When the first stars ignited, their ultraviolet light began stripping electrons from hydrogen atoms. The universe transformed from a neutral fog into transparent plasma. This process, cosmic reionization, proceeded unevenly: slowly at first, like a fire catching in damp wood, then faster as the flames spread through dry tinder.

Until now, calculations typically used a symmetric curve, as if the fog dissipated equally smoothly at the beginning and end. The new work replaces it with the Gompertz curve, originally devised to describe population growth: a slow start, a sharp rise, and a plateau. This approach better reflects the physics. Initially, there are few ultraviolet sources, and only when they become abundant does the process accelerate.

This allows for a more precise determination of the sum of neutrino masses — particles that barely interact with matter. The new data hint at a possible discrepancy with laboratory neutrino oscillation experiments. This discrepancy might have been hidden before due to the crude reionization model.

Thus, a growth curve invented for populations helps us peer into the universe's infancy and weigh its most elusive particles more accurately.