Suppose the most hesitant twinges at the heart of the Milky Way are not failed stars, but universe-powered cosmic labs powered by the universe’s most mysterious ingredient? New theoretical studies propose that dark dwarfs substellar objects radiating annihilation from dark matter can offer an exotic window into the dark matter nature, an unseen material that shapes galaxies without our telescopes.
At its core lies the mechanism whereby dark matter, in the form of Weakly Interacting Massive Particles (WIMPs), would be captured by brown dwarfs. These “failed stars,” with masses at around 8% that of the Sun, lack sufficient mass to burn hydrogen fusion. They shed very little light through gravitational contraction and sporadic deuterium fusion. But in regions saturated with dark matter such as the galactic center these objects could become repositories for WIMPs. As Jeremy Sakstein, Professor of Physics at the University of Hawai‘i, explains, “Dark matter interacts gravitationally, so it could be captured by stars and accumulate inside them. If that happens, it might also interact with itself and annihilate, releasing energy that heats the star”.
This is due to the unique nature of WIMPs. Unlike other dark matter candidates such as axions or sterile neutrinos, WIMPs are massive enough and self-interacting so that they can annihilate and release energy in the core of the star. The darker the matter trapped by the brown dwarf, the internally heated it will be, eventually turning into a dark dwarf. “For dark dwarfs to exist, dark matter has to be made of WIMPs, or any heavy particle that interacts with itself so strongly to produce visible matter,” Sakstein notes.
Spying on these missing objects, however, is a daunting task. Their defining feature may be their chemistry. Lithium-7, which is quickly burned away in the hot cores of ordinary stars, may last longer in brown dwarfs due to their lower core temperatures. But if a brown dwarf were heated by dark matter annihilation, it could heat up enough to destroy its lithium-7, doing the signature of a heavier, fusion-powered star. Sakstein explains, “There were a few markers, but we suggested the Lithium-7 because it would really be a unique effect. So if you were able to find an object which looked like a dark dwarf, you could look for the presence of this lithium because it wouldn’t be there if it were a brown dwarf or a similar object”.
The James Webb Space Telescope (JWST) is already constructed to search for such faint, cold objects. Its infrared sensitivity allows it to penetrate the galactic center full of dust, potentially distinguishing dark dwarfs from the stellar populous. Physicists propose not just direct detection of lithium-7 signatures, but statistical analysis of star populations, searching for anomalies indicative of a sub-population of dark dwarfs.
If a dark dwarf were discovered, the meaning would be profound. Sakstein says, “If we manage to find a dark dwarf, it would provide compelling evidence that dark matter is heavy, interacts strongly with itself, but only weakly with the Standard Model. This includes classes of WIMPs, but it would include some other more exotic models as well”. While not a smoking gun for WIMPs, the discovery would rightly narrow the field of dark matter suspects.
The search for dark dwarfs thus lies at the intersection of astrophysics and particle physics, leveraging advances in telescope technology and theoretical modeling. As new data arrive from JWST and other telescopes, the faint glow of these objects could well soon reveal secrets long hidden from science.
