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While astrophysicists celebrate the second detection of ripples in spacetime (SN Online: 6/15/16), they are also looking ahead to figuring out what led to these cosmic quakes. Black holes colliding in remote galaxies sent the gravitational waves our way. But how these duos ended up in an ill-fated embrace in the first place is unknown.

With only two clear detections from the Advanced Laser Interferometer Gravitational-Wave Observatory, and a third marginal candidate, there isn’t enough information to figure out for sure how these binary black holes formed. But there are two leading ideas.

One is that two heavyweight stars, each more than roughly 20 times as massive as the sun, are born, live and detonate together. Their deaths would leave behind a pair of black holes snuggled up to one another. They would eventually spiral together in a spectacular collision (SN: 3/19/16, p. 5).

Another idea is that the black holes find each other in the hustle and bustle of a dense star cluster. Within these crowded clusters, stars and black holes gravitationally shove each other around. “My graduate student calls it a black hole mosh pit,” Frederic Rasio, an astrophysicist at Northwestern University in Evanston, Ill., said June 15 during a news briefing at a meeting of the American Astronomical Society.

Rasio and colleagues developed computer simulations that investigate how denizens of these clusters interact with one another. Black holes settle into the center of the cluster, where some get caught in another’s gravitational embrace. Continued run-ins with other wandering black holes fling these pairings from the cluster, leaving the couple to soar across the galaxy and eventually merge into a single black hole.

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