NANOGRAV’s 15-year journey reveals a cosmic hum

Gravitational waves play a cosmic symphony as they pass through our galaxy. Today the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) Physics Frontiers Center released the results of 15 years of data in a set of papers published in The Astrophysical Journal Letters. This research is the first evidence of gravitational waves at very low frequencies. The team, comprised of 190 scientists, including SETI Institute researcher Dr. Michael Lam, transformed our region of the Milky Way Galaxy into an immense gravitational-wave antenna using pulsars. NANOGrav’s endeavor involved collecting data from 68 pulsars, fashioning a pulsar timing array—a distinctive type of detector.

“It’s incredibly exciting to have helped open a new window to the Universe,” said Lam.

A pulsar is a rotating neutron star, the remnant of a supernova explosion, with regular pulses of radiation. The pulsar’s intense magnetic field funnels jets of particles along the magnetic poles, producing a beam of radiation, like a beacon or lighthouse. If we are situated along the line of sight of the beams, we see a burst, or pulse of radiation as the pulsar spins. This pulse rate is incredible precise, making pulsars the ultimate cosmic timepieces.

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NANOGrav examined 15 years of data from three radio observatories: Arecibo Observatory in Puerto Rico, the Green Bank Telescope in West Virginia and the Very Large Array in New Mexico.

Albert Einstein’s theory of general relativity predicted the precise impact of gravitational waves on pulsar signals. By stretching and compressing the fabric of spacetime, these waves exert a discernible influence on the timing of each pulsar pulse, minutely delaying some while advancing others. The timing shifts are correlated among all pairs of pulsars, with the correlation pattern contingent on the angular separation of the two stars in the sky.

“The extensive array of pulsars analyzed by NANOGrav has empowered us to witness the initial signs of the correlation pattern foreseen by general relativity,” states Dr. Xavier Siemens, co-Director of the NANOGrav PFC.

In 2020, with over a decade of data, NANOGrav scientists detected hints of an additional enigmatic “hum” in the timing behavior of all the pulsars in their array. After exploring alternative explanations, they grew confident in the authenticity of this signal. Its detection became increasingly feasible with more extensive observations. However, at that stage, the gravitational-wave signature predicted by general relativity remained too faint to emerge. After fifteen years of pulsar observations, the evidence of gravitational waves, with periods spanning years to decades, emerges prominently.

Dr. Sarah Vigeland of the University of Wisconsin-Milwaukee, leading NANOGrav’s efforts to unveil the source of the signal, asserts, “With the confirmation of gravitational waves, our next objective is to employ these observations to scrutinize the sources generating this celestial hum. One possibility is that the signal emanates from pairs of supermassive black holes, each with masses surpassing millions or billions of times that of our Sun. As these colossal black holes orbit each other, they produce low-frequency gravitational waves.”

Scientists believe supermassive black holes reside at the centers of almost every large galaxy. When two galaxies merge, the supermassive black holes inevitably descend toward the center of the newly formed galaxy, coming together in a gravitational dance that lasts millions or billions of years. Eventually, these binary pairs of black holes will themselves merge, but until then, their gradual inspiral stretches and compresses the very fabric of spacetime, generating gravitational waves that ripple outward at the speed of light from their celestial abode, akin to ripples on a serene pond, eventually reaching our cosmic neighborhood.

The gravitational-wave signals from these colossal binaries overlap, akin to a harmonious crowd or an enthralling orchestral performance, producing an all-encompassing background “hum” that etches a distinctive pattern into pulsar timing data. This precise pattern has been the ultimate pursuit of NANOGrav scientists for nearly two decades. The suite of newly published papers by NANOGrav establishes the first evidence of this gravitational-wave background.

An in-depth analysis of this background hum is already unraveling the mysteries surrounding the growth and merging of supermassive black holes. The strength of the detected signal suggests that the population of such immensely massive black hole binaries in the universe amounts to hundreds of thousands, perhaps even millions.

Further investigation of this signal will enhance scientists’ comprehension of the universe’s evolution on a grand scale, providing insights into the frequency of galactic collisions and the mechanisms driving black hole mergers. Additionally, gravitational ripples stemming from the Big Bang may constitute a fraction of this signal, offering glimpses into the universe’s formation. Furthermore, these results hold implications even at the smallest scales, establishing boundaries on the existence of exotic particles within our cosmic domain. Michael Cavagnero, Program Director of NSF’s Physics Frontiers Centers, remarks, “This milestone represents a significant achievement in NSF’s multifaceted endeavor to exploit gravitational wave signals and gain deeper insights into astrophysical phenomena.”

Over time, NANOGrav anticipates distinguishing the contributions of relatively nearby individual supermassive black hole binaries. Dr. Scott Ransom from the National Radio Astronomy Observatory exclaims, “We are utilizing a gravitational-wave detector spanning the entire galaxy, crafted from exotic stars, and it simply astounds me. Our earlier data hinted at something extraordinary, but we were unaware of its true nature. Now we comprehend that it is the symphony of the gravitational universe. As we continue to listen, we will likely discern the melodies played by the cosmic orchestra. Integrating these gravitational-wave findings with studies of galaxy structure and evolution will revolutionize our understanding of the universe’s history.”

The set of papers is as follows:

  • Obervations and Timing of 68 Millisecond PulsarsDOI: 10.3847/2041-8213/acda9a
  • Detector Characterization and Noise Budget, DOI: 10.3847/2041-8213/acda88
  • Evidence for a Gravitational Wave Background, DOI: 10.3847/2041-8213/acdac6
  • Astrophysical Interpretation of a Gravitational Wave Background from Massive Black Hold Binaries (accepted for publication in ApJL)
  • Search for Signals from New Physics, DOI: 10.3847/2041-8213/acdc91
  • Bayesian Limits on GWs from Individual SMBHBs (accepted for publication in ApJL_

Support from the National Science Foundation (NSF) has been critical to NANOGrav’s success by providing support for scientific work through the Physics Frontiers Center program and through access to multiple world-class radio telescopes. Future NANOGrav results will incorporate data from Canada’s CHIME telescope, added to the project in 2019.

“The NSF NANOGrav team created, in essence, a galaxy-wide detector revealing the gravitational waves that permeate our universe,” says NSF Director Sethuraman Panchanathan. “The collaboration involving research institutions across the U.S. shows that world-class scientific innovation can, should and does reach every part of our nation.”

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 characterize the signal and search for new types of sources. “Our combined data will be much more powerful,” says Taylor. “We’re excited to discover what secrets they will reveal about our Universe.”

IMAGE CREDIT: Aurore Simonnet for the NANOGrav Collaboration.

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