Black holes are not eternal: they slowly lose energy, emitting entropy — a measure of hidden information — as heat. This process was predicted by Stephen Hawking. But until now, it remained a mystery what happens at the very end, when the hole has almost disappeared and must return the captured data.
Now physicists have shown: after the main mass evaporates, a tiny white hole remains. Like a dying ember, it slowly releases information. The time for this 'cleanup' grows with the mass of the original black hole far more steeply than previously thought.
Perhaps we will be able to see the faint radiation from these remnants — maybe they are hiding under the mask of dark matter.
as written by the authors arXiv:2605.03922v1
We derive bounds on the lifetime of an evaporating black hole. The bound follows from energy conservation and purification, within the framework of `asymptotically semiclassical spacetimes'. We use the recently derived expression for the Bondi flux of Hawking radiation, together with the expression for the entanglement entropy of Hawking radiation at null infinity, to investigate the purification phase after the last semiclassical ray. We discuss the energy-cost of entanglement purification and we find a lower bound on the purification time of the black hole, which scales as $$M_0^4/\hbar^{3/2}$$, where $$M_0$$ is the initial black hole mass. Additionally, motivated by quantum gravity considerations, we include the additional assumption that a Planck mass black hole is metastable. With this assumption, we find that the the purification time is extended to be exponential in the square of the initial black hole mass, i.e. in its initial area. We find that the redshift exponent is negative in this purification phase, which indicates the existence of a white-hole remnant which releases information slowly. We comment on phenomenological implications for primordial black hole remnants.
The complete evaporation time for a black hole with the mass of a mountain exceeds the age of the Universe by billions of times. And microscopic holes from the dawn of time may be burning out right now, and their last heat could be detected.