Now that we can detect gravitational waves, a new generation of stargazers is turning its attention to the Universe and using observatories that can be disrupted by a passing rabbit. Gravitational-wave detectors are going to give us an entirely new view of our place in the cosmos.

As with all new techniques, we are still in the age of crude and not-very-sensitive. That means we can only search for the biggest and baddest of events: black-hole mergers. So far, LIGO has detected three and a half mergers—the third merger is right on the edge of the detection limit, so it is provisional. But those black holes have been larger than expected, which raises an intriguing question: is that their first merger, or have they grown through previous mergers?