At the center of the Milky Way, like a matryoshka doll within a matryoshka doll, lies a supermassive black hole, surrounded by a swarm of stars and an invisible shell of dark matter. This shell, or "spike," is expected to be extremely dense right at the horizon. But stars and small black holes orbiting nearby could act like a spoon in a cup of coffee: stirring and scattering the particles. The authors of the new study set out to check how much this "spoon" damages the spike.
They used data on real stars in the galactic center — S2 and S38—which approach the black hole to about 300 times the radius of its horizon. It turned out that at such distances, the stars barely disturb the dark matter. Farther out, at 30,000 horizons, the stellar cluster does sweep particles away, but that's too far from where gravitational waves that detectors catch are born.
Next, the scientists simulated the history of small black holes of 10 solar masses falling into the central supermassive hole. Such events are called extreme mass ratio inspirals (EMRIs). Over 10 billion years, about 60 such falls would occur. Even this doesn't erase the spike: in the very core, at distances less than one ten-thousandth of the horizon, the dark matter density drops only to 82% of its original value. It's like running a vacuum cleaner over a carpet ten times and finding the pattern almost untouched.
The result matters for future observations: if the spike persists, it should slightly alter the shape of gravitational waves emitted by a small black hole as it approaches the giant. The space-based detector LISA, set to operate in the 2030s, could pick up these changes and thereby "weigh" the invisible shell around the black hole. So dark matter might not be so dark after all: we just need to listen to it, not look at it.
as written by the authors arXiv:2606.13761
Establishing realistic expectations for the dark matter (DM) distribution near a supermassive black hole (BH) is essential for assessing environmental imprints on gravitational wave (GW) signals. Using the Galactic Center as an observationally constrained case study, we investigate the evolution of DM density spikes under gravitational perturbations from the nuclear stellar and black hole populations surrounding the central BH. We find that scattering of DM particles by the nuclear star cluster depletes the DM distribution at radii $$r \sim 10^{-1}~\mathrm{pc}$$, far outside the region where the relevant GW signals are produced. At smaller radii, at $$r \sim 10^{-3}~\mathrm{pc}$$, the closest known stellar perturbers, S2 and S38, induce only negligible changes to the density profile. At still smaller radii, where no stellar perturbers are currently known, we assess the cumulative impact of past EMRIs by modelling successive mergers with stellar-mass BHs of mass $$\sim10~\mathrm{M}_\odot$$, in numbers consistent with the expected rate over $$10~\mathrm{Gyr}$$. We find that these events do not erase the central overdensity, reducing the density only to approximately $$82 \%$$ of its initial value at $$r \lesssim 10^{-5}~\mathrm{pc}$$. Our results indicate that, at least at the Galactic Center, DM overdensities around the central BH are expected to remain largely intact under stellar perturbations and plausible stellar-BH merger histories.
The density of dark matter at the horizon of a supermassive black hole could be a million times higher than the average in the galaxy, like all the air in a room compressed into a thimble.
In Peter Watts's novel Blindsight, an alien ship disguises itself as a clump of dark matter — perhaps such "spikes" are indeed the best hiding places in the universe.