Primordial black holes, hypothetical objects that emerged in the first instants after the Big Bang from the collapse of superdense fluctuations, should have vanished long ago according to Stephen Hawking's classic prediction: they would completely evaporate via quantum radiation, much like a puddle of water disappearing under the Sun's rays. However, new calculations show that evaporation is not so rapid. When a black hole radiates, the particles it produces don't fly away unimpeded — some interact with the black hole's own gravitational field, creating a feedback effect that scientists call the 'memory burden.' It's like a boiling pot with a thick cloud of steam hovering above; the steam shields the surface, slowing further boiling. As a result, an instantaneous flash of annihilation stretches into a long, slow fade that lasts far longer than previously assumed.
These 'under-boiled' black holes, whose lifetimes rival cosmological epochs, warmed the surrounding hydrogen in the young Universe for eons. Atoms of neutral hydrogen, absorbing this heat, change their 'radio voice'—the characteristic emission at a wavelength of 21 centimeters that arises from spontaneous transitions between hyperfine levels of the ground state. Astronomers detecting this ancient signal from the era before the first stars formed find it too cold: the gas temperature is lower than scenarios with an abundance of such black holes predict. If black holes made up even a tiny fraction of the total dark matter density, they would have heated the gas more strongly, distorting the observed radio picture. Consequently, these objects cannot constitute a significant part of dark matter: their total mass is capped at one ten-millionth of what would be needed to explain the hidden mass of galaxies.
The most sensitive instrument for hunting dark matter isn't a giant underground detector: it's an ordinary radio telescope listening to the echo of hydrogen from a time before the first stars ignited.