When the first gravitational wave (GW) was detected back in 2015, scientists said they had opened a new window into the Universe. While most of astronomy is based on detecting electromagnetic energy, GW are different. They're ripples in spacetime predicted by Einstein. GW detectors have let us detect mergers between black holes (BH), which emit GW when they collide. Astronomers can use these waves to determine the masses of the black holes. There have been hundreds of GW detections now, and collectively, they're like a census of BH masses. Astrophysical theory shows that massive stars between about 50 and 130 solar masses should collapse and become black holes. So there should be black holes detectable in this range. But gravitational wave observations show that stellar BH with more than about 45 solar masses are extremely rare. It's called the Forbidden Gap. What can account for this? New research in Nature may have figured it out.…