Dark dwarfs could be lurking out in the cosmos, according to a new study. These hypothetical, effectively eternal bodies would be powered by dark matter annihilation, and though dim, there is a way we could spot them.
Dark matter is expected to pervade the Universe, but being… well, dark, it’s hard to find. It doesn’t reflect or emit light – its existence is only inferred through its interactions with regular matter via gravity. No direct evidence of it has been found, despite decades of searching.
Now, astrophysicists in the UK and US propose a new place we might find dark matter – hiding in the hearts of brown dwarfs.
Bigger than gas giant planets but smaller than stars, these substellar objects never gather enough mass to kickstart the nuclear fusion process that powers stars. Instead, they float around space as large, dim, cold worlds.
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But in some cases, the new study proposes, brown dwarfs might transform into something more intriguing. In areas with a higher density of dark matter, the strange stuff could accumulate in the brown dwarf’s core. And if it’s a certain type of dark matter, the particles could interact with each other to produce energy that powers the brown dwarf.
The team calls these theoretical bodies “dark dwarfs”.
“These objects collect the dark matter that helps them become a dark dwarf,” says Jeremy Sakstein, astrophysicist at the University of Hawai’i.
“The more dark matter you have around, the more you can capture. And, the more dark matter ends up inside the star, the more energy will be produced through its annihilation.”
A hypothesis is only as strong as its testability, and the researchers include a way astronomers could verify the existence of dark dwarfs: look for lithium-7.
This particular isotope burns away quickly inside stars due to their intense heat. But in cooler objects like brown dwarfs, lithium-7 can stick around. Astronomers already use the presence of lithium-7 as a signal to confirm that an object is a brown dwarf.
However, if the extra energy from dark matter annihilation is powering a brown dwarf, it could appear larger and brighter than even red dwarfs of the same mass. So if you find something that looks like a red dwarf but has a lithium-7 signature, you might just have a black dwarf – and confirmation of dark matter.
Of course there are a lot of ifs. For one, dark matter would need to exist in a specific theorized form, made up of weakly interacting massive particles (WIMPs). One of the leading candidates, WIMPs would barely interact with regular matter, besides its gravitational influence, but would interact with themselves.
WIMPs would be their own antiparticles, meaning if two of them touch, they’d annihilate each other in a burst of energy.
When you cram them densely into a confined place – like the core of a brown dwarf – the energy released from all those collisions would power the object. Not to the extent of a star, of course, but more than your garden-variety brown dwarf.
Because this process gives them a constant size, temperature, and brightness, dark dwarfs would be effectively eternal, the team says.
However, if dark matter happens to take on a different form, like axions or dark photons, there’d be no way to tell from the outside that it’s accumulating inside brown dwarfs.
That’s if dark matter exists at all – there’s always the chance that the effects we attribute to it are caused by some other unknown physics.
Still, it’s important to come up with various ideas about what dark matter could be and how we might detect it. Different observatories and experiments can look out for different potential signatures of dark matter, allowing us to investigate a wide range of possibilities at once.
In this case, the researchers say that the best place to look for dark dwarf signatures would be toward the center of our galaxy, where dark matter would gather most densely.
The research is available on preprint server arXiv and will be published in the Journal of Cosmology and Astroparticle Physics.
