A black hole as light as 40 tons can exist inside a star if dark matter helps

In 1974, Stephen Hawking showed that black holes are not truly black and instead slowly lose energy through Hawking radiation, causing them to evaporate. The lighter the black hole, the faster this process occurs. Primordial black holes, which formed in the early universe, would not survive until today if they were as light as 1012 kg.

However, black holes that form inside compact stars, such as neutron stars and white dwarfs, may not have existed since the early universe. These black holes can form later due to the accumulation of hypothetical ultraheavy asymmetric dark matter particles within the star. Once formed, these tiny black holes can grow by accreting ordinary matter from their host star and from continued dark matter feeding.

While Hawking evaporation causes them to lose mass, the surrounding dense stellar matter can enable their growth under favorable conditions. A newly formed black hole with an initial mass of around 40 metric tons, roughly the weight of a loaded semi-truck, could overcome Hawking evaporation and continue to grow if it receives sufficient dark matter feeding.

The critical mass for a stellar environment is much lower than for primordial black holes, which must survive cosmic timescales in isolation. In a white dwarf, this critical mass is about 10,000 metric tons, while in a Galactic bulge with more dark matter, it can drop to just 40 metric tons. This demonstrates that a black hole with such a small initial mass can grow, eventually consuming its host star.