Long-lived black holes: how their slow boiling is heard in a radio signal from the past ⚡ экспресс
We investigate the impact of primordial black holes with the 'memory burden' effect on the global 21 cm signal during cosmic dawn. The memory effect, caused by the back-reaction of radiation on horizon microstates, slows the evaporation of low-mass black holes (M<10^15 g) and extends their lifetimes. This alters the energy injection rate into the intergalactic medium, thereby changing the thermal and ionization history of neutral hydrogen. By computing the modified energy input, we derive constraints on the dark matter fraction f_PBH from the amplitude of the 21 cm absorption signal at z≈17. For a slow transition into the memory-dominated phase (width δ=10^-2), black holes with masses 10^8–10^13 g are excluded for f_PBH≳10^-8. For a fast transition (k=1), evaporation is suppressed so effectively that no meaningful constraints exist for masses M≳10^7 g.
Tiny black holes, born in the first moments after the Big Bang, should have disappeared long ago according to Hawking's theory — evaporated like a puddle of water under the sun. But new calculations show evaporation isn't that fast. The hole seems to choke on its own radiation — like a boiling pot covered by a cloud of steam, slowing the boil. This 'memory burden' effect turns an instantaneous flash into a long, slow fading.
Such 'underboiled' holes spent eons warming the surrounding hydrogen in the young universe. Hydrogen atoms, feeling the heat, change their radio voice — 21-cm radiation. Astronomers catch this ancient signal and see it's too cold. If black holes were more than a tiny fraction, they would have heated the gas more, distorting the radio picture. So these objects cannot be dark matter — their number is less than one ten-millionth of the required amount.
🎯 The most sensitive tool for hunting dark matter isn't a giant underground detector, but an ordinary radio telescope listening to the echo of hydrogen from the time before the first stars lit up.