Neutrino Growth of Primordial Black Holes: Revisiting Mass Evolution in the Early Universe ✛ ✦
Context
Primordial black holes (PBH) are among the most intriguing candidates for dark matter. Their existence could explain not only the nature of hidden mass but also recent gravitational wave signals detected by LIGO. However, until now it was believed that during the radiation era, when the Universe was filled with plasma of particles and radiation, PBHs could not accrete enough matter due to their tiny gravitational radius compared to the mean free path of radiation. This belief was based on an argument by Stephen Hawking and colleagues, as well as classical solutions to Einstein's equations. The key scale is set by the Schwarzschild radius, which is negligibly small for light PBHs. The new study changes this paradigm by considering weakly interacting particles as a source of “food” for black holes.
Methods
The authors used a semiclassical approach, treating PBHs as black bodies that absorb neutrinos from the thermal bath of the early Universe. They computed the mass growth rate by solving an ordinary differential equation for the ratio R = $M_{abso}$rbed / $M_{init}$ial. The equation accounts for the geometric high-frequency radiation absorption cross-section and the kinetics of the primordial plasma, including changes in effective degrees of freedom. A key parameter is the neutrino mean free path, compared to the Schwarzschild radius; absorption is activated by a smooth weight function W = exp(-κ $r_s$/λ_ν). Calculations were performed for various values of parameter κ (0.5; 1; 2) and collapse fraction γ. The temperature range covers the era after the cosmological quark-hadron transition, where neutrinos contribute significantly to the energy density.
Results
The results show that significant mass growth occurs for PBHs with initial masses between $10^3$ and $10^7$ solar masses. Holes in the vicinity of $10^3$–$10^5$ M⊙ grow most actively — the “sweet spot” of absorption, where the available fraction of neutrino energy is maximal and the difference between formation time and absorption onset is minimal. For γ = 0.55 and κ = 1, the final growth factor R reaches hundreds for individual masses, and the dark matter fraction in PBHs $f_{PBH}$ increases from 0.1 to 0.1129. The mass spectrum is substantially deformed: the peak associated with e⁺e⁻ annihilation shifts to larger masses, and an additional peak appears in the intermediate range. For γ > 0.55, growth becomes uncontrolled — a divergence occurs, imposing an upper bound on collapse parameters. The results are insensitive to the choice of transition function model (κ).
Implications
This work demonstrates for the first time that PBHs can effectively grow during the radiation era through neutrino absorption. This changes predictions for the mass spectrum and PBH abundance, directly affecting the interpretation of observational data: from searches for gravitational lensing events to signals from merging black holes. Moreover, a new channel for probing early Universe physics opens up: the shape of the PBH spectrum can be used to reconstruct properties of the primordial plasma and the equation of state at the QCD phase transition epoch.
Future development
Future work plans to account for lepton and baryon asymmetries, which may alter the contribution of neutrinos to the energy density and mean free path. It is also necessary to consider more realistic models of the density fluctuation spectrum produced during inflation and the spatial clustering of PBHs. Observational tests with JWST and future gravitational wave observatories, such as LISA, will allow these predictions to be verified and model parameters to be refined.
Impact
The results will impact cosmology, dark matter physics, and gravitational wave astronomy. They also touch upon the formation of supermassive black holes in the early Universe and the interpretation of “little red dots” discovered by James Webb.
Next steps
A detailed comparison with observational constraints is needed, including data from microlensing and accretion radiation. The collapse criticality should also be incorporated into the calculation of the initial mass spectrum to refine the shape of the additional peak.
Key open problems
The article directly connects the nature of dark matter with Big Bang physics and inflationary cosmology. It also sheds light on the initial seeds for galaxy formation and the origin of massive black holes, which remains one of the unsolved problems in astrophysics.
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
The study examines the growth of primordial black holes by absorbing neutrinos from the thermal bath of the early Universe. The authors solved the equation for the mass ratio R, accounting for the geometric cross-section and the neutrino mean free path, and showed that for masses of 10³–10⁵ M⊙, the growth can be significant, and the mass spectrum becomes distorted.
