Natalie Wolchover in Quanta:
Once again, a gust of gravitational waves coming from the faraway collision of black holes has tickled the instruments of the Advanced Laser Interferometer Gravitational-Wave Observatory (Advanced LIGO), bringing the count of definitive gravitational-wave detections up to three. The new signal, detected in January and reported today in Physical Review Letters, deepens the riddle of how black holes come to collide.
Before Advanced LIGO switched on in the fall of 2015 and almost immediately detected gravitational waves from a black-hole merger, no one knew whether it would see merging black holes, merging neutron stars, black holes merging with neutron stars or none of the above. (As Albert Einstein figured out a century ago, pairs of dense, tightly orbiting objects are needed to generate ripples in the fabric of space-time, or gravitational waves.) But the three signals spotted by LIGO so far have all come from merging black holes, suggesting pairs of these ultradense, invisible objects abundantly populate the universe.
Astronomers have since been struggling mightily to understand how black holes (which, for the most part, are remnants of collapsed stars) can wind up so close to each other, without having been close enough to have merged during their stellar lifetimes. It’s a puzzle that has forced experts to think anew about many aspects of stars.
They’ll now have to think even harder.
More here. [Thanks to Jennifer Ouellette.]