Spinning primordial black holes: a new look at dark matter through cosmic dust heating ✛ ✦
Context
The mystery of dark matter remains one of the biggest unsolved problems in physics. Among many candidates, primordial black holes (PBHs) draw attention because they require no new physics beyond the Standard Model and naturally explain black hole mergers detected by gravitational-wave observatories. Our entire Galaxy becomes a laboratory to test their existence, with the key being humble interstellar dust—a component of the interstellar medium.
Methods
Instead of traditional lensing or gamma-ray background, the authors used the thermal balance of interstellar dust. PBHs with masses $10^{15}$–$10^{17}$ g emit Hawking radiation, predicted by Stephen Hawking—photons and electrons—which is absorbed by dust grains. If heating exceeds cooling via infrared emission, the dust will overheat. Cooling was calculated for two compositions: silicate and graphite, using Planck's law and the Stefan–Boltzmann law for equilibrium temperatures. Heating was estimated through the full photon spectrum of PBHs, including secondary photons from particle decays, with numerical simulations (BlackHawk code). For the first time in this method, PBH spin and five dark halo density profile models were taken into account, including isothermal, NFW, and Burkert, as well as a lognormal mass function.
Results
It turns out that even moderate spin significantly boosts the radiation flux, especially at a spin parameter a* ≈ 1. For instance, for a PBH of mass $10^{15}$ g, the spectrum extends into the hard region, increasing dust heating. Among profiles, the isothermal one gives the strongest constraints, while Burkert’s the weakest. For a monochromatic mass function and silicate dust grains, the upper limit on the PBH fraction in dark matter (fPBH) reaches $10^{-4}$, i.e., no more than 0.01%. For graphite, the limits are about twice as weak. With a lognormal distribution of width σ=2, an entire range of massive PBHs is ruled out as the dominant dark matter component. These results agree well with calculations for different halo structures, showing the method is universal though sensitive to the model choice.
Implications
The work shows that dust heating is a previously underestimated tool for probing PBH parameters. Accounting for spin opens up new constraints on nearly extremal spinning black holes, complementing data from X-ray telescopes and the gamma-ray background. The results are important for PBH formation theories: if future observations reveal that such holes have high spin, it would point to nontrivial processes in the early Universe.
Future development
Future studies could include a realistic dust distribution in the Galactic disk—this will tighten constraints in central regions by four orders of magnitude. Also promising is accounting for the electron contribution with their mean free path in the interstellar medium, and a detailed analysis of spectral features of heating by individual lines. Cross-correlation with infrared survey data will allow testing the predicted thermal anomalies.
Impact
The method will impact early Universe cosmology, black hole physics, and models of the interstellar medium. Astronomy, particle astrophysics, and dark matter theory gain a new bridge between micro- and macroscales.
Next steps
Next steps include applying to data from all-sky infrared and submillimeter surveys, and 3D modeling of radiative transfer in an inhomogeneous interstellar medium.
Key open problems
The constraints are directly linked to unsolved problems: the nature of dark matter, the mechanism of primordial black hole formation, and their spin distribution. They also touch on the black hole entropy paradox via Bekenstein–Hawking entropy, which underlies the radiation itself.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
This study examines the heating of interstellar dust by Hawking radiation from spinning primordial black holes (PBHs), and for the first time incorporates spin into this method, using the numerical code BlackHawk and five dark halo density profiles. Constraints are placed on the PBH fraction in dark matter, particularly stringent for nearly extreme spin and silicate dust grains.
Where its strength is
Including spin significantly enhances the heating and opens a new, independent way to constrain PBH parameters, sensitive to their rotation, complementing traditional methods and potentially shedding light on early Universe processes.
Where it could go next
- It might be interesting to compare dust heating with the slow warming of gas in the early Universe, which, as shown in a related work, revives PBHs as dark matter candidates; this could allow combining constraints across different epochs.based on: Cosmic sous-vide: How slow reheating resurrects primordial black holes arXiv:2605.11332v2
- Nearby lies a study of a neutrino shower that seeds dark matter; if PBHs are not dominant, their radiation could influence such processes, and a joint analysis could refine models with mixed dark matter composition.based on: Ghost Garden: How Neutrino Showers Sprout Dark Matter Seeds arXiv:2607.09285
Worth trying
- Investigate how dust heating changes when accounting for accretion onto PBHs, which could modify their spin and emission spectrum.
- Apply the method to dust data in other galaxies, where dark matter density profiles may differ from those considered here.
- Combine constraints from dust heating with gamma-ray background and gravitational lensing data in a unified Bayesian analysis.
Why this may matter
If spinning PBHs do exist, their contribution to dust heating could become a decisive argument in the debate over dark matter's nature, and the proposed method could become a standard tool alongside gravitational-wave and electromagnetic observations.
Nearby works in our archive
- How the First Black Holes Are Bornsimilarity 0.69
- Hot Spots from Tiny Black Holessimilarity 0.67
- Invisible Conductors of Galactic Diskssimilarity 0.67
- A Black Hole with Magnetic Chargesimilarity 0.66
- The Gentle Boiling of the Cosmossimilarity 0.66
- Cosmic sous-vide: How slow reheating resurrects primordial black holessimilarity 0.66
- Ghost Garden: How Neutrino Showers Sprout Dark Matter Seedssimilarity 0.64
The closest work in the archive sits at 0.69 by meaning — the usual level for our corpus.
Original abstract
as written by the authors arXiv:2607.15028
Primordial black holes (PBHs) are compelling dark matter candidates. PBHs with masses between $$10^{15}$$ and $$10^{18}\,\mathrm{g}$$ can heat interstellar dust via Hawking radiation. Previous studies of this dust heating mechanism mostly neglected PBH spin and adopted a single dark matter halo profile. In this work, we incorporate PBH spin, which substantially enhances the emitted radiation flux, and systematically investigate the dependence of constraints on the dark matter density distribution by considering five different halo models. We compute the complete photon spectra, including both primary and secondary emissions. Our results show that, for a fixed profile and mass function, larger spin parameters yield stronger constraints on the PBH fraction $$f_{\mathrm{PBH}}$$. Among the halo models, the Isothermal profile gives the most stringent limits, followed by Einasto, then NFW and Moore, while the Burkert profile yields the weakest constraints. For silicate grains, which cool less efficiently than graphite, the upper limits reach $$\mathcal{O}(10^{-4})$$ for high spin cases. We consider both monochromatic and lognormal mass functions, and find consistent trends between them. For the lognormal case, larger values of the width $$\sigma$$ lead to a broader mass range being excluded, in particular ruling out massive PBHs as the sole dark matter component. Our bounds are generally weaker than other existing limits, but they provide a complementary and independent constraint.
It is still unknown whether primordial black holes exist at all — none have been directly detected. But methods like dust heating turn the entire Galaxy into a kind of detector spanning a hundred kiloparsecs.
Key numbers
- PBH mass: 1015 – 1017 г
- upper limit f_PBH (silicate): ~10-4
- spin parameter a*: 0 – 0.9999
- dust grain equilibrium temperature: 17.9 K (силикат), 20.8 K (графит)
- Milky Way halo radius: ~100–180 кпк