Could Supermassive Black Holes Explain Our Universe's Gravitational-Wave 'Hum'?

"Earlier this year, after 15 years of searching, scientists finally heard the background hum of low-frequency gravitational waves that fill our universe," writes Space.com. "Now, the hard work of searching for the source of these ripples in spacetime can begin."Currently, the primary suspects in this case are pairings of supermassive black holes with masses millions, or even billions, of times that of the sun. However, that doesn't mean that there isn't room for a few unusual suspects, which could potentially point us toward new physics.... [G]ravitational waves detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) express wavelengths that are thousands of miles (or km) in length and hold frequencies of milliseconds to seconds. The new gravitational waves detected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav), by contrast, have wavelengths on a scale of trillions of miles (or km). This is similar to the distance between the sun and its neighboring star, Proxima Centauri, a staggering 20 light-years in length. Plus, NANOGrav gravitational wavelengths have frequencies on scales of years instead of mere seconds. Practically, what this means is scientists need to build over 15 years of NANOGrav data to confirm a low-frequency gravitational wave detection. But, when it happens, it's worth the wait. That's because these results have the capacity to point us toward new information about our universe... "The detection of low-frequency gravitational waves means they're from very different sources to the LIGO and Virgo sources, which are stellar mass black holes and neutron star mergers," Scott Ransom, a National Radio Astronomy Observatory astronomer and former chair of NANOGrav, told Space.com... Ransom is part of a collaboration of researchers that believe low-frequency gravitational waves, including those detected by NANOGrav, may originate from a pretty incredible source. They could come from, the team argues, hundreds of thousands of supermassive black hole pairings that, over the 13.8-billion-year course of cosmic history, came close enough together that they've merged... "For many decades, theorists have hypothesized that supermassive black hole binaries should produce a signal with characteristics just like what NANOGrav and other pulsar timing arrays are seeing," Luke Zoltan Kelly, a Northwestern University theoretical astrophysicist and NANOGrav researcher, told Space.com. "For most of the community, supermassive black hole binaries are a natural best guess for what's producing the gravitational wave background...." Zoltan Kelley pointed out to Space.com that besides binaries, there are a number of new models in cosmology and in particle physics that, under the right circumstances, could also produce a similar gravitational wave background to that detected by NANOGrav. For example, axion or 'fuzzy' dark matter, cosmic strings, inflationary phase transitions, and many others," the Northwestern astrophysicist said. "What's really exciting about these possibilities is that each of these models is an attempt to explain some of the biggest current mysteries of our universe."

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