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In 2020, after more than 12 years of gathering data, the NANOGrav team released results from monitoring the timing of 45 pulsars. Even then, Dr. Siemens said, the researchers saw tantalizing hints of a gravitational-wave background, but they needed to track more pulsars for longer amounts of time to confirm that they were indeed correlated, and to claim a discovery. So the NANOGrav team approached colleagues through the International Pulsar Timing Array — an umbrella organization that includes collaborations based in India, Europe, China and Australia — and coordinated an effort to uncover the gravitational-wave background together.
Fast-forward to Wednesday: Each collaboration is now publishing results from independently collected data, all of which support the existence of a gravitational-wave background. The NANOGrav team has the largest data set, with 15 years of measurements from 67 pulsars, each monitored for at least three years.
The findings carry a confidence level in the range of 3.5- to 4-sigma, just shy of the 5-sigma standard generally expected by physicists to claim a smoking-gun discovery. That means the odds of seeing a result like this randomly are about 1 in 1,000 years, Dr. Mingarelli said. “That’s good enough for me, but other people want once in a million years,” she said. “We’ll get there eventually.”
Marcelle Soares-Santos, an astrophysicist at the University of Michigan who was not involved in the work, acknowledged that while this was early evidence, the results were enticing. “This is something that the community has been anticipating for quite a while,” she said, adding that independent measurements from other pulsar timing collaborations strengthened the findings.
Still, Dr. Soares-Santos said, it was too soon to tell what impact a gravitational-wave background might have on future research. If the signal really was from the slow, inward spiraling of supermassive black holes, as many NANOGrav collaborators believe, it would augment what scientists understand about the way early galaxies merged, forming ever-larger systems of stars and dust that eventually settled into the complex structures observed today.
But if the ripples originated with the Big Bang, they might instead provide insight into the expansion of the cosmos or the nature of dark matter — the invisible glue scientists think holds the universe together — and perhaps even reveal new particles or forces that once existed. (Experts noted that the gravitational-wave background could also originate from multiple sources, in which case the challenge would be to disentangle how much comes from where.)
The NANOGrav team is already working on analyzing all the data from gravitational-wave collaborations around the world, equaling around 25 years’ worth of measurements from 115 pulsars. These results will be unveiled in a year or so, Dr. Siemens said, adding that he expected them to exceed the 5-sigma discovery level.
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