Until recently, black holes — those celestial spheres so dense that not even light can escape their gravitational pull — only seemed to come in size small or XXL. Astrophysicists inferred the presence of small “stellar” black holes weighing up to about 50 times the mass of the sun, as well as gargantuan black holes millions or billions of times heavier that sit in the centers of galaxies.

“It’s like seeing infants and then seeing adults, but you don’t see the teenagers,” said Priyamvada Natarajan, an astrophysicist at Yale University.

Then, on May 21, 2019, midsize black holes were unambiguously detected for the first time when the U.S.-based Laser Interferometer Gravitational-Wave Observatory (LIGO) and its European counterpart Virgo captured the tremor from a pair of black holes merging in the depths of space. According to their analysis, the pair weighed 66 and 85 solar masses.

Since the finding became official in September of this year, a debate has developed. The question is how intermediate-size black holes arise. Smaller black holes might grow to middleweight by guzzling gas and dust. Or they might inflate by consuming one another, enlarging with each successive merger. “Whether one of these processes is relevant, or both, is unclear,” said Imre Bartos, a physicist at the University of Florida. The genesis of intermediate-size black holes matters because it intersects with a number of other astrophysical plotlines.

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