A new study suggests some primordial black holes could survive much longer than scientists once believed. Instead of completely evaporating through Hawking radiation, these tiny objects may eventually settle into a stable state that resembles a hypothetical white hole.
Primordial black holes are thought to have formed in the first moments after the Big Bang, when the universe was far hotter and denser than it is today. Unlike the black holes produced by dying stars, they have never been observed, making them one of the most intriguing unresolved ideas in cosmology.
For years, researchers have assumed that the smallest black holes would eventually evaporate away through Hawking radiation, a phenomenon proposed by Stephen Hawking in the 1970s. The new research takes a closer look at the final moments of that process and arrives at a different conclusion.
The Mystery Begins At The Planck Mass
The work was led by Daniel Paraizo and his colleagues, who explored what happens when a primordial black hole shrinks to the Planck mass, about 20 micrograms. That number may sound abstract, but it corresponds to something surprisingly familiar: roughly the mass of a human eyebrow hair or a flea egg. In physics, the Planck mass is a special scale where the effects of gravity and quantum mechanics become equally important.
“We found that the lifetime of black holes is much longer than previously thought,” explained Paraizo. “The phenomena that we identify are relevant for black holes possibly formed in the early universe. These objects have not been observed yet, but their search is a topic of intense interest as dark matter candidates.”
Black holes gradually lose mass by emitting Hawking radiation, and the process accelerates as they become smaller. Yet their ultimate fate remains one of the biggest unresolved questions in theoretical physics.
A Black Hole That Refuses To Vanish
To investigate the problem, the researchers calculated how primordial black holes of different masses would evolve over time. Their results, presented in the study available on the arXivrepository, show that a primordial black hole with an initial mass of around one billion tons, about the mass of a medium-sized asteroid, would take roughly one billion years to shrink to the Planck scale.
Much smaller primordial black holes behave differently. A black hole with a starting mass of around one ton would reach the Planck-mass stage almost immediately. Earlier studies suggested that the final 20 micrograms would quickly radiate away. Paraizo noted that:
“It is then that our results predict something new: previous arguments indicated that the remaining 20 micrograms are radiated in at least 1 second; our estimate shows instead that these 20 microgram remnants are practically stable.”
If correct, that means the evaporation process may effectively stall once the black hole reaches this tiny mass, leaving behind an object that can persist rather than vanish.
What Remains After a Black Hole Evaporates?
The study proposes that something unusual happens once the black hole reaches the Planck-mass threshold. Paraizo explained that the event horizon, the boundary that prevents light and radiation from escaping, gradually disappears.
“The mechanism that we study for the death of this Planck-sized black hole is the gradual disappearance of the horizon that traps radiation,” he said.
As that process unfolds, the remnant begins emitting what the researchers call “purifying radiation.” The behavior resembles that expected from a white hole, a hypothetical object often described as the opposite of a black hole.
While black holes trap matter and radiation, white holes are theorized to expel them. He emphasized that physicists still do not understand the underlying physics of white holes, but said the object identified by the team has “exactly the same properties from far away.”
The researchers caution that many questions remain unanswered. Understanding the ultimate fate of these remnants would require a theory of quantum gravity capable of linking quantum mechanics with general relativity.
