Scientific American reports:

After nearly two decades of listening, astronomers are finally starting to “hear” the rumbles of gravitational waves they believe emanate from the behemoths of our universe: supermassive black holes.

The result comes from a National Science Foundation–sponsored initiative known as the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Since 2004 NANOGrav has monitored metronomelike flashes of light from a Milky Way–spanning network of dead stars known as pulsars. Forged from the hearts of exploding massive stars, these city-size orbs weigh as much as an entire sun and can spin thousands of times per second. This makes them remarkably accurate timekeepers—and ideal sentinels for the especially large ripples in spacetime predicted to emerge from merging supermassive black holes.

Such gravitational waves are distinct from the kinds that were previously reported from the Laser Interferometer Gravitational-Wave Observatory (LIGO) and other Earth-based detectors. For one thing, the waves spotted via pulsars wouldn’t all be traceable to individual merger events: they would form the so-called gravitational-wave background, the ambient rustling of spacetime built up from cumulative mergers throughout the cosmos. Another important distinction is that in their crest-to-trough span, each of these waves should be approximately the size of our solar system—which counterintuitively makes them much harder to detect. Washing over pulsar-strewn space, these gargantuan swells in spacetime could betray their presence via minuscule offsets to the dead stars’ spins, allowing observers to glimpse them through painstaking measurements. In a collection of five papers released today, that is essentially what NANOGrav claims to have done.

“It’s incredibly exciting because we think we’re starting to open up this new window on the gravitational-wave universe,” says Sarah Vigeland, an astrophysicist at the University of Wisconsin–Milwaukee and a member of NANOGrav. [Continue reading…]