Where its strength is
For the first time, the possibility of efficient PBH growth during the radiation era via weakly interacting particles is demonstrated, which challenges standard arguments against accretion. This opens a new channel for mass evolution based on plasma kinetics and provides concrete predictions for the shift of peaks in the spectrum.
Where it could go next
- It might be interesting to see how the constraints on f_PBH change due to the additional heating of the plasma by evaporating PBHs, if we account for mass growth via neutrinos. In our archive, there is a related paper showing that tiny black holes heat the Universe for longer than previously thought.based on: Tiny Black Holes Warm the Universe Longer Than Thought arXiv:2605.15772
- It would be curious to compare the proposed growth mechanism with the evolution of PBHs that have not evaporated by the present epoch, and to check how neutrino accretion affects their survival. We have a quick analysis on this topic.based on: Black Holes That Didn't Evaporate arXiv:2608.08144v1
- Since the work points to the growth of massive PBHs, one could investigate whether such grown objects serve as seeds for supermassive black holes. Nearby, there is an analysis where tiny black holes act as stellar detonators.based on: Dark Matter's Invisible Sparks: Tiny Black Holes as Stellar Detonators arXiv:2606.07505v2
Worth trying
- Test combining neutrino accretion with Hawking radiation in a unified mass evolution code to see the competition between the processes.
- Check how predictions for the gamma-ray background from PBH evaporation change if some of them have grown and stopped evaporating.
- Consider neutrino absorption in the context of models with non-trivial metrics, for example, with slowed expansion at early stages.
Why this may matter
If neutrino feeding is indeed efficient, it alleviates the tension between observational constraints on f_PBH and the need to explain all dark matter as PBHs. Moreover, the shift in the mass spectrum could be critical for interpreting LIGO events as PBH mergers.
Nearby works in our archive
- Tiny Black Holes Warm the Universe Longer Than Thoughtsimilarity 0.72
- Black Holes That Didn't Evaporatesimilarity 0.70
- How Tiny Black Holes Are Born from a Slowed-Down Expansionsimilarity 0.68
- Tiny Black Holes That Survived the Universe's Youthsimilarity 0.67
- Dark Matter's Invisible Sparks: Tiny Black Holes as Stellar Detonatorssimilarity 0.67
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The archive is crowded around this work: 9 reviews nearby, closest at 0.72. This ground is well walked — and there is plenty to compare against.
Original abstract
as written by the authors arXiv:2607.09285
We present a new picture of primordial black holes mass evolution through neutrino absorption. Using semi-classical approach and a closer look at the kinetic of the early plasma we revisit the thermal absorption of radiation by a population of primordial black holes ranging from $$10^{-3}-10^9 M_\odot$$ embedded in a thermal bath. We find significant mass growth of intermediate mass and supermassive PBHs, the effect shift the predicted $$e^+e^-$$ peak from thermal history. Depending on the value of the collapse fraction an additional peak around the intermediate mass range, might become significant. Moreover, because PBH grow from the thermal bath the fraction of DM in PBH $$f_{\rm PBH}$$ also change. These results revise the previous view on mass evolution of PBH and have implications for dark matter PBH observations.
Neutrinos are so ghostly that they can travel through a light-year of lead without a single interaction, but in the dense, hot early Universe they become a meal for hungry black holes, helping them grow like crazy.
The idea of black holes growing by absorbing radiation or exotic particles echoes science fiction concepts of spacetime engineering. For example, in Carl Sagan’s “Contact,” spacetime curvature is used for interstellar travel, and in Iain M. Banks’ “Culture” series, black holes are sources of colossal energy.
Key numbers
- range of initial PBH masses: 10-3 – 109 M⊙
- temperature of neutrino absorption onset: ~10 MeV (for masses around 103 M⊙)
- masses of maximum growth ("sweet spot"): 103 – 105 M⊙
- dark matter fraction in PBHs after absorption (γ=0.55, κ=1): 0.1129 (initial 0.1)
- critical value of collapse parameter γ_div: ≈0.55