A tiny black hole zips through a gas cloud. Instead of swallowing the gas, it heats it up, like an iron left on a damp cloth. Radiation streams from the black hole, creating a hot spot — a region where the temperature is hundreds of times higher than in the surrounding space.

Previously, it was thought that such spots could live almost forever, cooling slowly according to a single law. But the new work takes into account that the universe is expanding. Expansion acts like a fan: it carries heat away faster than it would spread on its own. A hot spot first loses temperature sharply, then cools following a steeper law — not like in a static world.

In the end, all hot spots disappear in a finite time. This is an important conclusion: if such spots really existed in the early universe, their traces could have survived to this day only under special conditions. Now it is clear that we need to look for them differently — not as eternal beacons, but as short flashes that went out long ago.