A black hole weighing only a few dozen tons could survive and grow at the heart of a star.
A study describes what could happen inside certain very dense stars, such as neutron stars and white dwarfs. Over time, these objects could capture particles of dark matter, the invisible substance whose nature remains unknown.
In the scenario studied, these particles would be extremely massive and would not easily annihilate one another. They could therefore accumulate near the star’s center. If their concentration became high enough, their own gravity would eventually draw them together until they formed a tiny black hole inside the star.
This black hole would then have two possible fates. It could gain mass by absorbing the extremely dense matter surrounding it and new dark matter particles. But it would also lose mass through Hawking radiation. This phenomenon, predicted by quantum physics, gradually causes black holes to lose energy and acts all the faster when they are lightweight.
One might therefore think that such a small black hole would disappear almost immediately. However, the calculations by H. A. Adarsha, Chandrachur Chakraborty, and Sudip Bhattacharyya take these gains and losses into account simultaneously. When the influx of matter exceeds evaporation, the black hole can continue to grow instead of disappearing.
Dark matter significantly changes the threshold. Without a continuous supply of it, the mass needed to sustain growth can reach around 10 billion kilograms in some of the cases studied. In a region of the Milky Way rich in dark matter, the calculations fall to around 40,000 kg for a black hole formed in a white dwarf. That is approximately the mass of a fully loaded heavy truck.
This value therefore does not apply to just any star. In a white dwarf located in the galactic disk, where the Solar System is found, the calculated threshold is closer to 10 million kilograms. For a neutron star located in the central region of the Galaxy, it is around 600,000 kg. The amount of dark matter available around the star thus plays a major role.
If the black hole crosses this threshold and continues to grow, it could eventually absorb its host star. This possibility primarily offers an indirect way to search for dark matter. Very old pulsars and white dwarfs still exist today. Some forms of dark matter capable of destroying them too quickly therefore become less plausible.
The authors use this longevity to test extremely massive dark matter particles and the frequency with which they might interact with ordinary matter. However, the model remains dependent on each star’s environment. The researchers note in particular that its rotation and magnetic field could alter the results and will need to be better incorporated into future calculations.