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Tiny, almost undetectable primordial black holes could be one of the mysterious sources of mass contributing to dark matter. There are significant limits to their lifespan in open space, but in recent years astrophysicists have wondered: what if these black holes are at the heart of neutron stars?
Gradually, these black holes would accrete the neutron star, devouring it from within. These hypothetical systems have yet to be verified, but a new pre-print article, published on arXiv and not yet peer-reviewed, calculated how long this devouring would take.
This, in turn, could be used to analyze the current population of neutron stars to constrain the nature of black holes considered a candidate for dark matter – whether they are primordial, dating back to the Big Bang, or black holes. which formed inside neutron stars.
While we don’t know what dark matter is, it’s pretty fundamental to our understanding of the Universe: there just isn’t enough matter we can directly detect – normal matter – to make account for all gravity. In fact, there is so much gravity that scientists have calculated that about 75-80% of all matter is dark.
There are a number of candidate particles that could be dark matter. The primordial black holes that formed just after the Big Bang are not one of the main candidates, because if they were above a certain mass, we would have noticed them already; but, below this mass, they would have evaporated via the emission of the Hawking radiation long before now.
Black holes, however, are an attractive candidate for dark matter – they’re also extremely difficult to detect if they’re just sitting in space doing nothing. Astronomers therefore continue to look for them.
One idea that has been explored recently is the endoparasitic black hole. There are two scenarios for this. The first is that the primordial black holes have been captured by neutron stars and are sinking deep into the core. The other is that dark matter particles are captured inside a neutron star; if the conditions are right, then these could come together and collapse into a black hole.
These black holes are small, but they wouldn’t stay that way. From their cozy position, installed inside the neutron star, these small black holes would parasitize their host.
The team of physicists at Bowdoin College and the University of Illinois at Urbana-Champaign calculated the rate of accretion – that is, the rate at which the black hole would devour the neutron star – to a range of black hole mass ratios, three to nine orders of magnitude less massive than the neutron star host.
Neutron stars have a theoretical upper mass limit of 2.3 times the mass of the Sun, so black hole masses extend downward into the range of dwarf planets.
For a non-rotating neutron star hosting a non-rotating black hole, the accretion would be spherical. At accretion rates calculated by the team, black holes as small as 10-21 times the mass of the Sun would fully accrete a neutron star well during the lifetime of the Universe.
This suggests that primordial black holes, since the beginning of the Universe, would have fully accreted their host neutron stars before now. These timescales are in direct conflict with the age of ancient populations of neutron stars, the researchers said.
“As an important application, our results corroborate arguments that use the current existence of populations of neutron stars to constrain either the contribution of primordial black holes to the dark matter content of the Universe or that of particles. of dark matter that can form black holes in the center of neutron stars after capture, ”they wrote in their article.
The result is therefore another blow to the primordial black holes; but it does not entirely exclude endoparasitic black holes. If there are globes of dark matter particles floating in space and being thrown into neutron stars, they could collapse into black holes and convert neutron stars into black hole stuff even as you read this phrasing.
And it’s really awesome.
The team’s article was published on arXiv.
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