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Unlike stellar black holes, which form from the collapse of massive stars, primordial black holes would have formed from density fluctuations in the early universe. Their existence remains unconfirmed, but they are attracting attention as possible candidates for dark matter.
The team of Daniel Paraizo, from the Eberly College of Science, studied their evolution taking into account Hawking radiation, the process by which black holes lose mass in the form of evaporation. The smaller a black hole, the faster it evaporates.
Mathematical calculations show that a primordial black hole with an initial mass of about 1.1 billion US tons (1 billion metric tonnes) would take a billion years to reach the Planck mass, about 20 micrograms. At this stage, previous studies predicted rapid evaporation within a second. But the new results indicate that these 20 micrograms become practically stable and the black hole then behaves like a white hole. This change in behavior is a surprise: the event horizon, the boundary that traps light, gradually disappears.
Primordial black holes could therefore still exist today in the form of white holes.
White holes are the exact opposite of black holes: instead of attracting, they endlessly repel matter and radiation. This transition to an object with white hole properties opens new perspectives for understanding the fate of primordial black holes. However, to predict their ultimate destiny, a theory of quantum gravity would be needed, unifying general relativity and quantum mechanics. This "theory of everything" remains to be discovered despite decades of research.
Their study, available on arXiv, adds a new piece to the edifice of theoretical physics.
Hawking radiation
Proposed by Stephen Hawking in 1974, this theoretical radiation is an emission of particles due to quantum effects near a black hole's horizon. It causes a slow loss of mass until complete evaporation.
The smaller the black hole, the more it radiates. For stellar black holes, this process is extremely slow, but much lighter primordial black holes could evaporate in a few billion years.
Planck mass
The Planck mass (about 20 micrograms) is a fundamental unit in physics where quantum and gravitational effects become equally important. It represents the upper mass limit for an elementary particle: beyond it, any object would collapse into a microscopic black hole.
In everyday life, 20 micrograms is equivalent to the mass of a human eyelash or a flea egg. It is at this scale that primordial black holes could stabilize into white holes.