Black holes are often imagined as enormous objects formed when massive stars collapse. But physics allows something far stranger: a black hole so small that its initial mass could be comparable to that of a loaded truck.
A new theoretical study suggests that, under the right conditions, a black hole weighing only about 40 metric tons could form inside a compact star and continue growing rather than disappearing. The surprising ingredient that could make this possible is dark matter.
The idea connects three mysterious areas of modern physics: black holes, dark matter and the extreme environments inside neutron stars and white dwarfs.
Why Tiny Black Holes Should Disappear
In 1974, physicist Stephen Hawking showed that black holes are not completely black. Because of quantum effects near their event horizons, they can slowly release energy in the form of what is now called Hawking radiation.
As a black hole loses energy, it also loses mass. And the smaller the black hole becomes, the faster this process occurs.
This creates a major problem for extremely small black holes. A primordial black hole with a mass of roughly 10¹² kilograms would have an evaporation lifetime comparable to the age of the universe. Smaller black holes would generally be expected to have disappeared long ago if they were isolated.
So how could a black hole weighing only a few dozen tons survive?
The answer proposed by the study is that it does not have to be isolated.
Instead, it could be born inside a star.
Dark Matter Could Create a Black Hole Inside a Star
Neutron stars and white dwarfs are incredibly dense objects. Their powerful gravity could potentially capture certain types of hypothetical dark matter particles.
The study focuses on a type of ultraheavy asymmetric dark matter. Unlike ordinary matter and some dark-matter models, asymmetric dark matter does not efficiently annihilate with itself.
That means captured particles could continue accumulating inside the star.
Over extremely long periods, the dark matter could become concentrated near the stellar core. Eventually, if enough dark matter collects in one place, its own gravity could become strong enough to cause the accumulated material to collapse into a tiny black hole.
This creates what researchers describe as an endoparasitic black hole—a black hole living inside its host star.
But simply creating the black hole does not guarantee that the star will be destroyed.
A competition immediately begins.
A Race Between Growth and Evaporation
The newly formed black hole faces two opposing processes.
On one side is Hawking radiation, which removes mass and causes the black hole to shrink.
On the other side is accretion. The black hole can gain mass by absorbing material from the surrounding star. It can also potentially continue receiving dark matter.
The final outcome depends on which process wins.
If Hawking evaporation is stronger, the tiny black hole becomes smaller and eventually disappears.
If the supply of matter is strong enough, however, the black hole can begin growing.
Once growth gets ahead of evaporation, the situation changes dramatically. As the black hole becomes larger, its ability to absorb surrounding matter increases, allowing growth to continue.
This creates a critical mass.
Below that mass, evaporation wins. Above it, growth can win.
Why Dark Matter Changes the Picture
Without continued dark matter feeding, the critical mass can be extremely large.
For representative compact-star conditions, calculations considering only ordinary stellar accretion and Hawking evaporation can produce a characteristic critical mass of around 10¹⁰ kilograms.
But adding a continuous supply of dark matter changes the calculation.
The critical mass can become dramatically smaller because the black hole receives an additional source of incoming mass.
For a white dwarf in the Galactic disk, the critical initial mass is estimated to be around 10,000 metric tons under the conditions studied.
But the environment becomes much more interesting in the Galactic bulge, where dark matter is expected to be considerably more abundant.
There, the calculated critical mass for a black hole inside a white dwarf can fall to approximately 40 metric tons.
For a neutron star in the Galactic bulge, the corresponding value is around 600 metric tons.
That 40-ton figure is roughly comparable to the mass of a heavily loaded semitruck.
But There Is an Important Catch
This does not mean scientists have discovered a 40-ton black hole.
There is currently no observation showing that such a black hole exists inside a star.
The result is a theoretical calculation showing that, under particular assumptions about dark matter and the stellar environment, a black hole with an initial mass of around 40 metric tons could have a positive growth rate.
In other words, it could gain mass faster than it loses mass through Hawking radiation.
Given enough time, such a black hole could continue growing.
Eventually, it could consume increasing amounts of the surrounding star. In the extreme end state, the host star could be destroyed and the system could become a much larger black hole.
The Physics Gets Even More Complicated
At these incredibly small scales, researchers cannot simply assume that matter behaves like a smooth fluid flowing into the black hole.
For a tiny black hole, the wavelength associated with incoming particles can become comparable to the black hole's characteristic size. Under these circumstances, the usual classical description of accretion may no longer be sufficient.
A quantum description of particle absorption is needed.
As the black hole grows, however, the situation changes. Eventually, its size becomes large enough for the familiar fluid-like description of accretion to become appropriate.
The calculations therefore have to account for both regimes while simultaneously considering stellar matter, dark matter feeding and Hawking evaporation.
Stars Could Become Natural Dark Matter Detectors
Perhaps the most interesting consequence of this idea is that it provides a possible way to test dark matter indirectly.
If certain ultraheavy dark matter particles were capable of accumulating inside stars and creating growing black holes, those black holes could eventually destroy their host stars.
That creates a natural constraint.
Astronomers observe extremely old white dwarfs and neutron stars, including long-lived millisecond pulsars. Some of these objects have survived for billions of years.
Their survival suggests that whatever dark matter is made of cannot interact with ordinary matter in just any way.
If a particular dark matter model predicted that old compact stars should have been destroyed long ago, but such stars are still observed, that model could potentially be ruled out or strongly constrained.
In this sense, stars can act as enormous natural laboratories.
They have been accumulating and interacting with matter for billions of years—far longer than any human experiment could operate.
A Remarkable Possibility, Not an Observed Object
The idea of a 40-ton black hole sounds almost impossible because black holes are normally associated with enormous masses.
But the physics of black holes does not require them to be stellar-sized. Extremely small black holes are possible in theory; the challenge is keeping them from evaporating.
The study suggests that the environment inside a compact star can change that balance dramatically.
Hawking radiation tries to destroy the tiny black hole, while stellar matter and dark matter can feed it.
Under favorable conditions, dark matter can provide enough additional material to push the balance toward growth—even when the initial black hole weighs only about 40 metric tons.
The result is not evidence that such black holes exist. Instead, it highlights a fascinating possibility: the smallest black holes might not necessarily live alone in space. Some could, in principle, be born deep inside stars, where dark matter helps them survive and grow.
The findings, published in Physical Review D, also show how observations of ancient stars could help physicists investigate one of the biggest mysteries in the universe: what dark matter actually is.
Reference: H. A. Adarsha et al, Transmutation timescales for the dark matter induced collapse of compact stars into black holes, Physical Review D (2026). DOI: 10.1103/zb1m-762n.
Comments
Post a Comment