Black Holes with Memory: Forgetfulness as the Engine of Dark Matter express
Black holes lose mass like water from a bucket in the sun — an evaporation discovered by Stephen Hawking. But tiny primordial black holes, born right after the Big Bang, behave differently: once they lose half their mass, a memory effect kicks in — as if the bucket covers itself with a lid. Jacob Bekenstein calculated this measure of disorder, entropy, for black holes.
Previously, it was thought the lid snaps shut instantly. Now physicists have modeled a smooth transition: the evaporation rates can add up or multiply. Comparing with data on the birth of light elements in the first minutes after the Big Bang showed that with addition, up to 10% of holes survive and could make up dark matter. With multiplication, only 1%.
A detail that changes the picture: a tiny black hole with the mass of a mountain and the size of a proton can survive to this day solely thanks to this memory. The slightest nuance in 'forgetfulness' determines whether it's the invisible foundation of the universe.
The smallest primordial black hole could weigh as much as a mountain, yet be the size of a proton, and it's still alive today thanks to the memory effect.