For decades, black holes have been viewed as cosmic objects from which nothing can escape—not even light. Once matter crosses the event horizon, it is believed to be trapped forever. But a fascinating new theoretical idea is challenging this traditional picture. Researchers are now exploring the possibility that black holes may not simply fade away over time. Instead, they might eventually transform into something completely different: a white hole.
In a recent theoretical study, physicist Mattia Villani and collaborators developed a model describing what might happen when a black hole changes into a white hole. Their work suggests that this dramatic transition could release a huge amount of high-energy radiation, potentially creating detectable gamma-ray bursts across the Universe.
From Black Holes to White Holes
The idea that black holes might eventually become white holes was originally proposed within the framework of Loop Quantum Gravity. This theory attempts to unite quantum mechanics and gravity into a single description of nature.
The concept was first suggested by Carlo Rovelli and Francesca Vidotto in 2014, building on earlier work in Loop Quantum Cosmology.
According to this idea, when a massive star collapses, it forms a black hole as expected. However, instead of collapsing forever into an infinitely dense point, the matter inside eventually reaches densities close to the Planck scale, where quantum effects become extremely strong.
At that point, quantum gravity may stop the collapse and reverse it.
Rather than continuing inward, matter would suddenly rebound outward. In effect, the black hole would transform into a white hole—an object theoretically capable of ejecting matter and energy.
Time Behaves Differently Around Black Holes
One of the most surprising aspects of this theory involves time itself.
Near a black hole, gravity becomes so intense that time slows dramatically for outside observers. For someone falling with the collapsing matter, the bounce from black hole to white hole could happen very quickly.
But for an observer far away, the process could appear to take millions or even billions of years.
This means that black holes created in the early Universe might only be exploding today.
Previous calculations suggested that the lifetime of such black holes depends on the square of their original mass. Objects with masses roughly comparable to the Moon—about 10²⁶ grams—are especially interesting because they may be reaching the end of their lives around the present cosmic era.
These ancient objects are known as primordial black holes.
A Giant Fireball Could Form
If a black hole suddenly turns into a white hole and ejects matter outward, the escaping material would be extremely hot and moving at speeds close to the speed of light.
Such conditions naturally create what astronomers call a fireball.
Unlike some black hole systems that generate narrow jets of material, researchers think white-hole explosions may behave differently. Because strong background magnetic fields may be absent, a focused jet might never form.
Instead, a roughly spherical shell of matter would expand rapidly into surrounding space.
As this shell crashes into the interstellar environment, shock waves would form and release enormous amounts of energy.
What Happens Immediately After the Explosion?
Scientists still do not know exactly what matter looks like during the first moments after the explosion because a complete theory of quantum gravity remains unfinished.
However, after the earliest stage, normal particles should begin to appear.
Initially, the expanding material would likely contain:
Electrons
Positrons
Quarks
Photons
Protons
Neutrons
Quarks would rapidly combine into larger particles through a process called hadronization. Many unstable particles would decay almost immediately.
Eventually, the remaining cloud would mostly consist of photons, electrons, positrons, and a relatively small number of protons and neutrons.
Researchers believe the number of electrons and positrons would greatly exceed the number of heavier particles.
This is important because it creates a system dominated by radiation rather than ordinary matter.
Producing Light Through a Photospheric Model
To understand how such an explosion might appear, Villani used a photospheric emission model.
This model assumes that photons and electron-positron pairs continuously interact inside the expanding fireball.
Several physical processes occur simultaneously:
Pair production creates electrons and positrons from energetic photons.
Pair annihilation converts particles back into photons.
Compton scattering transfers energy between particles and radiation.
Adiabatic cooling reduces particle energies as the shell expands.
Together, these effects determine the radiation that finally escapes once the expanding material becomes transparent.
Initially, the particles follow a thermal distribution, meaning their energies are spread according to temperature.
As the system evolves, interactions reshape the energy distribution, eventually producing radiation that differs significantly from ordinary thermal emission.
Unexpected Gamma-Ray Signatures
One of the most interesting findings of the study is that the emitted radiation does not follow a simple thermal pattern.
Instead, researchers predict non-thermal spectra with very sharp energy cutoffs.
Their calculations suggest these cutoffs appear around:
1 MeV in some cases
3 MeV in others
These values fall within the gamma-ray region of the electromagnetic spectrum.
For explosions occurring at extremely large cosmological distances, however, another effect becomes important.
As light travels through an expanding Universe, its wavelength stretches because of cosmological redshift.
This would shift the emission peak toward lower energies, moving it closer to the hard X-ray region.
As a result, future telescopes searching for mysterious gamma-ray or X-ray bursts might potentially detect these events.
An Enormous Release of Energy
The study estimates that these explosions could release more than 10⁴³ ergs of energy.
To understand how enormous that number is, it represents an energy output far beyond anything humanity can produce.
Researchers also found that larger black holes would generate even stronger bursts.
Since the radiation is expected to spread in nearly all directions rather than being concentrated into jets, these explosions could potentially be visible from many viewing angles.
Can Primordial Black Holes Explain Dark Matter?
The team also investigated how many primordial black holes could exist in the Universe.
Their calculations suggest a cosmological density between approximately:
10⁻⁸ to 2.5 × 10⁻⁸
This value is much smaller than the total density required for Dark Matter.
This means primordial black holes may still exist, but they probably cannot account for all dark matter in the Universe.
The Next Step
The current model is intentionally simplified. Some physical effects remain excluded, particularly the influence of magnetic fields.
Future work will include radiation generated through processes such as synchrotron emission caused by magnetic-field amplification through Weibel instability.
If future observations detect the predicted signals, scientists could gain something extraordinary: direct evidence that black holes do not simply disappear, but may instead experience one final explosive transformation.
Such a discovery would not only change our understanding of black holes—it could provide one of the first observational windows into quantum gravity itself.
Reference: Villani, M. Electromagnetic emission from a black-to-white hole transition - Photospheric emission. Gen Relativ Gravit 58, 61 (2026). https://doi.org/10.1007/s10714-026-03564-9
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