Darkish matter is one among Nature’s most confounding mysteries. It retains particle physicists up at night time and cosmologists glued to their supercomputer simulations. We all know it is actual as a result of its mass prevents galaxies from falling aside. However we do not know what it’s.
Darkish matter would not like different matter and will favor its personal firm. Whereas it would not appear to work together with common baryonic matter, it may presumably react with itself and self-annihilate. It wants a tightly-packed setting to try this, and that will result in a approach astrophysicists can lastly detect it.
New theoretical analysis outlines how this might occur and states that sub-stellar objects, principally brown dwarfs, may host the method. The analysis is titled “Dark dwarfs: dark matter-powered sub-stellar objects awaiting discovery at the galactic center,” and it is printed within the Journal of Cosmology and Astroparticle Physics. The lead creator is Djuna Croon, a theoretical physicist and assistant professor within the Institute for Particle Physics Phenomenology within the Division of Physics at Durham College.
Brown dwarfs are sub-stellar objects bigger than planets however not huge sufficient to set off hydrogen fusion. When brown dwarfs type, they comply with the identical course of as stars. However sadly for them, their gasoline accretion stalls and stunts their progress. They can not change into huge sufficient to set off hydrogen fusion and are doomed to a lifetime of dimness. They generate some warmth by briefly fusing deuterium and thru gravitational contraction, however they may by no means shine brightly like stars do. When it comes to luminosity, they’re dimmer than white dwarfs however barely brighter than gasoline giants like Jupiter. Over time, brown dwarfs calm down and change into dimmer and dimmer. Brown dwarfs are notoriously tough to detect because of their dimness and low mass.
An artist’s depiction of the relative sizes of the Solar, a low mass star, a brown dwarf, Jupiter, and the Earth. Sizes are to scale, however distances usually are not. Credit score: Jupiter: NASA, ESA, and A. Simon (NASA, GSFC). Solar and Low-Mass Star: NASA, SDO. Brown Dwarf: NASA, ESA, and JPL-Caltech. Earth: NASA. Infographic: NASA and E. Wheatley (STScI).
Croon and her co-researchers recommend that darkish matter annihilation may make some brown dwarfs detectable. Darkish matter particles could also be their very own anti-particles and may annihilate each other when their density is excessive. In line with E=mc2, their annihilation converts their mass to vitality and produce particles from the Commonplace Mannequin like photons, electrons, and positrons.
“Darkish matter interacts gravitationally, so it might be captured by stars and accumulate inside them,” defined co-author Jeremy Sakstein in a press release. “If that occurs, it may also work together with itself and annihilate, releasing vitality that heats the star.” Sakstein is Professor of Physics on the College of Hawai‘i.
There’s extra darkish matter close to the galactic heart, and the authors suppose that is the place sufficient of it may accumulate in brown dwarfs for self-annihilation to happen. When this occurs, it creates one other kind of sub-stellar object: darkish dwarfs.
“These objects accumulate the darkish matter that helps them change into a darkish dwarf. The extra darkish matter you have got round, the extra you may seize,” Sakstein explains. “And, the extra darkish matter finally ends up contained in the star, the extra vitality shall be produced by its annihilation.”
There are a lot of candidate particles for darkish matter. Certainly one of them is Weakly Interacting Massive Particles (WIMPs), and this theoretical mannequin solely works if WIMPs are certainly what darkish matter is. “For darkish dwarfs to exist, darkish matter must be made from WIMPs, or any heavy particle that interacts with itself so strongly to supply seen matter,” Sakstein says.
“Observing a darkish dwarf wouldn’t conclusively inform us that darkish matter is a WIMP, however it could imply that it’s both a WIMP or one thing that, for all intents and functions, behaves like a WIMP.” – Jeremy Sakstein, co-author, College of Hawaii.
If we will detect these darkish dwarfs, we’re principally detecting darkish matter. Detection depends on Lithium-7, a naturally-occurring lithium isotope that’s the most considerable and steady of all lithium isotopes. Regular brown dwarfs would deplete their lithium-7 whereas darkish dwarfs would protect it.
“DDs are bodily distinct from brown/purple dwarfs in a number of methods,” the authors write. They’re barely extra huge, and are predominantly powered by DM annihilation, with an added element of steady hydrogen fusion. Their luminosities, radii, and efficient temperatures are fixed by time. The authors additionally clarify that these darkish dwarfs would retain their lithium whereas it could be depleted by nuclear burning in brown dwarfs.
“The lithium take a look at is a major methodology for confirming that an object is a brown dwarf,” the researchers write. Astronomers use lithium strains in stellar spectra to hint the core temperature histories of brown dwarfs and younger stars and to find out what evolutionary epoch they’re in.
“The detection of lithium-7 in objects heavier than the lithium burning restrict would supply proof for the existence of DM heating,” the researchers write of their paper. “A consequence of that is that DDs may be recognized by their enhanced lithium abundance regardless of a comparatively giant mass, and outdated stellar age.” In addition they present that the quantity of lithium retained over time is a perform of mass and darkish matter densities.
This determine reveals Lithium survival after one billion years as a perform of mass for various DM densities. “Subsequently, given an noticed mass and estimated age, the spectroscopic detection of lithium can function a marker for DM heating,” the researchers write of their paper. Picture Credit score: Croon et al. 2025. Journal of Cosmology and Astroparticle Physics
“There have been a couple of markers, however we urged the Lithium-7 as a result of it could actually be a singular impact,” Sakstein defined. Bizarre stars shortly eat Lithium-7. “So if you happen to have been capable of finding an object which seemed like a darkish dwarf, you might search for the presence of this lithium as a result of it wouldn’t be there if it was a brown dwarf or the same object.”
Darkish dwarfs are extraordinarily chilly objects, and whereas the JWST could possibly detect them, there could also be one other approach, in response to Sakstein. “The opposite factor you might do is to have a look at an entire inhabitants of objects and ask, in a statistical method, whether it is higher described by having a sub-population of darkish dwarfs or not.”
Discovering darkish dwarfs would go an extended solution to answering the query concerning the nature of darkish matter. If we may detect some darkish dwarfs, it could bolster the concept that darkish matter is WIMPs.
“With mild darkish matter candidates, one thing like an axion, I don’t suppose you’d be capable to get one thing like a darkish dwarf,” mentioned Sakstein. “They don’t accumulate inside stars. If we handle to discover a darkish dwarf, it could present compelling proof that darkish matter is heavy, interacts strongly with itself, however solely weakly with the Commonplace Mannequin. This contains courses of WIMPs, however it could embrace another extra unique fashions as effectively,” concludes Sakstein.
“Observing a darkish dwarf wouldn’t conclusively inform us that darkish matter is a WIMP, however it could imply that it’s both a WIMP or one thing that, for all intents and functions, behaves like a WIMP.”
