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.
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