How to spot invisible black holes at the universe's peak age?

Astronomers propose using line-intensity mapping—essentially a cosmic blur where you add up faint signals from millions of galaxies—to detect black holes that individual telescopes can't see. By targeting a specific ultraviolet emission line called [Ne V], they can measure the total power output from accreting black holes and trace their growth when the universe was only 2–3 billion years old. This method reveals hidden black holes and constrains how these objects seed and grow.