A Black Hole the Weight of a Loaded Semitruck Can Live Inside a Star, If Dark Matter Feeds It

A black hole with the mass of a loaded semitruck should not last long enough to matter. Stephen Hawking's 1974 result says the smaller a black hole is, the faster it radiates itself away, and a 40-tonne object would evaporate essentially the moment it formed. Three physicists have now worked out the conditions under which one survives instead: put it inside a dense star sitting in a part of the galaxy thick with dark matter, and the material falling in can outrun the radiation leaking out.

The paper, published in Physical Review D by Chandrachur Chakraborty and H. A. Adarsha of the Manipal Centre for Natural Sciences with Sudip Bhattacharyya of the Tata Institute of Fundamental Research in Mumbai, calls the objects endoparasitic black holes — a black hole living inside a host star and feeding on it. The formation route is not stellar collapse. If dark matter consists of very heavy particles, a neutron star or white dwarf will capture them over billions of years, concentrate them in its core, and eventually accumulate enough in a small enough volume for the collection to collapse on its own.

The number that matters is the threshold mass at which growth beats evaporation. It depends entirely on how much dark matter is available to eat. In the galactic bulge, where dark matter density is highest, a black hole inside a white dwarf needs only about 40 tonnes to tip into runaway growth. Inside a neutron star in the same region the threshold is roughly 600 tonnes. Move the same white dwarf out to the galactic disk, where the dark matter is thinner, and the requirement jumps to about 10,000 tonnes.

Below the threshold the black hole evaporates and nothing happens. Above it, the black hole grows without limit and eats the star from the inside — a process that leaves no supernova, no burst of light, just a compact star that quietly becomes a black hole of its own mass.

That is where the result turns into a measurement rather than a curiosity. If this process worked easily, very old compact stars should not still be here. The galaxy is full of millisecond pulsars and white dwarfs that have been sitting in dark-matter-rich regions for billions of years without being consumed, and every one of them puts an upper bound on how readily heavy dark matter particles can accumulate and collapse. The authors make the point directly: the stars have already run the experiment, acting as natural dark matter detectors that have been collecting data far longer than any tank of liquid xenon under a mountain.

The caveat is the usual one for dark matter work. The whole calculation assumes a particular kind of dark matter — heavy, capable of being captured by ordinary matter, and not annihilating with itself once it collects. If dark matter turns out to be light, or to annihilate efficiently, the mechanism never starts. What the paper provides is a clean way to convert the continued existence of ancient stars into a constraint on the parameters, which is a rare thing in a field where most searches come back empty.