a collection of cosmic clocks, exotic stars called pulsars, in our galaxy.
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 United States 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 the beginning of a golden age of gravitational wave astrophysics for supermassive black holes,” said Kayhan Gultekin, U-M astrophysicist and co-author of the set of papers. “Up until now, we have only had limits on the amount of gravitational waves produced by supermassive black holes, but now that we have evidence for the gravitational wave background, we can combine traditional astrophysical observations using light and telescopes with gravitational waves, called multimessenger astrophysics.
“When you combine these different techniques, it allows us to learn more about the evolution of supermassive black holes from very early in our universe’s history than we could by using just gravitational waves or just electromagnetic waves alone.”
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 Earth. To meet this need, astronomers turned our sector of the Milky Way Galaxy into a huge gravitational-wave antenna by making use of 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 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.
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.
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 optimized for modern computing architectures.
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 behavior 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 was 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.
Supermassive black holes are believed to reside at the centers of the largest galaxies in the universe. When two galaxies merge, the black holes from each wind up sinking to the center 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 an overall background “hum” that imprints a unique pattern in pulsar timing data. This pattern is what NANOGrav scientists have been seeking for almost 20 years. In its suite of newly published papers, NANOGrav demonstrates evidence for this gravitational-wave background.
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.
Gultekin was a key part of the paper that examined whether the scientists could explain the gravitational wave background with supermassive blackhole binaries.
“Here we were using everything we know about astrophysics, using what we know about galaxy evolution and galaxy merging to form supermassive black hole binaries, and how these binaries would tighten and how close they would get together before gravitational waves would really pull them together, to come up with the population of supermassive black hole binaries across the universe that best match the data,” Gultekin said. “We were able to show that massive black hole binaries can produce a gravitational wave background very similar to what we see.
“Furthermore, in order to match the data, we need a population of supermassive black holes that are, on average, more massive than your garden-variety supermassive black holes, something that was hinted at in NANOGrav’s previous results.”
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.
Astrophysicists around the globe have been busy chasing this gravitational-wave signal. Several papers released Wednesday 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 characterize the signal and search for new types of sources.
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 (NSERC) Discovery Grant, and the Canadian Institute for Advanced Research (CIFAR). The Arecibo Observatory is a facility of the National Science Foundation operated under cooperative agreement (#AST-1744119) by the University of Central Florida 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.
This research was supported in part through computational resources and services provided by Advanced Research Computing, a division of Information and Technology Services at U-M.