![]() They can then look for tiny deviations from that expected arrival time. The intervals are so regular that scientists can predict exactly when a pulse should arrive at Earth. "These pulses arrive at stunningly regular intervals." "Each time their beam crosses our line of sight, we see a pulse signal," says NANOGrav collaboration member Thankful Cromartie of Cornell University. Each pulsar is small, about the size of a city, but it spins hundreds of times a second, sending out beams of radio emissions that regularly sweep the sky. NANOGrav's technique relies on monitoring pulsars, which are the super-dense, spinning cores of dead stars. "That is one of the most exciting things about this project for me." "We get to hack the galaxy," says Hazboun, a member of the NANOGrav team, which has nearly 100 members from the U. So researchers decided to turn the galaxy itself into a kind of detector, by taking advantage of its existing weirdness. To catch wavelengths that long, a detector would have to have "arms" that stretched as long as half of the galaxy. But this wouldn't work to find the kind of long-wavelength gravitational waves created by supermassive black holes - the kind whose wavelength is 4 light years long, or "20 million million miles," says Hazboun. That approach worked to find gravitational waves that stretched roughly 2,000 miles long, says Jeff Hazboun, an astrophysicist at Oregon State University. When a gravitational wave rolled through and stretched space, these detectors could catch the incredibly slight change in the distance traveled by the lasers. The initial detection of those gravitational waves relied on a pair of specially-built devices, in Louisiana and Washington, that sent lasers down two 2.5-mile "arms," or tubes. ![]() That landmark discovery showed that gravitational waves truly existed, fulfilling a prediction made by Albert Einstein in 1916 and giving researchers a new way to study exotic phenomena like black holes and neutron stars. The first were seen in 2015, when a research consortium registered the waves created by the merger of two black holes that were each about 30 times as massive as the sun. Until now, scientists have only been able to detect gravitational waves created by much smaller black holes. ![]() Other research groups using telescopes in Europe, Australia, India, and China also say they're starting to see hints of these waves. "We're very happy to announce that our hard work has paid off." "We've been on a mission for the last fifteen years to find a low-pitched hum of gravitational waves resounding throughout the universe," says Stephen Taylor, a Vanderbilt University astrophysicist who serves as the chair of a team of researchers known as the North American Nanohertz Observatory for Gravitational Waves ( NANOGrav). These waves are like the ripples that move through a pond if you toss in a rock - only these waves move through the very fabric of the universe, and researchers have been eager to study them. When two galaxies merge, the enormous black holes at their centers are thought to come together and circle each other in a spinning dance that sends giant waves spiraling out. These mysterious, extremely dense objects, millions to billions of times more massive than the sun, sit at the center of galaxies like our own. The discovery means that astrophysicists may have opened a whole new window onto supermassive black holes. Scientists say they are starting to find signs of an elusive type of rumbling through space that could be created by the biggest, baddest black holes in the universe.
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