Just last year, three American physicists shared the Nobel Prize in Physics for their role in the historic detection of gravitational waves. The signals came from cosmic ripples in space-time created by some of the most violent events in the universe: colliding black holes.Scientists have now detected six gravitational-wave signals — five from merging pairs of stellar-mass black holes, and one from a merging pair of neutron stars. But strangely, most of the stellar-mass black holes involved were more than 20 times as massive as the Sun. The find perplexed astronomers. Stellar-mass black holes, which form when massive stars collapse, typically top out at about 10 to 15 times the mass of the Sun.So, how did these relatively small black holes bulk up before merging?In the past, scientists suspected these black holes grew larger because they started their lives as giant stars with very few metals — or elements besides hydrogen and helium. Since low-metallicity stars produce weak solar winds, they keep most of their mass before collapsing into black holes.But according to a new study published in The Astrophysical Journal Letters, there may be more than one way to make a black hole balloon in size — and it doesn’t involve a low-metal diet.Instead, the authors outline a way that average stellar-mass black hole can grow by gobbling up the material circling a galaxy’s supermassive black hole. Furthermore, this new mechanism also may predict a fresh source of gravitational waves.