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A firefly blinks for one ten-billionth of a second somewhere above the Siberian taiga. Seeing such a thing is impossible, but the TAIGA-HiSCORE array, spread across a valley in the Tunkinsky district of Buryatia, tried to catch exactly such flashes — not from insects, but from the depths of space.

Usually, this system of many light-sensitive detectors catches cosmic-ray particles that slam into Earth's atmosphere. But its wide field of view, covering about one steradian of sky, also allows it to spot very short flashes of visible light — lasting only ten nanoseconds. If more than three thousand photons arrive per square meter in that time, the detectors trigger.

Where could such flashes come from? One idea is the evaporation of primordial black holes: tiny objects that, according to calculations, could have been born in the first instants after the Big Bang and are now ending their lives, releasing their last bursts of energy. Another possibility is magnetic reconnection in the disks of matter around black holes, when magnetic field lines suddenly rearrange and release energy. And there is an even more exotic option: signals from distant lasers of extraterrestrial civilizations.

Over four winter seasons — from 2018 to 2022—the array found no reliable flashes. This does not mean they do not exist at all: perhaps they are too rare or too faint. But now we know an upper limit: no more than one event per thousand steradians per hour. It is as if you stared at a huge patch of sky for four years and never saw a single "firefly," but you know for sure they are not blinking every minute.

This result is an important step in charting the boundaries of the unknown. An instrument built for one task has shown it can search for something entirely different. And although the cosmic "fireflies" remain silent, the very fact that we have learned to listen for them brings the sky a little closer.