The Universe Is Full of 'Impossible' Black Holes. Scientists Now Know Why

Gravitational-wave detections show that black holes can form through repeated mergers inside star clusters. Stellar evolution predicts classic black holes from massive-star collapse, with masses around 10 to 40 solar masses, while supermassive black holes arise from early-universe processes. Black holes with masses between 40 and 100 solar masses are difficult to explain with conventional stellar physics because they are too heavy for stellar collapse yet not easily produced by direct collapse of massive clouds. Merging smaller ultradense objects can create these heavier remnants, but evidence was limited until gravitational-wave detectors began operating. Laser interferometers measure micro-distortions in space-time from collisions, confirming black hole mergers and revealing that such events occur frequently. Some of the heaviest black holes show signatures consistent with second-generation origins.

"Between these two extremes lies a contested category: black holes with masses between 40 and 100 solar masses. They are too heavy to be born after the death of a star, but they do not reach the necessary dimensions to emerge from the collapse of a gigantic cloud of matter. Conventional stellar physics considers them "impossible," yet they appear frequently in detections."

"Astrophysicists propose that these massive black holes could form by the merging of two or more smaller, ultradense objects. The idea was plausible, but it needed evidence. Until relatively recently, there was no way to obtain it."

"Then gravitational wave detectors came on the scene. These instruments use lasers to measure the micro-distortion of space-time generated by the collision of extremely dense objects. The first detection, in 2015, confirmed a merger between black holes. Since then, each new signal has allowed for a better characterization of these structures and revealed that these collisions occur much more frequently than previously imagined."

"Gravitational waves recorded in recent years show that some of the heaviest black holes within star clusters exhibit clear signs of being "second-generation" black holes-products of past collisions-and therefore could not have originated from the collapse of a massive star."

Read at WIRED