An Enormous Gravity ‘Hum’ Moves Through the Universe

Astronomers have found a background din of exceptionally long-wavelength gravitational waves pervading the cosmos. The cause? Probably supermassive black hole collisions, but more exotic options can’t be ruled out.

Jonathan O’Callaghan at Quanta Magazine: Astronomers have found an extra-low hum rumbling through the universe. The discovery, announced today, shows that extra-large ripples in space-time are constantly squashing and changing the shape of space. These gravitational waves are cousins to the echoes from black hole collisions first picked up by the Laser Interferometer Gravitational-Wave Observatory (LIGO) experiment in 2015. But whereas LIGO’s waves might vibrate a few hundred times a second, it might take years or decades for a single one of these gravitational waves to pass by at the speed of light.

The finding has opened a wholly new window on the universe, one that promises to reveal previously hidden phenomena such as the cosmic whirling of black holes that have the mass of billions of suns, or possibly even more exotic (and still hypothetical) celestial specters.

“It’s beautiful,” said Chiara Caprini, a theoretical physicist at the University of Geneva and CERN in Switzerland who was not directly involved in the work. “A new era in the observation of the universe has opened up.”

The results come from studies that stretch back more than a decade by four teams based in the U.S., Europe, Australia and China. Today, in a coordinated data release, the teams present evidence for a background “hum” of gravitational waves that were detected by tracking changes in the impossibly regular beats of objects called pulsars.

As long-wavelength gravitational waves pass through our cosmic neighborhood, they distort the space-time around us, which changes the arrival time of a pulsar’s pulses. Researchers had to map the correlations of these arrival times across dozens of different pulsars for decades in order to pick up the signal. “I had butterflies when I first saw this,” said Stephen Taylor, an astrophysicist at Vanderbilt University and chair of the team known as the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav. “I’m so excited we can finally talk about it.”

Most likely, the gravitational waves come from pairs of supermassive black holes that are spiraling around each other inside merging galaxies. But we might be seeing something else entirely, perhaps something exotic such as ruptures in space-time itself resulting from loops of energy called cosmic strings.

“Finding for the first time the suggestion of background gravitational waves is fascinating,” said Juan García-Bellido, a theoretical cosmologist from the Autonomous University of Madrid who was not involved in the work. “It’s really Nobel Prize-winning research.”

More here.

Jonathan O’Callaghan is a freelance space and science journalist based in London. He writes regularly for a number of publications including The New York Times, Scientific American, New Scientist, Forbes and Wired.

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