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upstart writes:

After Decades of Observations, Astronomers have Finally Sensed the Pervasive Background Hum of Merging Supermassive Black Holes:

We've become familiar with LIGO/VIRGO's detections of colliding black holes and neutron stars that create gravitational waves, or ripples in the fabric of space-time. However, the mergers between supermassive black holes – billions of times the mass of the Sun — generate gravitational waves too long to register with these instruments.

But now, after decades of careful observations, astronomers around the world using a different type of gravitational wave detection method have finally gathered enough data to measure what is essentially a gravitational wave background hum of the Universe, mostly from supermassive black holes spiraling toward collision.

Scientists say the newly detected gravitational waves are by far the most powerful ever measured, and they persist for years to decades. They carry roughly a million times as much energy as the one-off bursts of gravitational waves from black hole and neutron star mergers detected by LIGO and Virgo.

"It's like a choir, with all these supermassive black hole pairs chiming in at different frequencies," said scientist Chiara Mingarelli, who worked about 190 other scientists with the NANOGrav (North American Nanohertz Observatory for Gravitational Waves). "This is the first-ever evidence for the gravitational wave background. We've opened a new window of observation on the universe."

[...] For this collaboration, 25 years of observing 25 pulsars revealed the gravitational waves with wavelengths much longer than those seen by other experiments.

[...] Since they are long-lasting, the 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.