During the first billion years after the Big Bang, as the first stars and galaxies switched on and filled the darkness with new light, they also caused another transformation: ionizing the neutral hydrogen gas filling the universe. Until recently, astronomers struggled to explain how it could happen. Ionizing hydrogen requires high-energy ultraviolet (UV) light, and the sparse, fledgling galaxies did not seem up to the task. Now, JWST, NASA’s orbiting infrared observatory, has turned that on its head. It is finding so many bright young galaxies and luminous black holes that there may be an oversupply of UV light.

“We’re beginning to call this an overproduction crisis, because it seems like we just have too many ionizing photons in the universe."