A tiny black hole — the size of an atom but as massive as a mountain. It wasn't born from a star but in the first instants after the big bang, and it has been quietly losing mass ever since, radiating heat. Toward the end of its life, it heats up so much that in seconds it releases a burst of particles comparable in energy to an entire stellar explosion. Such a 'farewell firework' was suspected in the neutrino event recorded by the KM3NeT detector. But the universe isn't a silent film: if a black hole flared up, gamma-ray telescopes and cosmic-ray catchers should have detected the accompanying glow.
The authors of the study built a picture of what different instruments would see when a black hole evaporates nearby. They accounted for where the 'gazes' of gamma-ray observatories were pointed at each moment and calculated how many particles would reach Earth. It turned out that before the main neutrino punch, there should have been a noticeable 'warm-up'—weaker neutrinos and gamma rays accumulating over hours and days. The real archives are silent: no gamma-ray bursts, no cosmic-ray showers at the right time.
This doesn't close the door on other explanations for the mysterious neutrino, but it sets a hard condition: if you're looking for a culprit among primordial black holes, you'll have to abandon the simplest model. Maybe nature added something extra to the scenario — an extra dimension or nontrivial quantum physics. Meanwhile, the KM3NeT detector keeps listening to the deep hum of the cosmos, and each new signal brings us closer to the answer, or to a new surprise.
The KM3NeT detector sits on the floor of the Mediterranean Sea. Its 'eyes'—thousands of light-sensitive spheres strung on cables — catch faint flashes of light from particles piercing the water.
In Stanisław Lem's novel 'His Master's Voice,' a mysterious neutrino signal from space forces humanity to rethink its understanding of physics — much like the KM3-230213A event.